Acids, bases and alkalis in terms of hydrogen and hydroxide ions, the pH scale and indicators, neutralisation, the reactions of acids with metals, oxides, hydroxides and carbonates, and preparing soluble salts.

What this dot point is asking

CCEA Double Award wants acids and alkalis described in terms of ions, the pH scale and indicators, neutralisation, the four reactions of acids, and how to prepare a soluble salt. The salt-preparation method is a classic exam question, so learn it as a sequence.

Acids, bases and alkalis

The pH scale and indicators

The pH scale runs from 0 to 14. Below 7 is acidic, 7 is neutral, and above 7 is alkaline. Indicators show pH by changing colour: universal indicator goes red in strong acid, green at neutral and purple in strong alkali. Litmus turns red in acid and blue in alkali.

Neutralisation

The four reactions of acids

Acids react in four standard ways:

• Acid + metal gives a salt + hydrogen (test: squeaky pop).
• Acid + metal oxide (base) gives a salt + water.
• Acid + metal hydroxide (base) gives a salt + water.
• Acid + carbonate gives a salt + water + carbon dioxide (test: turns limewater milky).

The salt formed depends on the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, and nitric acid makes nitrates. The metal part of the salt comes from the metal, oxide, hydroxide or carbonate that reacted. So sulfuric acid with zinc oxide gives zinc sulfate, and hydrochloric acid with calcium carbonate gives calcium chloride.

A base is any substance that neutralises an acid; an alkali is a base that dissolves in water. Metal oxides and hydroxides are bases, and the soluble ones (such as sodium hydroxide) are alkalis. This is why the salt-preparation method below uses an insoluble base, which can be filtered off once the acid is used up.

Preparing a soluble salt

To make a soluble salt from an acid and an insoluble base:

1. Add excess base to warm acid and stir until no more reacts (so all the acid is used up).
2. Filter off the excess base.
3. Crystallise the salt by evaporating some water and leaving it to cool.
4. Filter and dry the crystals.

Examples in context

Example 1. Indigestion remedies. Antacids contain a base such as magnesium hydroxide that neutralises excess stomach acid, forming a salt and water and easing the pain. This is neutralisation in everyday life.

Example 2. Testing for a carbonate. Adding dilute acid to a carbonate produces fizzing as carbon dioxide is released, which turns limewater milky. This both makes a salt and provides a test for carbonates, linking reactions to analysis.

Try this

Q1. What ion do all acids release in water? [1 mark]

• Cue. Hydrogen ions, H+.

Q2. Acid + carbonate gives which three products? [1 mark]

• Cue. A salt, water and carbon dioxide.