AHMADCHEM

Hujan asam didefinisikan sebagai segala macam hujan dengan pH di bawah 5,6. Hujan secara alami bersifat asam (pH sedikit di bawah 6) karena karbondioksida (CO2) di udara yang larut dengan air hujan memiliki bentuk sebagai asam lemah. Jenis asam dalam hujan ini sangat bermanfaat karena membantu melarutkan mineral dalam tanah yang dibutuhkan oleh tumbuhan dan binatang.

Hujan asam disebabkan oleh belerang (sulfur) yang merupakan pengotor dalam bahan bakar fosil serta nitrogen di udara yang bereaksi dengan oksigen membentuk sulfur dioksida dan nitrogen oksida. Zat-zat ini berdifusi ke atmosfer dan bereaksi dengan air untuk membentuk asam sulfat dan asam nitrat yang mudah larut sehingga jatuh bersama air hujan. Air hujan yang asam tersebut akan meningkatkan kadar keasaman tanah dan air permukaan yang terbukti berbahaya bagi kehidupan ikan dan tanaman. Usaha untuk mengatasi hal ini saat ini sedang gencar dilaksanakan.

[sunting] Sumber
Secara alami hujan asam dapat terjadi akibat semburan dari gunung berapi dan dari proses biologis di tanah, rawa, dan laut. Akan tetapi, mayoritas hujan asam disebabkan oleh aktivitas manusia seperti industri, pembangkit tenaga listrik, kendaraan bermotor dan pabrik pengolahan pertanian (terutama amonia). Gas-gas yang dihasilkan oleh proses ini dapat terbawa angin hingga ratusan kilometer di atmosfer sebelum berubah menjadi asam dan terdeposit ke tanah.

Hujan asam karena proses industri telah menjadi masalah yang penting di Republik Rakyat Cina, Eropa Barat, Rusia dan daerah-daerah di arahan anginnya. Hujan asam dari pembangkit tenaga listrik di Amerika Serikat bagian Barat telah merusak hutan-hutan di New York dan New England. Pembangkit tenaga listrik ini umumnya menggunakan batu bara sebagai bahan bakarnya.


Proses yang terlibat dalam pemecahan Asam ( catatan: bahwa hanya SO2 dan NOX memegang peran penting dalam hujan asam).
[sunting] Pembentukan hujan asam
Secara sedehana, reaksi pembentukan hujan asam sebagai berikut:



Bukti terjadinya peningkatan hujan asam diperoleh dari analisa es kutub. Terlihat turunnya kadar pH sejak dimulainya Revolusi Industri dari 6 menjadi 4,5 atau 4. Informasi lain diperoleh dari organisme yang dikenal sebagai diatom yang menghuni kolam-kolam. Setelah bertahun-tahun, organisme-organisme yang mati akan mengendap dalam lapisan-lapisan sedimen di dasar kolam. Pertumbuhan diatom akan meningkat pada pH tertentu, sehingga jumlah diatom yang ditemukan di dasar kolam akan memperlihatkan perubahan pH secara tahunan bila kita melihat ke masing-masing lapisan tersebut.

Sejak dimulainya Revolusi Industri, jumlah emisi sulfur dioksida dan nitrogen oksida ke atmosfer turut meningkat. Industri yang menggunakan bahan bakar fosil, terutama batu bara, merupakan sumber utama meningkatnya oksida belerang ini. Pembacaan pH di area industri terkadang tercatat hingga 2,4 (tingkat keasaman cuka). Sumber-sumber ini, ditambah oleh transportasi, merupakan penyumbang-penyumbang utama hujan asam.

Masalah hujan asam tidak hanya meningkat sejalan dengan pertumbuhan populasi dan industri tetapi telah berkembang menjadi lebih luas. Penggunaan cerobong asap yang tinggi untuk mengurangi polusi lokal berkontribusi dalam penyebaran hujan asam, karena emisi gas yang dikeluarkannya akan masuk ke sirkulasi udara regional yang memiliki jangkauan lebih luas. Sering sekali, hujan asam terjadi di daerah yang jauh dari lokasi sumbernya, di mana daerah pegunungan cenderung memperoleh lebih banyak karena tingginya curah hujan di sini.

Terdapat hubungan yang erat antara rendahnya pH dengan berkurangnya populasi ikan di danau-danau. pH di bawah 4,5 tidak memungkinkan bagi ikan untuk hidup, sementara pH 6 atau lebih tinggi akan membantu pertumbuhan populasi ikan. Asam di dalam air akan menghambat produksi enzim dari larva ikan trout untuk keluar dari telurnya. Asam juga mengikat logam beracun seperi alumunium di danau. Alumunium akan menyebabkan beberapa ikan mengeluarkan lendir berlebihan di sekitar insangnya sehingga ikan sulit bernafas. Pertumbuhan Phytoplankton yang menjadi sumber makanan ikan juga dihambat oleh tingginya kadar pH.

Tanaman dipengaruhi oleh hujan asam dalam berbagai macam cara. Lapisan lilin pada daun rusak sehingga nutrisi menghilang sehingga tanaman tidak tahan terhadap keadaan dingin, jamur dan serangga. Pertumbuhan akar menjadi lambat sehingga lebih sedikit nutrisi yang bisa diambil, dan mineral-mineral penting menjadi hilang.

Ion-ion beracun yang terlepas akibat hujan asam menjadi ancaman yang besar bagi manusia. Tembaga di air berdampak pada timbulnya wabah diare pada anak dan air tercemar alumunium dapat menyebabkan penyakit Alzheimer.


