• Revise first an ionic compound such as sodium chloride (NaCl) consists of a positive ion (a cation, for example Na⁺) and a negative ion (an anion, for example Cl⁻).
• A double replacement reaction is a reaction between two ionic compounds, usually dissolved in water, in which the cations and anions exchange partners to make two new compounds.
• General pattern AB + CD → AD + CB
• Three important kinds are studied.

1.5.1 Neutralisation: acid + alkali

• Activity.

• Put a little dilute sodium hydroxide solution in a test tube and add drops of universal indicator.
• Then add dilute hydrochloric acid drop by drop.
• The universal indicator gives a colour that depends on how acidic or alkaline the solution is:

• Observations NaOH solution is colourless but turns the indicator violet.
• As acid is added gradually the colour changes through blue to green, the neutralisation point.
• If we continue to add acid the colour moves to yellow, orange and finally red.
• NaOH + HCl → NaCl + H₂O

• The acid and the alkali exchange partners Na⁺ combines with Cl⁻ (salt) and H⁺ combines with OH⁻ (water).
• A reaction between an acid and an alkali that gives a salt and water only is a neutralisation reaction.

1.5.2 Acid + salt

• Activity.

• Put sodium carbonate powder in a flask.
• Add dilute hydrochloric acid slowly from a funnel with a stopcock.
• Pass the gas through clear limewater using a delivery tube.
• Observation effervescence occurs and limewater turns milky (turbid).
• Conclusion the gas is carbon dioxide. The reaction is:

Na₂CO₃ + 2HCl → 2NaCl + H₂CO₃

• H₂CO₃ → H₂O + CO₂↑ (carbonic acid is unstable and breaks down)
• Overall Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑

• So the marble reaction is a double replacement reaction, whereas the zinc reaction is a single replacement reaction.
• Not every reaction of dilute hydrochloric acid produces a gas: with sodium hydroxide, for example, the products are a salt and water and no gas is produced.

• The two solutions are exchanging partners Na⁺ with NO₃⁻ (stays dissolved) and Ag⁺ with Cl⁻ (insoluble solid).
• A reaction between two soluble salts in solution that forms an insoluble solid is a precipitation reaction.

• Check your understanding. Two test tubes each contain silver nitrate solution.
• Sodium chloride solution is added to the first, and a piece of aluminium is put in the second.
• In the first, a white precipitate shows that a reaction happened.
• In the second, a grey/silvery deposit of silver forms on the aluminium (aluminium is more reactive than silver and replaces it), and the colourless solution slowly changes because aluminium nitrate forms.
• Life application treating hard water. Underground water may contain a lot of calcium and magnesium salts; this is hard water. It leaves hard deposits (scale) inside pipes, boilers and washing machines and finally damages them. At treatment plants, sodium carbonate is added. Carbonate ions combine with calcium and magnesium ions to make insoluble carbonates, which can be filtered or settle out:

Ca²⁺ + CO₃²⁻ → CaCO₃↓

Mg²⁺ + CO₃²⁻ → MgCO₃↓

• Model activity. In class you can design models of the different types of reactions using coloured paper clips: each colour is an atom, and linked clips are bonded atoms.
• Taking apart and re-joining the clips shows how combination (join), decomposition (split), single replacement (one clip changes partner) and double replacement (two pairs swap partners) rearrange atoms.