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When atoms or groups of atoms lose or gain electrons, charged particles called ions are formed. Ions can be either positively or negatively charged.

Acids

When acids dissolve in water they produce hydrogen ions, H⁺. These are sometimes called protons, because hydrogen ions are the same as a hydrogen nucleus (which is a proton).

For example, take a look at the equation for hydrochloric acid.

HCl(aq) → H⁺(aq) + Cl^–(aq)

Note that (aq) stands for aqueous and means that the substance is in solution.

Acids are often produced from non-metal oxides. For example, sulfur oxides make sulfuric acid.

Alkalis

When alkalis dissolve in water they produce hydroxide ions, OH^–.

For example, take a look at the equation for sodium hydroxide.

NaOH(aq) → Na⁺(aq) + OH^–(aq)

Ammonia is slightly different. This is the equation for ammonia in solution.

NH₃(aq) + H₂O(l) → NH₄⁺(aq) + OH^–(aq)

Bases

A base is chemically opposite to an acid. Some bases dissolve in water and are called alkalis. Other bases, including many metal oxides, do not dissolve in water.

Neutralisation reaction [Higher tier only]

When the H⁺ ions from an acid react with the OH^– ions from an alkali, a neutralisation reaction occurs to form water. This is the equation for the reaction.

H⁺(aq) + OH^–(aq) → H₂O(l)

For example, hydrochloric acid and sodium hydroxide solution react together to form water and sodium chloride solution. The acid contains H⁺ ions and Cl^– ions, and the alkali contains Na⁺ ions and OH^– ions. The H⁺ ions and OH^– ions produce the water, and the Na⁺ ions and Cl^– ions produce the sodium chloride, NaCl(aq).

Because neutralisation reactions involve the loss and gain of hydrogen ions, this process is sometimes referred to as ‘proton transfer’.