[sunting] Sejarah
Hujan asam dilaporkan pertama kali di Manchester, Inggris, yang menjadi kota penting dalam Revolusi Industri. Pada tahun 1852, Robert Angus Smith menemukan hubungan antara hujan asam dengan polusi udara. Istilah hujan asam tersebut mulai digunakannya pada tahun 1872. Ia mengamati bahwa hujan asam dapat mengarah pada kehancuran alam.

Walaupun hujan asam ditemukan di tahun 1852, baru pada tahun 1970-an para ilmuwan mulai mengadakan banyak melakukan penelitian mengenai fenomena ini. Kesadaran masyarakat akan hujan asam di Amerika Serikat meningkat di tahun 1990-an setelah di New York Times memuat laporan dari Hubbard Brook Experimental Forest di New Hampshire tentang of the banyaknya kerusakan lingkungan yang diakibatkan oleh hujan asam.


Upaya Pengendalian Hujan Asam

Usaha untuk mengendalikan hujan asam ialah menggunakan bahan bakar yang mengandung sedikit zat pencemae, menghindari terbentuknya zat pencemar saar terjadinya pembakaran, menangkap zat pencemar dari gas buangan dan penghematan energi.

a. Bahan Bakar Dengan kandungan Belerang Rendah
Kandungan belerang dalam bahan bakar bervariasi. Masalahnya ialah sampai saat ini Indonesia sangat tergantung dengan minyak bumi dan batubara, sedangkan minyak bumi merupakan sumber bahan bakar dengan kandungan belerang yang tinggi.
Penggunaan gas asalm akan mengurangi emisi zat pembentuk asam, akan tetapi kebocoran gas ini dapat menambah emisi metan. Usaha lain yaitu dengan menggunakan bahan bakar non-belerang misalnya metanol, etanol dan hidrogen. Akan tetapi penggantian jenis bahan bakar ini haruslah dilakukan dengan hati-hati, jika tidak akan menimbulkan masalah yang lain. Misalnya pembakaran metanol menghasilkan dua sampai lima kali formaldehide daripada pembakaran bensin. Zat ini mempunyai sifat karsinogenik (pemicu kanker).

b. Mengurangi kandungan Belerang sebelum Pembakaran
Kadar belarang dalam bahan bakar dapat dikurangi dengan menggunakan teknologi tertentu. Dalam proses produksi, misalnya batubara, batubara diasanya dicuci untukk membersihkan batubara dari pasir, tanah dan kotoran lain, serta mengurangi kadar belerang yang berupa pirit (belerang dalam bentuk besi sulfida( sampai 50-90% (Soemarwoto, 1992).

c. pengendalian Pencemaran Selama Pembakaran
Beberapa teknologi untuk mengurangi emisi SO2 dan Nox pada waktu pembakaran telah dikembangkan. Slah satu teknologi ialah lime injection in multiple burners (LIMB). Dengan teknologi ini, emisi SO2 dapat dikurangi sampai 80% dan NOx 50%.

Caranya dengan menginjeksikan kapur dalam dapur pembakaran dan suhu pembakaran diturunkan dengan alat pembakar khusus. Kapur akan bereaksi dengan belerang dan membentuk gipsum (kalsium sulfat dihidrat). Penuruna suhu mengakibatkan penurunan pembentukan Nox baik dari nitrogen yang ada dalam bahan bakar maupun dari nitrogen udara.

Pemisahan polutan dapat dilakukan menggunakan penyerap batu kapur atau Ca(OH)2. Gas buang dari cerobong dimasukkan ke dalam fasilitas FGD. Ke dalam alat ini kemudian disemprotkan udara sehingga SO2 dalam gas buang teroksidasi oleh oksigen menjadi SO3. Gas buang selanjutnya "didinginkan" dengan air, sehingga SO3 bereaksi dengan air (H2O) membentuk asam sulfat (H2SO4). Asam sulfat selanjutnya direaksikan dengan Ca(OH)2 sehingga diperoleh hasil pemisahan berupa gipsum (gypsum). Gas buang yang keluar dari sistem FGD sudah terbebas dari oksida sulfur. Hasil samping proses FGD disebut gipsum sintetis karena memiliki senyawa kimia yang sama dengan gipsum alam.

d. Pengendalian Setelah Pembakaran
Zat pencemar juga dapat dikurangi dengan gas ilmiah hasil pembakaran. Teknologi yang sudah banyak dipakai ialah fle gas desulfurization (FGD) (Akhadi, 2000. Prinsip teknologi ini ialah untuk mengikat SO2 di dalam gas limbah di cerobong asap dengan absorben, yang disebut scubbing (Sudrajad, 2006). Dengan cara ini 70-95% SO2 yang terbentuk dapat diikat. Kerugian dari cara ini ialah terbentuknya limbah. Akan tetapi limbah itu dapat pula diubah menjadi gipsum yang dapat digunakan dalam berbagai industri. Cara lain ialah dengan menggunakan amonia sebagai zat pengikatnya sehingga limbah yang dihasilkan dapat dipergunakan sebagi pupuk.
Selain dapat mengurangi sumber polutan penyebab hujan asam, gipsum yang dihasilkan melalui proses FGD ternyata juga memiliki nilai ekonomi karena dapat dimanfaatkan untuk berbagai keperluan, misal untuk bahan bangunan. Sebagai bahan bangunan, gipsum tampil dalam bentuk papan gipsum (gypsum boards) yang umumnya dipakai sebagai plafon atau langit-langit rumah (ceiling boards), dinding penyekat atau pemisah ruangan (partition boards) dan pelapis dinding (wall boards).

