Concentrations and strengths of acids - Higher
Dilute and concentrated solutions
A solutionMixture formed by a solute and a solvent. forms when a soluteThe dissolved substance in a solution. dissolveWhen a substance breaks up and mixes completely with a solvent to produce a solution. in a solventThe liquid in which the solute dissolves to form a solution.. The concentrationA measure of the mass or amount of solute dissolved in a given volume of solvent or solution. of a solution is a measure of how 'crowded' the solute particles are. The more concentrated the solution, the more particles it contains in a given volumeThe volume of a three-dimensional shape is a measure of the amount of space or capacity it occupies, eg an average can of fizzy drink has a volume of 330 ml..
When solutions are described as dilute or concentrated:
- a dilute solution contains a relatively small amount of dissolvedA substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution. solute
- a concentrated solution contains a relatively large amount of dissolved solute
Take care to use the word 'dilute' correctly. It can be used as an adjective to describe the concentration of a solution (as here), or as a verb to describe the process of adding more water to a solution to decrease its concentration.
Strong and weak acids
acidSubstance producing more hydrogen ions than hydroxide ions when dissolved in water. in solution are a source of hydrogen ionElectrically charged particle, formed when an atom or molecule gains or loses electrons., H+. The hydrogen ions are produced when the acid dissociationThe breaking up of a molecule into ions when dissolved in water. or breaks down to form ions.
Strong acids
Strong acids completely dissociate into ions in solution. For example, hydrochloric acid is a strong acid. It ionises completely to form hydrogen ions and chloride ions:
HCl(aq) → H+(aq) + Cl-(aq)
Nitric acid and sulfuric acid are also strong acids.
Weak acids
Weak acids only partially dissociate in solution. For example, ethanoic acid is a weak acid. It is only partially ionised to form hydrogen ions and ethanoate ions:
CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
The ⇌ symbol is used in the equation to show that the reaction is a reversible reactionA chemical reaction which can go both ways. and does not go to completion.
pH and hydrogen ion concentration
The pHScale of acidity or alkalinity. A pH (power of hydrogen) value below 7 is acidic, a pH value above 7 is alkaline. of a solution is a measure of its concentration of hydrogen ions:
- the higher the concentration of H+ ions in an acidic solution, the lower the pH
- the lower the concentration of H+ ions in an acidic solution, the higher the pH
This means that, for a given concentration in aqueous solution, the stronger an acid, the lower the pH.
If the hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1 unit.
pH of alkaline solutions
The higher the concentration of OH- ions in an alkaline solution, the higher the pH.
Question
A solution of 1 g/dm3 hydrochloric acid has a pH of 1.6. Predict its pH when it is diluted to 0.1 g/dm3.
The hydrogen ion concentration decreases by a factor of 10, so the pH increases by 1 from 1.6 to 2.6.
Question
A solution of 2 g/dm3 hydrochloric acid has a pH of 1.3. Predict its pH when it is diluted to 0.02 g/dm3.
The hydrogen ion concentration decreases by a factor of 100, so the pH increases by 2 from 1.3 to 3.3.