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Acid

In chemistry, compound that releases hydrogen ions (H⁺ or protons) in the presence of an ionizing solvent (usually water). Acids react with bases to form salts, and they act as solvents. Strong acids are corrosive; dilute acids have a sour or sharp taste, although in some organic acids this may be partially masked by other flavour characteristics. The strength of an acid is measured by its hydrogen-ion concentration, indicated by the pH value. Solutions dominated by acids have a pH below 7.0.

Acids can be classified as monobasic, dibasic, tribasic, and so forth, according to their basicity (the number of hydrogen atoms available to react with a base) and degree of ionization (how many of the available hydrogen atoms dissociate in water). Dilute sulphuric acid is classified as a strong (highly ionized), dibasic acid.

Inorganic acids include boric, carbonic, hydrochloric, hydrofluoric, nitric, phosphoric, and sulphuric. Organic acids include ethanoic (acetic), benzoic, citric, methanoic (formic), lactic, oxalic, and salicylic, as well as complex substances such as nucleic acids and amino acids.

Sulphuric, nitric, and hydrochloric acids are sometimes referred to as the mineral acids. Most naturally occurring acids are found as organic compounds, such as the fatty acids R-COOH and sulphonic acids R-SO₃H, where R is an organic group.

All acids produce hydrogen ions when dissolved in water. For example with hydrochloric acid, which is produced when hydrogen chloride gas reacts with water:

HCl_(g) + aq ⇌ H⁺_(aq) + Cl⁻_(aq)

The reactions of acids are the reactions of the H⁺_(aq) ion. These are as follows.

With indicators

They give a specific colour reaction with indicators; for example, litmus turns red.

With alkalis

They react with alkalis to form a salt and water (neutralization). For example, hydrochloric acid added to sodium hydroxide gives the salt sodium chloride plus water.

HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

With carbonates

With carbonates and hydrogencarbonates, acids form a salt and displace carbon dioxide. For example, when nitric acid is added to sodium hydrogencarbonate:

HNO₃ + NaHCO₃ → NaNO₃ + CO₂ + H₂O

With metals

Acids react with metals to give off hydrogen and form a salt. For example, with magnesium and sulphuric acid the products are magnesium sulphate and hydrogen.

Mg + H₂SO₄ → MgSO₄ + H₂

Acids react with many bases, such as oxides and hydroxides, but the product is not always soluble in water so the reaction soon ceases, as when sulphuric acid reacts with calcium oxide, hydroxide, or carbonate.

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