GCSE Chemistry revision

Neutralisation of acids and salt production

Reactions of acids

AQA 4.4.2.2 Foundation & Higher

Your specification

AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

MOST 🎯🎯

SOME 🎯🎯🎯

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

Neutralisation Reactions: The Big Picture

Neutralisation reactions occur when an acid reacts with a base (metal oxide, metal hydroxide, or metal carbonate) to form a salt and water.
The general rule is to combine the negative ion from the acid with the positive ion from the base to form the salt product.
Balancing the equation correctly is essential to ensure the charges on the ions in the salt formula are equal and opposite.

1. Neutralisation Reactions: The Big Picture AQA

Notes

Neutralisation reactions occur when an acid reacts with a base (metal oxide, metal hydroxide, or metal carbonate) to form a salt and water.
The general rule is to combine the negative ion from the acid with the positive ion from the base to form the salt product.
Balancing the equation correctly is essential to ensure the charges on the ions in the salt formula are equal and opposite.

Acids Reacting with Metal Oxides

When an acid reacts with a metal oxide, the products are always a salt and water.
For example, hydrochloric acid reacting with sodium oxide produces sodium chloride (the salt) and water: Na₂O + 2HCl → 2NaCl + H₂O The positive metal ion from the oxide combines with the negative ion from the acid to form the salt.

2. Acids Reacting with Metal Oxides AQA

Notes

When an acid reacts with a metal oxide, the products are always a salt and water.
For example, hydrochloric acid reacting with sodium oxide produces sodium chloride (the salt) and water: Na₂O + 2HCl → 2NaCl + H₂O The positive metal ion from the oxide combines with the negative ion from the acid to form the salt.

Acids Reacting with Metal Hydroxides

Metal hydroxides also react with acids to produce a salt and water, following the same neutralisation pattern.
For example, sulfuric acid reacting with potassium hydroxide produces potassium sulfate and water: 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O Care must be taken with ion charges — the sulfate ion is 2−, so two potassium ions (each 1+) are needed to balance the formula.

3. Acids Reacting with Metal Hydroxides AQA

Notes

Metal hydroxides also react with acids to produce a salt and water, following the same neutralisation pattern.
For example, sulfuric acid reacting with potassium hydroxide produces potassium sulfate and water: 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O Care must be taken with ion charges — the sulfate ion is 2−, so two potassium ions (each 1+) are needed to balance the formula.

Acids Reacting with Metal Carbonates

When an acid reacts with a metal carbonate, the products are a salt, water, and carbon dioxide gas.
For example, nitric acid reacting with calcium carbonate produces calcium nitrate, water, and carbon dioxide: CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂ The release of carbon dioxide is the key difference between carbonate reactions and oxide/hydroxide reactions.

4. Acids Reacting with Metal Carbonates AQA

Notes

When an acid reacts with a metal carbonate, the products are a salt, water, and carbon dioxide gas.
For example, nitric acid reacting with calcium carbonate produces calcium nitrate, water, and carbon dioxide: CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂ The release of carbon dioxide is the key difference between carbonate reactions and oxide/hydroxide reactions.

Making a Soluble Salt: Setting Up the Reaction

To make a soluble salt, an acid is reacted with an insoluble base such as a metal oxide, hydroxide, or carbonate.
Dilute acid (e.g. hydrochloric acid) is placed in a beaker and gently heated using a Bunsen burner.
The insoluble base (e.g. copper oxide) is added a little at a time; when it stops disappearing, the base is in excess and all the acid has been neutralised.

5. Making a Soluble Salt: Setting Up the Reaction AQA

Notes

To make a soluble salt, an acid is reacted with an insoluble base such as a metal oxide, hydroxide, or carbonate.
Dilute acid (e.g. hydrochloric acid) is placed in a beaker and gently heated using a Bunsen burner.
The insoluble base (e.g. copper oxide) is added a little at a time; when it stops disappearing, the base is in excess and all the acid has been neutralised.

Filtering Out the Excess Base

Once the base is in excess, the mixture is filtered using filter paper and a filter funnel to remove the unreacted solid base.
The liquid that passes through (the filtrate) contains the dissolved soluble salt, such as aqueous copper chloride.
Filtration ensures the final salt solution is free from excess insoluble base before crystallisation begins.

6. Filtering Out the Excess Base AQA

Notes

Once the base is in excess, the mixture is filtered using filter paper and a filter funnel to remove the unreacted solid base.
The liquid that passes through (the filtrate) contains the dissolved soluble salt, such as aqueous copper chloride.
Filtration ensures the final salt solution is free from excess insoluble base before crystallisation begins.

Crystallisation: Obtaining Pure Salt Crystals

The filtered salt solution is gently heated using a water bath or electric heater (not a Bunsen burner) to evaporate water and begin rystallisation.
Gentle heating is important to avoid overheating and decomposing or damaging the salt crystals.
Once crystals begin to form, heating is stopped and the solution is left to cool, allowing more crystals to form.

7. Crystallisation: Obtaining Pure Salt Crystals AQA

Notes

The filtered salt solution is gently heated using a water bath or electric heater (not a Bunsen burner) to evaporate water and begin rystallisation.
Gentle heating is important to avoid overheating and decomposing or damaging the salt crystals.
Once crystals begin to form, heating is stopped and the solution is left to cool, allowing more crystals to form.

Drying and Collecting the Crystals

The crystals are collected by filtering the cooled solution again using filter paper and a funnel.
The crystals are dried either by dabbing with filter paper or leaving them in a warm place.
The final product is pure, dry crystals of the soluble salt, e.g. copper(II) chloride, made from an insoluble base and an acid.

8. Drying and Collecting the Crystals AQA

Notes

The crystals are collected by filtering the cooled solution again using filter paper and a funnel.
The crystals are dried either by dabbing with filter paper or leaving them in a warm place.
The final product is pure, dry crystals of the soluble salt, e.g. copper(II) chloride, made from an insoluble base and an acid.