Amerika Serikat merupakan negara perintis dalam memproduksi gipsum sintetis ini. Pabrik wallboard dari gipsum sintetis yang pertama di AS didirikan oleh Standard Gypsum LLC mulai November tahun 1997 lalu. Lokasi pabriknya berdekatan dengan stasiun pembangkit listrik Tennessee Valley Authority (TVA) di Cumberland yang berkapasitas 2600 megawatt.

Produksi gipsum sintetis merupakan suatu terobosan yang mampu mengubah bahan buangan yang mencemari lingkungan menjadi suatu produk baru yang bernilai ekonomi. Sebagai bahan wallboard, gipsum sintetis yang diproduksi secara benar ternyata memiliki kualitas yang lebih baik dibandingkan gipsum yang diperoleh dari penambangan. Gipsum hasil proses FGD ini memiliki ukuran butiran yang seragam. Mengingat dampak positifnya cukup besar, tidak mustahil suatu saat nanti, setiap PLTU batu bara akan dilengkapi dengan pabrik gipsum sintetis.

d. Mengaplikasikan prinsip 3R (Reuse, Recycle, Reduce)
Hendaknya prinsip ini dijadikan landasan saat memproduksi suatu barang, dimana produk itu harus dapat digunakan kembali atau dapat didaur ulang sehingga jumlah sampah atau limbah yang dihasilkan dapat dikurangi. Teknologi yang digunakan juga harus diperhatikan, teknologi yang berpotensi mengeluarkan emisi hendaknya diganti dengan teknologi yang lebih baik dan bersifat ramah lingkungan. Hal ini juga berkaitan dengan perubahan gaya hidup, kita sering kali berlomba membeli kendaraan pribadi, padahal transportasilah yang merupakan penyebab tertinggi pencemaran udara. Oleh karena itu kita harus memenuhi kadar baku mutu emisi, baik di industri maupun transportasi.
Di beberapa area, sulfat tersebut dijual ke pabrik kimia sebagai gipsum bila kadar kalsium sulfatnya tinggi. Di tempat lain, sulfat tersebut ditempatkan di land-fill.


Di Amerika Serikat, banyak pembangkit tenaga listrik tenaga batu bara menggunakan Flue gas desulfurization (FGD) untuk menghilangkan gas yang mengandung belerang dari cerobong mereka. Sebagai contoh FGD adalah wet scrubber yang umum digunakan di Amerika Serikat dan negara-negara lainnya. Wet scrubber pada dasarnya adalah tower yang dilengkapi dengan kipas yang mengambil gas asap dari cerobong ke tower tersebut. Kapur atau batu kapur dalam bentuk bubur juga diinjeksikan ke ke dalam tower sehingga bercampur dengan gas cerobong serta bereaksi dengan sulfur dioksida yang ada, Kalsium karbonat dalam batu kapur menghasilkan kalsium sulfat ber pH netral yang secara fisik dapat dikeluarkan dari scrubber. Oleh karena itu, scrubber mengubah polusi menjadi sulfat industri.

Di beberapa area, sulfat tersebut dijual ke pabrik kimia sebagai gipsum bila kadar kalsium sulfatnya tinggi. Di tempat lain, sulfat tersebut ditempatkan di land-fill.
Diposkan oleh rahmanto di 18:55 1 komentar

For past climate change, see paleoclimatology and geologic temperature record.
Global mean surface temperature difference from the average for 1961–1990

Mean surface temperature change for the period 1999 to 2008 relative to the average temperatures from 1940 to 1980

Global warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the last century.[1][A] The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation caused most of the observed temperature increase since the middle of the 20th century.[1] The IPCC also concludes that variations in natural phenomena such as solar radiation and volcanoes produced most of the warming from pre-industrial times to 1950 and had a small cooling effect afterward.[2][3] These basic conclusions have been endorsed by more than 45 scientific societies and academies of science,[B] including all of the national academies of science of the major industrialized countries.[4] A small number of scientists dispute the consensus view.

Climate model projections summarized in the latest IPCC report indicate that the global surface temperature will probably rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first century.[1] The uncertainty in this estimate arises from the use of models with differing sensitivity to greenhouse gas concentrations and the use of differing estimates of future greenhouse gas emissions. Some other uncertainties include how warming and related changes will vary from region to region around the globe. Most studies focus on the period up to the year 2100. However, warming is expected to continue beyond 2100 even if emissions stop, because of the large heat capacity of the oceans and the long lifetime of carbon dioxide in the atmosphere.[5][6]

An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts.[7] The continuing retreat of glaciers, permafrost and sea ice is expected, with warming being strongest in the Arctic. Other likely effects include increases in the intensity of extreme weather events, species extinctions, and changes in agricultural yields.

Political and public debate continues regarding climate change, and what actions (if any) to take in response. The available options are mitigation to reduce further emissions; adaptation to reduce the damage caused by warming; and, more speculatively, geoengineering to reverse global warming. Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissio
Temperature changes
Main article: Temperature record
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

The most commonly discussed measure of global warming is the trend in globally averaged temperature near the Earth's surface. Expressed as a linear trend, this temperature rose by 0.74°C ±0.18°C over the period 1906-2005. The rate of warming over the last 50 years of that period was almost double that for the period as a whole (0.13°C ±0.03°C per decade, versus 0.07°C ± 0.02°C per decade). The urban heat island effect is estimated to account for about 0.002 °C of warming per decade since 1900.[8] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with regionally-varying fluctuations such as the Medieval Warm Period or the Little Ice Age.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[9] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[10][11] Temperatures in 1998 were unusually warm because the strongest El Niño in the past century occurred during that year.[12]

Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[13] Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.[14] The Northern Hemisphere warms faster than the Southern Hemisphere because it has more land and because it has extensive areas of seasonal snow and sea-ice cover subject to the ice-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.[15]

The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[16]

Radiative forcing
Main article: Radiative forcing

External forcing is a term used in climate science for processes external to the climate system (though not necessarily external to Earth). Climate responds to several types of external forcing, such as changes in greenhouse gas concentrations, changes in solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.[2] Attribution of recent climate change focuses on the first three types of forcing. Orbital cycles vary slowly over tens of thousands of years and thus are too gradual to have caused the temperature changes observed in the past century.

