Strong and weak acids

When an acid dissolves in water, some or all of its particles release hydrogen ions, $$\ce{H+}$$. This formation of ions is called ionisation. The strength of an acid describes the degree to which it ionises in water.

• A strong acid ionises completely: essentially all of its acid particles release hydrogen ions.
• A weak acid ionises only partially: some of its particles release hydrogen ions, while many remain un-ionised.

For example, hydrochloric acid, nitric acid and sulfuric acid are strong acids. Ethanoic acid, citric acid and carbonic acid are weak acids.

Acid strength and concentration

Strength describes the proportion of acid particles that ionise. Concentration describes how much acid is present in a given volume of solution. A solution can therefore be dilute but contain a strong acid, or concentrated but contain a weak acid.

If two acidic solutions have the same concentration, the stronger acid produces more hydrogen ions in solution. It therefore has a lower pH. Comparing pH alone does not establish which acid is stronger unless the concentrations are also considered.

Acid strength and concentration

An acid solution has two different properties: strength describes how much of the acid ionises in water, while concentration describes how much acid is present in a given volume of solution.

Acid strength: degree of ionisation

When an acid dissolves in water, some or all of its particles form ions. A strong acid ionises completely in water: essentially all of its acid particles form ions. A weak acid ionises only partially: some acid particles form ions, while others remain un-ionised.

Strong and weak therefore describe the fraction of the acid that ionises, not the total amount of acid present.

Acid concentration: amount in solution

Concentration describes the amount of acid substance dissolved in a given volume of solution. A concentrated acid solution contains a larger amount of acid per unit volume than a dilute acid solution.

Concentration does not by itself say what fraction of the acid has ionised. A solution can contain a large amount of a weak acid, most of which remains un-ionised, or a small amount of a strong acid that is completely ionised.

Combining strength and concentration

Because strength and concentration describe different things, all four combinations are possible:

• Strong and dilute: a small amount of acid per volume, ionised completely.
• Strong and concentrated: a large amount of acid per volume, ionised completely.
• Weak and dilute: a small amount of acid per volume, ionised partially.
• Weak and concentrated: a large amount of acid per volume, ionised partially.

Use strong or weak to describe ionisation, and concentrated or dilute to describe the amount of acid in the solution.

pH, neutrality and hydrogen ion concentration

The pH of a solution indicates its concentration of hydrogen ions, $$\ce{H+}$$. A solution with a higher hydrogen ion concentration is more acidic and has a lower pH. A solution with a lower hydrogen ion concentration has a higher pH.

A neutral solution is neither acidic nor alkaline. At 25 °C, a neutral solution has pH 7: solutions below pH 7 are acidic, and those above pH 7 are alkaline. An alkaline solution still contains hydrogen ions, but at a lower concentration than a neutral solution.

Comparing hydrogen ion concentrations

The pH scale is logarithmic: each decrease of one pH unit means the hydrogen ion concentration increases by a factor of 10. For example, a solution at pH 3 has ten times the hydrogen ion concentration of a solution at pH 4. Compared with a solution at pH 5, its concentration is $$10 \times 10 = 100$$ times as high.

In the other direction, an increase of one pH unit means the hydrogen ion concentration falls by a factor of 10. A difference of two pH units therefore represents a hundredfold difference in concentration, not a twofold difference.