Students should be able to: use and explain the terms dilute and…

The idea

This skill combines two pairs of terms. Dilute and concentrated describe how much acid is dissolved per unit volume: a dilute solution has a small amount of substance in a given volume, a concentrated solution has a large amount. Weak and strong describe the degree of ionisation: a strong acid ionises completely, a weak acid only partially. The pairs are independent, so a weak acid can be concentrated and a strong acid can be dilute. Neutrality means [H⁺] equals [OH⁻], giving pH 7 for aqueous solutions at the usual laboratory temperature. Relative acidity depends on hydrogen ion concentration: the greater the [H⁺], the lower the numerical pH. At this level, pH values are whole numbers, so you compare and order solutions using values such as pH 1, 4 and 7.

What earns the marks

• Dilute and concentrated refer to the amount of substance dissolved per unit volume of solution.
• Weak and strong refer to the degree of ionisation of the acid in aqueous solution.
• The two descriptions are independent: a weak acid can be concentrated, and a strong acid can be dilute.
• Neutrality corresponds to equal concentrations of H⁺ and OH⁻ ions, giving pH 7 in aqueous solution at the usual laboratory temperature.
• Relative acidity increases as hydrogen ion concentration increases, which corresponds to a lower numerical pH.
• At this level, pH values are compared as whole numbers, for example pH 2 is more acidic than pH 5.

Common mistakes and how to put them right

• Using dilute and weak as if they mean the same thing.

  Put it right: Dilute concerns amount of substance, weak concerns degree of ionisation.

• Assuming all acids have pH values below 7 and all alkalis above 7 without linking this to H⁺ and OH⁻ concentrations.

  Put it right: Neutrality needs equal concentrations of both ions.

• Treating pH as a linear measure of acidity.

  Put it right: A change of one pH unit represents a tenfold change in H⁺ concentration.

Exam tips

• Define each term separately and then state clearly that concentration and strength are independent.
• When describing neutrality, refer to equal H⁺ and OH⁻ concentrations and give pH 7 for aqueous solutions at the usual laboratory temperature.
• Use whole-number pH values when ordering solutions by acidity, and justify the order using H⁺ concentration.