Greenhouse gases
Main articles: Greenhouse gas and Greenhouse effect
Recent increases in atmospheric carbon dioxide (CO2). Monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Diagram of greenhouse effect.

The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in the atmosphere warm a planet's lower atmosphere and surface. It was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896.[17] Existence of the greenhouse effect as such is not disputed, even by those who do not agree that the recent temperature increase is attributable to human activity. The question is instead how the strength of the greenhouse effect changes when human activity increases the concentrations of greenhouse gases in the atmosphere.

Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F).[18][C] The major greenhouse gases are water vapor, which causes about 36–70 percent of the greenhouse effect; carbon dioxide (CO2), which causes 9–26 percent; methane (CH4), which causes 4–9 percent[not in citation given]; and ozone (O3), which causes 3–7 percent.[19][20] Clouds also affect the radiation balance, but they are composed of liquid water or ice and so are considered separately from water vapor and other gases.

Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO2, methane, tropospheric ozone, CFCs and nitrous oxide. The concentrations of CO2 and methane have increased by 36% and 148% respectively since the mid-1700s.[21] These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores.[22] Less direct geological evidence indicates that CO2 values this high were last seen approximately 20 million years ago.[23] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, particularly deforestation.[24]

CO2 concentrations are continuing to rise due to burning of fossil fuels and land-use change. The future rate of rise will depend on uncertain economic, sociological, technological, and natural developments. Accordingly, the IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[25] Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if coal, tar sands or methane clathrates are extensively exploited.[26]

The destruction of stratospheric ozone by chlorofluorocarbons is sometimes mentioned in relation to global warming. Although there are a few areas of linkage, the relationship between the two is not strong. Reduction of stratospheric ozone has a cooling influence, but substantial ozone depletion did not occur until the late 1970s.[27] Tropospheric ozone contributes to surface warming.[28]

Aerosols and soot
Ship tracks over the Atlantic Ocean on the east coast of the United States. The climatic impacts from aerosol forcing could have a large effect on climate through the indirect effect.

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, has partially counteracted global warming from 1960 to the present.[29] The main cause of this dimming is aerosols produced by volcanoes and pollutants. These aerosols exert a cooling effect by increasing the reflection of incoming sunlight. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another in recent decades, so that net warming has been driven mainly by non-CO2 greenhouse gases.[30]

In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget.[31] Sulfate aerosols act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets.[32] This effect also causes droplets to be of more uniform size, which reduces growth of raindrops by collision-coalescence. Clouds modified by pollution have been shown to produce less drizzle, making the cloud brighter and more reflective to incoming sunlight, especially in the near-infrared part of the spectrum.[33]

Soot may cool or warm, depending on whether it is airborne or deposited. Atmospheric soot aerosols directly absorb solar radiation, which heats the atmosphere and cools the surface. Regionally (but not globally), as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.[34] When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.[35] The influences of aerosols, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.[36]

Solar variation
Main article: Solar variation
Solar variation over the last thirty years.

Variations in solar output have been the cause of past climate changes.[37] Although solar forcing is generally thought to be too small to account for a significant part of global warming in recent decades,[38][39] a few studies disagree, such as a recent phenomenological analysis that indicates the contribution of solar forcing may be underestimated.[40]

Greenhouse gases and solar forcing affect temperatures in different ways. While both increased solar activity and increased greenhouse gases are expected to warm the troposphere, an increase in solar activity should warm the stratosphere while an increase in greenhouse gases should cool the stratosphere.[2] Observations show that temperatures in the stratosphere have been steady or cooling since 1979, when satellite measurements became available. Radiosonde (weather balloon) data from the pre-satellite era show cooling since 1958, though there is greater uncertainty in the early radiosonde record.[41]

A related hypothesis, proposed by Henrik Svensmark, is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[42] Other research has found no relation between warming in recent decades and cosmic rays.[43][44] A recent study concluded that the influence of cosmic rays on cloud cover is about a factor of 100 lower than needed to explain the observed changes in clouds or to be a significant contributor to present-day climate change.[45]

Feedback
Main article: Effects of global warming

A positive feedback is a process that amplifies some change. Thus, when a warming trend results in effects that induce further warming, the result is a positive feedback; when the warming results in effects that reduce the original warming, the result is a negative feedback. The main positive feedback in global warming involves the tendency of warming to increase the amount of water vapor in the atmosphere. The main negative feedback in global warming is the effect of temperature on emission of infrared radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature.

