Chemical Reactions · Grade 10

What actually happens when an acid meets a base?

The hydrogen ion the acid released and the hydroxide ion the base released join to make water. What is left in solution is a salt — the base's positive ion with the acid's negative ion. That is why neutralisation always gives a salt and water, and why the pH moves towards seven.

Learning objectives

The pH scale

The scale runs from 0 to 14. Below 7 is acidic, above 7 is alkaline, and exactly 7 is neutral — pure water. Lemon juice sits near 2, water near 7, a washing soda solution near 11.

The measurement is made with an indicator, a substance whose colour changes with acidity. Litmus paper only says acidic or alkaline; universal indicator gives a range of colours and an approximate number.

The important thing is that the scale is not linear: each step is a factor of ten. A solution at pH 3 is ten times more acidic than one at pH 4 and a hundred times more than one at pH 5.

An acid's three reactions

Acid with a base gives a salt and water. That is neutralisation, and underneath it is a hydrogen ion meeting a hydroxide ion.

Acid with a metal gives a salt and hydrogen gas. The hydrogen can be identified by a small squeaky pop at a flame — the same alkali metal behaviour met in the periodic table hub, with an acid in place of water.

Acid with a carbonate gives a salt, water and carbon dioxide. That is the reaction that fizzes when vinegar meets bicarbonate of soda, and the one a farmer sets off when spreading lime on acidic soil.

Families worth recognising

Combination: two substances go in and one comes out. Decomposition: one goes in and several come out. Displacement: a more reactive element pushes a less reactive one out of its compound. Neutralisation: an acid and a base. Combustion: a substance reacts with oxygen and releases energy.

Classifying is not an exercise in memory but a shortcut. Recognise a reaction as neutralisation and you already know the products without calculating anything; recognise a displacement and you can predict from the elements' positions whether it will happen at all.

Worked examples

  1. What forms when hydrochloric acid reacts with sodium hydroxide?

    1. Acid with base is neutralisation
    2. The hydrogen ion and the hydroxide ion make water
    3. A sodium ion and a chloride ion are left

    Answer: Sodium chloride and water — table salt and water

  2. A drop of universal indicator turns the solution red. What can you say?

    1. Red on universal indicator means high acidity
    2. That is a low pH, around 1 to 3

    Answer: The solution is strongly acidic

  3. Classify: CaCO3 → CaO + CO2

    1. One substance goes in
    2. Two substances come out

    Answer: A decomposition reaction

Common mistakes

Thinking a salt always means table salt
In chemistry a salt is any ionic compound formed by neutralisation. Sodium chloride is only one of them, and most are not edible.
Reading the pH scale as an ordinary scale
Each step is a factor of ten. The gap between 2 and 4 is not double but a hundredfold, and that changes every judgement about dilution and safety.
Expecting a gas from every acid reaction
A metal gives hydrogen and a carbonate gives carbon dioxide, but neutralisation with a base gives only water and a salt. Wait for bubbles and you will conclude nothing happened.

What to remember

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