pH, strong acids and weak acids – GCSE Combined Science Chemistry

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Acids are all around you. There is acid in lemon juice, in vinegar and in your stomach. In chemistry we sort acids by how strong they are and by how acidic their solutions are.

This matters because two acids can look the same but behave very differently. One may fizz fast with a metal and the other may be slow. The reason is the particles inside the liquid.

By the end you will be able to explain what ionisation is and tell strong acids from weak acids. You will explain why strength is not the same as concentration. You will also use the pH scale to compare hydrogen ion concentrations.

Acids make hydrogen ions

When an acid dissolves in water, its molecules split up into ions. An ion is an atom or group of atoms with an electric charge. This splitting is called ionisation.

Every acid makes hydrogen ions, written H⁺. Hydrochloric acid does this: HCl → H⁺ + Cl⁻. The H⁺ is really joined to a water molecule, but we write H⁺ to keep equations simple.

The more hydrogen ions a solution has, the lower its pH. Alkalis are different. They release hydroxide ions, OH⁻.

Strong acids

A strong acid is completely ionised in water. That means every molecule splits into ions. Hydrochloric, sulfuric and nitric acid are strong acids.

Because the reaction goes all the way, we write it with a single arrow: HCl → H⁺ + Cl⁻. Think of a class where every pupil leaves the room at the bell.

A strong acid makes lots of hydrogen ions, so it has a low pH, such as pH 1. It also conducts electricity well, because moving ions carry the current.

Weak acids

A weak acid is only partly ionised in water. Most of its molecules stay whole. Only a few split into ions. Ethanoic, citric and carbonic acid are weak acids.

Because only some molecules split, the change can go both ways. We show this with the reversible arrow: CH₃COOH ⇌ CH₃COO⁻ + H⁺.

A weak acid has fewer hydrogen ions than a strong acid of the same concentration. So its pH is higher. It also reacts more slowly with metals and carbonates, and it conducts less well.

Strength and concentration

These two words sound alike, but they mean different things. Strength is how much of the acid ionises. Concentration is how much acid is dissolved in a given volume.

Think of squash. The amount of squash you put in the glass is like concentration. A little gives a dilute solution. A lot gives a concentrated solution.

The label strong or weak does not change when you add water. So you can have a dilute solution of a strong acid. You can also have a concentrated solution of a weak acid. Never use strong to mean concentrated.

Comparing equal concentrations

Take 1 mol/dm³ hydrochloric acid and 1 mol/dm³ ethanoic acid. The unit mol/dm³ measures concentration, so both have the same amount of acid dissolved. The hydrochloric acid still has the lower pH, because it is completely ionised.

Add magnesium to each. The hydrochloric acid bubbles faster, because it has more hydrogen ions. The rate depends on the concentration of hydrogen ions.

If there is plenty of magnesium, both acids make the same total volume of hydrogen. The weak acid keeps ionising as its hydrogen ions are used up. The total gas depends on the moles of acid, not its strength.

The pH scale

The pH scale measures the concentration of hydrogen ions. A pH below 7 is acidic, 7 is neutral and above 7 is alkaline. A lower pH means more hydrogen ions.

Each step of 1 on the scale is a factor of 10. A solution of pH 2 has 10 times the hydrogen ions of a solution of pH 3.

Two units apart is 10 × 10 = 100 times. Four units apart is 10 000 times. So a solution of pH 5 has 10 000 times fewer hydrogen ions than one of pH 1.

Recap

Acids ionise in water to make hydrogen ions, H⁺. More H⁺ means a lower pH.

A strong acid is completely ionised and uses a single arrow. A weak acid is partly ionised and uses ⇌.

Strength is about ionisation. Concentration is about how much acid is dissolved.

Each pH unit is a tenfold change in hydrogen ion concentration.