Water vapor feedback
If the atmosphere is warmed, the saturation vapor pressure increases, and the amount of water vapor in the atmosphere will tend to increase. Since water vapor is a greenhouse gas, the increase in water vapor content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapor (a positive feedback), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO2 alone. Although this feedback process causes an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[46]
Cloud feedback
Warming is expected to change the distribution and type of clouds. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud, details that are difficult to represent in climate models.[46]
Lapse rate
The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with the fourth power of temperature, longwave radiation escaping to space from the relatively cold upper atmosphere is less than that emitted toward the ground from the lower atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height. Both theory and climate models indicate that global warming will reduce the rate of temperature decrease with height, producing a negative lapse rate feedback that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.[47]
Ice-albedo feedback
Aerial photograph showing a section of sea ice. The lighter blue areas are melt ponds and the darkest areas are open water, both have a lower albedo than the white sea ice. The melting ice contributes to the ice-albedo feedback.
When ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.[48]
Arctic methane release
Warming is also the triggering variable for the release of methane from sources both on land and on the deep ocean floor, making both of these possible feedback effects. Thawing permafrost, such as the frozen peat bogs in Siberia, creates a positive feedback due to the release of CO2 and methane.[49]
Reduced absorption of CO2 by the oceans
Ocean ecosystems' ability to sequester carbon is expected to decline as the oceans warm. This is because warming reduces the nutrient levels of the mesopelagic zone (about 200 to 1000 m deep), which limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[50]
Gas release
Release of miscellaneous gases of biological origin may be affected by global warming, but research into such effects is at an early stage. Such releases may have direct climate effects, such as Nitrous oxide[51] released from peat and indirect effects, such as Dimethyl sulfide[52] released from oceans.

Climate models
Main article: Global climate model
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions and regionally divided economic development.

The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

The main tools for projecting future climate changes are computer models of the climate. These models are based on physical principles including fluid dynamics and radiative transfer. Although they attempt to include as many processes as possible, simplifications of the actual climate system are inevitable because of the constraints of available computer power and limitations in knowledge of the climate system. All modern climate models are in fact combinations of models for different parts of the Earth that are coupled to one another. These include an atmospheric model for air movement, temperature, clouds, and other atmospheric properties; an ocean model that predicts temperature, salt content, and circulation of ocean waters; models for ice cover on land and sea; and a model of heat and moisture transfer from soil and vegetation to the atmosphere. Some models also include treatments of chemical and biological processes.[53] Climate models project a warmer climate due to increasing levels of greenhouse gases.[54] Although much of the variation in model outcomes depends on the greenhouse gas emissions used as inputs, the temperature effect of a specific greenhouse gas concentration (climate sensitivity) varies depending on the model used. The representation of clouds is one of the main sources of uncertainty in present-generation models.[55]

Global climate model projections of future climate most often have used estimates of greenhouse gas emissions from the IPCC Special Report on Emissions Scenarios (SRES). In addition to human-caused emissions, some models also include a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain. Some observational studies also show a positive feedback.[56][57][58]

Including uncertainties in future greenhouse gas concentrations and climate sensitivity, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] A 2008 paper predicts that the global temperature may not increase during the next decade because short-term natural fluctuations may temporarily outweigh greenhouse gas-induced warming.[59]

Models are also used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes. Although these models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, they do indicate that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[24] The physical realism of models is tested by examining their ability to simulate current or past climates.[60] While a 2007 study by David Douglass and colleagues found that the models did not accurately predict observed changes in the tropical troposphere,[61] a 2008 paper published by a 17-member team led by Ben Santer noted errors in the Douglass study, and found instead that the models and observations were not statistically different.[62] Not all effects of global warming are accurately predicted by the climate models used by the IPCC. For example, observed Arctic shrinkage has been faster than that predicted.[63]

Attributed and expected effects

Environmental
Main articles: Effects of global warming and Regional effects of global warming
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

It usually is impossible to connect specific weather events to global warming. Instead, global warming is expected to cause changes in the overall distribution and intensity of events, such as changes to the frequency and intensity of heavy precipitation. Broader effects are expected to include glacial retreat, Arctic shrinkage, and worldwide sea level rise. Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events are attributable in part to global warming.[64] Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and some adverse health effects from warmer temperatures.[65]

Social and economic effects of global warming may be exacerbated by growing population densities in affected areas. Temperate regions are projected to experience some benefits, such as fewer cold-related deaths.[66] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[64] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature (see Atlantic Multidecadal Oscillation), but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 0.18 to 0.59 meters (0.59 to 1.9 ft) in 2090-2100 relative to 1980-1999,[1] new trade routes resulting from arctic shrinkage,[67] possible thermohaline circulation slowing, increasingly intense (but less frequent) hurricanes and extreme weather events,[68] reductions in the ozone layer, changes in agriculture yields, changes in the range of climate-dependent disease vectors,[69] which has been linked to increases in the prevalence of malaria and dengue fever,[70] and ocean oxygen depletion.[71] Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[72] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in ocean acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[73] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][74] Heat and carbon dioxide trapped in the oceans may still take hundreds years to be re-emitted, even after greenhouse gas emissions are eventually reduced.[6] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns and disruptions in food webs.[75] One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[76] However, few mechanistic studies have documented extinctions due to recent climate change,[77] and one study suggests that projected rates of extinction are uncertain.[78]

Economic
Main articles: Economics of global warming and Low-carbon economy
Projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From IPCC AR4.

