5 · Reactions of acids — Chemical Changes

1 · Acid + alkali → salt + water

When an acid is mixed with an alkali, a neutralisation reaction occurs. The two cancel each other out, and the products are always a salt and water.

acid + alkali → salt + water

hydrochloric acid + sodium hydroxide → sodium chloride + water

When an alkali is added to an acid, the pH of the solution rises as the alkali neutralises the acid. When an acid is added to an alkali, the pH falls. On complete neutralisation the solution formed has a pH of 7 — neither acidic nor alkaline.

Take note — what "salt" means here In chemistry a salt is a general term for a compound containing a metal, not just the table salt you eat. Table salt is one particular salt, sodium chloride.

Naming the salt The salt's name is built from two parts — the metal, then the acid used:

hydrochloric acid always makes a chloride
sulfuric acid always makes a sulfate
nitric acid always makes a nitrate

So potassium hydroxide with sulfuric acid gives potassium sulfate; magnesium with hydrochloric acid gives magnesium chloride.

2 · Acid + metal → salt + hydrogen

An acid can react with a metal to form a salt and hydrogen gas. Two products are always formed.

acid + metal → salt + hydrogen

hydrochloric acid + magnesium → magnesium chloride + hydrogen

However, not all metals react with an acid in the same way. Some metals — copper, silver and gold — are relatively unreactive and do not react with dilute acids at all. This is where the phrase "the acid test" comes from: miners in the 1850s used nitric acid to check whether a metal was really gold.

3 · Acid + carbonate → salt + carbon dioxide + water

An acid can react with a carbonate to form a salt, carbon dioxide gas and water. Three products are always formed.

acid + carbonate → salt + carbon dioxide + water

hydrochloric acid + magnesium carbonate → magnesium chloride + carbon dioxide + water

Marble and limestone are largely calcium carbonate, which is why acid rain slowly eats away at statues and limestone buildings.

Run all three at the bench

Choose an acid and something to react it with. Watch what happens, read off the word equation, then test the gas — and notice what happens if you choose the wrong test.

Reactions of acids

1 · Choose the acid

2 · Choose what to add

In the test tube

Tests for the four gases

These come up constantly in exam questions, and each needs the observation stated exactly:

Gas	Test	Positive result
Hydrogen	Introduce a burning splint	Extinguishes with a squeaky "pop"
Oxygen	Introduce a glowing splint	The splint rekindles into a flame
Carbon dioxide	Bubble through limewater	A white precipitate forms — limewater turns chalky
Ammonia	Introduce damp red litmus paper	The paper turns blue

Where neutralisation is used

Toothpaste. Bacteria on our teeth produce acids that cause tooth decay. Toothpaste is weakly alkaline and neutralises any acid present in the mouth.
Antacids. These medicines contain alkaline compounds that neutralise excess acid produced by the stomach, relieving discomfort.
Treating soil. Soil that is too acidic or too alkaline may stop plants growing well. Adding a suitable substance neutralises it.

Bases and alkalis Extension

A metal oxide is known as a base. Like an alkali, it reacts with an acid to form a salt and water. Most bases (for example copper(II) oxide, CuO) are insoluble in water. Some are soluble — sodium oxide dissolves in water to form the alkali sodium hydroxide.

So: alkalis are simply soluble bases.

1 · Acid + alkali → salt + water

2 · Acid + metal → salt + hydrogen

3 · Acid + carbonate → salt + carbon dioxide + water

Run all three at the bench

Reactions of acids

1 · Choose the acid

2 · Choose what to add

In the test tube

Tests for the four gases

Where neutralisation is used

Bases and alkalis Extension

Check yourself