• An acid is a substance that neutralises a base forming a salt and water:

2HCl(aq) + CaO(s) ⇌ CaCl₂(aq) + H₂O(l)

acid                                salt

Acids react with bases to form salt and water

• Acids are also substances that release hydrogen ions when they dissolve in water:

HCl(g) → H⁺(aq) + Cl^– (aq)

Acids dissociate in water to release a hydrogen ion

Names & formulae of some common acids table

• In organic acids (such as carboxylic acids) only some of the hydrogen atoms can form ions when the acid dissociates

• A base is a compound that neutralises an acid forming a salt and water

2HCl(aq) + CaO(s) ⇌ CaCl₂(aq) + H₂O(l)

 base          salt

• A base is a substance that accepts hydrogen ions or a compound that contains oxide or hydroxide ions
  For example:

NH₃(g) + H₂O(l) → NH₄⁺(aq) + OH^–(aq)

base                                hydroxide ions

For example:

NaOH(s) + aq → Na⁺(aq) + OH^–(aq)

base                                hydroxide ions

• A base that is soluble in water is called an alkali

Names & formulae of some common alkalis table