The IPCC reports the aggregate net economic costs of damages from climate change globally (discounted to the specified year). In 2005, the average social cost of carbon from 100 peer-reviewed estimates is US$12 per tonne of CO2, but range -$3 to $95/tCO2. The IPCC's gives these cost estimates with the caveats, "Aggregate estimates of costs mask significant differences in impacts across sectors, regions and populations and very likely underestimate damage costs because they cannot include many non-quantifiable impacts."[79]

One widely publicized report on potential economic impact is the Stern Review, written by Sir Nicholas Stern. It suggests that extreme weather might reduce global gross domestic product by up to one percent, and that in a worst-case scenario global per capita consumption could fall by the equivalent of 20 percent.[80] The response to the Stern Review was mixed. The Review's methodology, advocacy and conclusions were criticized by several economists, including Richard Tol, Gary Yohe,[81] Robert Mendelsohn[82] and William Nordhaus.[83] Economists that have generally supported the Review include Terry Barker,[84] William Cline,[85] and Frank Ackerman.[86] According to Barker, the costs of mitigating climate change are 'insignificant' relative to the risks of unmitigated climate change.[87]

According to United Nations Environment Programme (UNEP), economic sectors likely to face difficulties related to climate change include banks, agriculture, transport and others.[88] Developing countries dependent upon agriculture will be particularly harmed by global warming.[89]

Responses to global warming

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement responses. These responses to global warming can be divided into mitigation of the causes and effects of global warming, adaptation to the changing global environment, and geoengineering to reverse global warming.

Mitigation
Main article: Mitigation of global warming
Carbon capture and storage (CCS) is an approach to mitigation. Emissions may be sequestered from fossil fuel power plants, or removed during processing in hydrogen production. When used on plants, it is known as bio-energy with carbon capture and storage.

Mitigation of global warming is accomplished through reductions in the rate of anthropogenic greenhouse gas release. Models suggest that mitigation can quickly begin to slow global warming, but that temperatures will appreciably decrease only after several centuries.[90] The world's primary international agreement on reducing greenhouse gas emissions is the Kyoto Protocol, an amendment to the UNFCCC negotiated in 1997. The Protocol now covers more than 160 countries and over 55 percent of global greenhouse gas emissions.[91] As of June 2009, only the United States, historically the world's largest emitter of greenhouse gases, has refused to ratify the treaty. The treaty expires in 2012. International talks began in May 2007 on a future treaty to succeed the current one.[92] UN negotiations are now gathering pace in advance of a meeting in Copenhagen in December 2009.[93]

Many environmental groups encourage individual action against global warming, as well as community and regional actions. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[94][95]

There has also been business action on climate change, including efforts to improve energy efficiency and limited moves towards use of alternative fuels. In January 2005 the European Union introduced its European Union Emission Trading Scheme, through which companies in conjunction with government agree to cap their emissions or to purchase credits from those below their allowances. Australia announced its Carbon Pollution Reduction Scheme in 2008. United States President Barack Obama has announced plans to introduce an economy-wide cap and trade scheme.[96]

The IPCC's Working Group III is responsible for crafting reports on mitigation of global warming and the costs and benefits of different approaches. The 2007 IPCC Fourth Assessment Report concludes that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6 percent increase and three percent decrease in global gross domestic product.[97]

Adaptation
Main article: Adaptation to global warming

A wide variety of measures have been suggested for adaptation to global warming. These measures range from the trivial, such as the installation of air-conditioning equipment, to major infrastructure projects, such as abandoning settlements threatened by sea level rise.

Measures including water conservation,[98] water rationing, adaptive agricultural practices,[99] construction of flood defences,[100] Martian colonization,[101] changes to medical care,[102] and interventions to protect threatened species[103] have all been suggested. A wide-ranging study of the possible opportunities for adaptation of infrastructure has been published by the Institute of Mechanical Engineers.[104]

Geoengineering
Main article: Geoengineering

Geoengineering is the deliberate modification of Earth's natural environment on a large scale to suit human needs.[105] An example is greenhouse gas remediation, which removes greenhouse gases from the atmosphere, usually through carbon sequestration techniques such as carbon dioxide air capture.[106] Solar radiation management reduces insolation, such as by the addition of stratospheric sulfur aerosols.[107] No large-scale geoengineering projects have yet been undertaken.

Debate and skepticism
Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Scientific opinion on climate change, Climate change denial, List of countries by greenhouse gas emissions per capita, List of countries by carbon dioxide emissions per capita, List of countries by carbon dioxide emissions, and List of countries by ratio of GDP to carbon dioxide emissions
Per capita greenhouse gas emissions in 2000, including land-use change.

Per country greenhouse gas emissions in 2000, including land-use change.

Increased publicity of the scientific findings surrounding global warming has resulted in political and economic debate.[108] Poor regions, particularly Africa, appear at greatest risk from the projected effects of global warming, while their emissions have been small compared to the developed world.[109] The exemption of developing countries from Kyoto Protocol restrictions has been used to rationalize non-ratification by the U.S. and criticism from Austria.[110] Another point of contention is the degree to which emerging economies such as India and China should be expected to constrain their emissions.[111] The U.S. contends that if it must bear the cost of reducing emissions, then China should do the same[112][113] since China's gross national CO2 emissions now exceeds those of the U.S.[114][115][116] However, China has contended that it is less of an obligated to reduce emissions since its per capita responsibility and per capita emissions are less that of the U.S.[117] India, also exempt, has made similar contentions.[118]

In 2007-2008 the Gallup Polls surveyed 127 countries. Over a third of the world's population were unaware of global warming, developing countries were less aware than developed, and Africa was the least aware. Awareness does not equate to belief that global warming is a result of human activities. Of those aware, Latin America leads in belief that temperature changes are a result of human activities while Africa, parts of Asia and the Middle East, and a few countries from the Former Soviet Union lead in the opposite.[119] In the western world, the concept and the appropriate responses are contested. Nick Pidgeon of Cardiff University finds that "results show the different stages of engagement about global warming on each side of the Atlantic" where Europe debates the appropriate responses while the United States debates whether climate change is happening.[120]

