What makes an acid

An acid is a molecule that gives away a hydrogen ion, H⁺. Vinegar, lemon juice and sour sweets all taste sour because of the H⁺ their acids give away.

A hydrogen atom is one proton with one electron. Take the electron away and a single proton is left, with a positive charge. That is a hydrogen ion, so chemists also call H⁺ a proton. An atom or molecule with an electric charge is called an ion.

Which hydrogen goes

Ethanoic acid, the acid in vinegar, carries a -⁠COOH group. In lesson 11, Uneven sharing, you saw that oxygen pulls on shared electrons harder than hydrogen does. In the O–⁠H bond of the -⁠COOH group, the shared pair sits closer to the oxygen, so the oxygen carries a small negative charge, δ−, and the hydrogen a small positive charge, δ+.

In water, that bond can break with both of its electrons staying on the oxygen. The hydrogen leaves as H⁺, and the rest of the molecule keeps the extra electron as a negative charge. It is now the ethanoate ion, CH₃COO⁻.

The three hydrogens on the CH₃ carbon stay put, because carbon and hydrogen share their electrons almost evenly. Ethanol has an O–⁠H bond too, and it holds on to its H⁺ far more tightly. In the -⁠COOH group, the C=O oxygen next door pulls on electrons as well, and once the H⁺ has gone, the negative charge spreads over both oxygen atoms. Spread out, the charge sits more comfortably, so the acid lets go of its H⁺ more easily.

An H⁺ in water joins a water molecule at once, making H₃O⁺, and chemists write it as H⁺ for short. Ethanoic acid is a weak acid: in vinegar, fewer than 1 molecule in 100 has given away its H⁺ at any moment. A strong acid, such as the hydrochloric acid in your stomach, gives away all of its H⁺.

Acids in food

Many sour foods owe their taste to carboxylic acids. Lemon and lime juice are about 5% citric acid, and each citric acid molecule has three -⁠COOH groups. Apples are sharp with malic acid, and yoghurt is tangy with lactic acid.