Acids are substances such as hydrochloric acid, sulfuric acid and the ethanoic acid in vinegar. Their solutions in water share several properties, whatever the acid.
A drop of dilute hydrochloric acid turns blue litmus paper red. Metals such as zinc and magnesium react with the acid and give off hydrogen, and carbonates such as the calcium carbonate in marble give off carbon dioxide.
Acids in food, such as citric acid in lemons and ethanoic acid in vinegar, give them their sour taste, but no chemical in a laboratory should ever be tasted.
Bases are substances that react with acids and cancel their properties. Sodium hydroxide and ammonia are common examples, and both dissolve well in water.
Their solutions turn red litmus paper blue and turn phenolphthalein, a colourless indicator, pink. Sodium hydroxide solution and aqueous ammonia give the same results, even though sodium hydroxide is a solid and ammonia is a gas.
Solutions of bases also feel slippery, because they react with oils on the skin, and concentrated ones damage the skin and the eyes.
In the 1880s, the Swedish chemist Svante Arrhenius proposed that an acid is a substance that gives hydrogen ions, \mathrm{H^+}, when it dissolves in water. The properties shared by all acids are the properties of these ions.
Hydrogen chloride is a gas made of molecules, but in water its molecules ionize, each giving a hydrogen ion and a chloride ion: \mathrm{HCl \longrightarrow H^+ + Cl^-}. Nitric acid and sulfuric acid give hydrogen ions in the same way.
A hydrogen ion is a bare proton, so it does not stay alone in water. It bonds to a water molecule and forms an oxonium ion, \mathrm{H_3O^+}, although \mathrm{H^+} is often written for short.
Arrhenius defined a base as a substance that gives hydroxide ions, \mathrm{OH^-}, in water. Sodium hydroxide is an ionic compound, and when it dissolves its ions simply separate: \mathrm{NaOH \longrightarrow Na^+ + OH^-}.
Ammonia, \mathrm{NH_3}, has no hydroxide ions of its own, yet its solution is basic. Some ammonia molecules take a hydrogen ion from a water molecule and become ammonium ions, \mathrm{NH_4^+}, and each water molecule that loses a hydrogen ion is left as a hydroxide ion.
Only a small fraction of the ammonia molecules react, and the reverse reaction also happens, so aqueous ammonia contains far more \mathrm{NH_3} molecules than ions.
A base that dissolves in water is called an alkali. Sodium hydroxide, potassium hydroxide and calcium hydroxide are alkalis, and their solutions are described as alkaline.
Calcium hydroxide dissolves only a little, but the solution, limewater, still turns phenolphthalein pink. Copper(II) hydroxide and iron(III) hydroxide hardly dissolve at all, so they are bases but not alkalis.
Such insoluble hydroxides still react with acids, and the phenolphthalein stays colourless only because so few hydroxide ions reach the water around them.