Debates weigh the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail.[97] Using economic incentives, alternative and renewable energy have been promoted to reduce emissions while building infrastructure.[121][122] Business-centered organizations such as the Competitive Enterprise Institute, conservative commentators, and companies such as ExxonMobil have downplayed IPCC climate change scenarios, funded scientists who disagree with the scientific consensus, and provided their own projections of the economic cost of stricter controls.[123][124][125][126] Environmental organizations and public figures have emphasized changes in the current climate and the risks they entail, while promoting adaptation to changes in infrastructural needs and emissions reductions.[127] Some fossil fuel companies have scaled back their efforts in recent years,[128] or called for policies to reduce global warming.[129]

Some global warming skeptics in the science or political community dispute all or some of the global warming scientific consensus objecting to whether global warming is actually occurring, if human activity is truly to blame, and if the threat is as great a threat as has been alleged. Prominent global warming skeptics include Frederick Seitz, Freeman Dyson, Richard Lindzen, Fred Singer, Patrick Michaels, John Christy, Harrison Schmitt, and Robert Balling.[130][131][132]

Global Warming is defined as the increase of the average temperature on Earth. As the Earth is getting hotter, disasters like hurricanes, droughts and floods are getting more frequent.
Over the last 100 years, the average temperature of the air near the Earth´s surface has risen a little less than 1° Celsius (0.74 ± 0.18°C, or 1.3 ± 0.32° Fahrenheit). Does not seem all that much? It is responsible for the conspicuous increase in storms, floods and raging forest fires we have seen in the last ten years, though, say scientists.



Their data show that an increase of one degree Celsius makes the Earth warmer now than it has been for at least a thousand years. Out of the 20 warmest years on record, 19 have occurred since 1980. The three hottest years ever observed have all occurred in the last eight years, even.
Earth should be in cool-down-period

But it is not only about how much the Earth is warming, it is also about how fast it is warming. There have always been natural climate changes – Ice Ages and the warm intermediate times between them – but those evolved over periods of 50,000 to 100,000 years.



A temperature rise as fast as the one we have seen over the last 30 years has never happened before, as far as scientists can ascertain. Moreover, normally the Earth should now be in a cool-down-period, according to natural effects like solar cycles and volcano activity, not in a heating-up phase.



Earth should be in cool-down-period

But it is not only about how much the Earth is warming, it is also about how fast it is warming. There have always been natural climate changes – Ice Ages and the warm intermediate times between them – but those evolved over periods of 50,000 to 100,000 years.



A temperature rise as fast as the one we have seen over the last 30 years has never happened before, as far as scientists can ascertain. Moreover, normally the Earth should now be in a cool-down-period, according to natural effects like solar cycles and volcano activity, not in a heating-up phase.



Earth should be in cool-down-period

But it is not only about how much the Earth is warming, it is also about how fast it is warming. There have always been natural climate changes – Ice Ages and the warm intermediate times between them – but those evolved over periods of 50,000 to 100,000 years.



A temperature rise as fast as the one we have seen over the last 30 years has never happened before, as far as scientists can ascertain. Moreover, normally the Earth should now be in a cool-down-period, according to natural effects like solar cycles and volcano activity, not in a heating-up phase.

Asam
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Asam dan Basa:


Untuk nama buah atau pohon, lihat: Asam (tumbuhan), sedangkan untuk negara bagian di India, lihat: Assam.

Asam (yang sering diwakili dengan rumus umum HA) secara umum merupakan senyawa kimia yang bila dilarutkan dalam air akan menghasilkan larutan dengan pH lebih kecil dari 7. Dalam definisi modern, asam adalah suatu zat yang dapat memberi proton (ion H+) kepada zat lain (yang disebut basa), atau dapat menerima pasangan elektron bebas dari suatu basa. Suatu asam bereaksi dengan suatu basa dalam reaksi penetralan untuk membentuk garam. Contoh asam adalah asam asetat (ditemukan dalam cuka) dan asam sulfat (digunakan dalam baterai atau aki mobil). Asam umumnya berasa masam; walaupun demikian, mencicipi rasa asam, terutama asam pekat, dapat berbahaya dan tidak dianjurkan.
Daftar isi
[sembunyikan]

* 1 Berbagai definisi asam
* 2 Sifat-sifat
o 2.1 Sifat kimia
* 3 Sejarah
* 4 Penggunaan asam
* 5 Lihat pula

[sunting] Berbagai definisi asam

Istilah "asam" merupakan terjemahan dari istilah yang digunakan untuk hal yang sama dalam bahasa-bahasa Eropa seperti acid (bahasa Inggris), zuur (bahasa Belanda), atau Säure (bahasa Jerman) yang secara harfiah berhubungan dengan rasa masam. Dalam kimia, istilah asam memiliki arti yang lebih khusus. Terdapat tiga definisi asam yang umum diterima dalam kimia, yaitu definisi Arrhenius, Brønsted-Lowry, dan Lewis.

* Arrhenius: Menurut definisi ini, asam adalah suatu zat yang meningkatkan konsentrasi ion hidronium (H3O+) ketika dilarutkan dalam air. Definisi yang pertama kali dikemukakan oleh Svante Arrhenius ini membatasi asam dan basa untuk zat-zat yang dapat larut dalam air.
* Brønsted-Lowry: Menurut definisi ini, asam adalah pemberi proton kepada basa. Asam dan basa bersangkutan disebut sebagai pasangan asam-basa konjugat. Brønsted dan Lowry secara terpisah mengemukakan definisi ini, yang mencakup zat-zat yang tak larut dalam air (tidak seperti pada definisi Arrhenius).
* Lewis: Menurut definisi ini, asam adalah penerima pasangan elektron dari basa. Definisi yang dikemukakan oleh Gilbert N. Lewis ini dapat mencakup asam yang tak mengandung hidrogen atau proton yang dapat dipindahkan, seperti besi(III) klorida. Definisi Lewis dapat pula dijelaskan dengan teori orbital molekul. Secara umum, suatu asam dapat menerima pasangan elektron pada orbital kosongnya yang paling rendah (LUMO) dari orbital terisi yang tertinggi (HOMO) dari suatu basa. Jadi, HOMO dari basa dan LUMO dari asam bergabung membentuk orbital molekul ikatan.

Walaupun bukan merupakan teori yang paling luas cakupannya, definisi Brønsted-Lowry merupakan definisi yang paling umum digunakan. Dalam definisi ini, keasaman suatu senyawa ditentukan oleh kestabilan ion hidronium dan basa konjugat terlarutnya ketika senyawa tersebut telah memberi proton ke dalam larutan tempat asam itu berada. Stabilitas basa konjugat yang lebih tinggi menunjukkan keasaman senyawa bersangkutan yang lebih tinggi.

Sistem asam/basa berbeda dengan reaksi redoks; tak ada perubahan bilangan oksidasi dalam reaksi asam-basa.

[sunting] Sifat-sifat

Secara umum, asam memiliki sifat sebagai berikut:

* Rasa: masam ketika dilarutkan dalam air.
* Sentuhan: asam terasa menyengat bila disentuh, terutama bila asamnya asam kuat.
* Kereaktifan: asam bereaksi hebat dengan kebanyakan logam, yaitu korosif terhadap logam.
* Hantaran listrik: asam, walaupun tidak selalu ionik, merupakan elektrolit.

[sunting] Sifat kimia

Dalam air, reaksi kesetimbangan berikut terjadi antara suatu asam (HA) dan air, yang berperan sebagai basa,

HA + H2O ↔ A- + H3O+

Tetapan asam adalah tetapan kesetimbangan untuk reaksi HA dengan air:

K_a = \frac{[\mbox{H}_{3}\mbox{O}^+][ \mbox{A}^-]}{[\mbox{HA}]}

Asam kuat mempunyai nilai Ka yang besar (yaitu, kesetimbangan reaksi berada jauh di kanan, terdapat banyak H3O+; hampir seluruh asam terurai). Misalnya, nilai Ka untuk asam klorida (HCl) adalah 107.

Asam lemah mempunyai nilai Ka yang kecil (yaitu, sejumlah cukup banyak HA dan A- terdapat bersama-sama dalam larutan; sejumlah kecil H3O+ ada dalam larutan; asam hanya terurai sebagian). Misalnya, nilai Ka untuk asam asetat adalah 1,8 × 10-5.

Asam kuat mencakup asam halida - HCl, HBr, dan HI. (Tetapi, asam fluorida, HF, relatif lemah.) Asam-asam okso, yang umumnya mengandung atom pusat ber-bilangan oksidasi tinggi yang dikelilingi oksigen, juga cukup kuat; mencakup HNO3, H2SO4, dan HClO4. Kebanyakan asam organik merupakan asam lemah.

Larutan asam lemah dan garam dari basa konjugatnya membentuk larutan penyangga.

[sunting] Sejarah

Sekitar tahun 1800, banyak kimiawan Prancis, termasuk Antoine Lavoisier, secara keliru berkeyakinan bahwa semua asam mengandung oksigen. Lavoisier mendefinisikan asam sebagai zat mengandung oksigen karena pengetahuannya akan asam kuat hanya terbatas pada asam-asam okso dan karena ia tidak mengetahui komposisi sesungguhnya dari asam-asam halida, HCl, HBr, dan HI. Lavoisier-lah yang memberi nama oksigen dari kata bahasa Yunani yang berarti "pembentuk asam". Setelah unsur klorin, bromin, dan iodin teridentifikasi dan ketiadaan oksigen dalam asam-asam halida ditemukan oleh Sir Humphry Davy pada tahun 1810, definisi oleh Lavoisier tersebut harus ditinggalkan.

Kimiawan Inggris pada waktu itu, termasuk Humphry Davy, berkeyakinan bahwa semua asam mengandung hidrogen. Kimiawan Swedia Svante Arrhenius lalu menggunakan landasan ini untuk mengembangkan definisinya tentang asam. Ia mengemukakan teorinya pada tahun 1884.

Pada tahun 1923, Johannes Nicolaus Brønsted dari Denmark dan Martin Lowry dari Inggris masing-masing mengemukakan definisi protonik asam-basa yang kemudian dikenal dengan nama kedua ilmuwan ini. Definisi yang lebih umum diajukan oleh Lewis pada tahun yang sama, menjelaskan reaksi asam-basa sebagai proses transfer pasangan elektron.

[sunting] Penggunaan asam

Asam memiliki berbagai kegunaan. Asam sering digunakan untuk menghilangkan karat dari logam dalam proses yang disebut "pengawetasaman" (pickling). Asam dapat digunakan sebagai elektrolit di dalam baterai sel basah, seperti asam sulfat yang digunakan di dalam baterai mobil. Pada tubuh manusia dan berbagai hewan, asam klorida merupakan bagian dari asam lambung yang disekresikan di dalam lambung untuk membantu memecah protein dan polisakarida maupun mengubah proenzim pepsinogen yang inaktif menjadi enzim pepsin. Asam juga digunakan sebagai katalis; misalnya, asam sulfat sangat banyak digunakan dalam proses alkilasi pada pembuatan bensin.