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A solution forms when a solute dissolves in a solvent. The concentration of a solution is a measure of how ‘crowded’ the solute particles are. The more concentrated the solution, the more solute particles it contains in a given volume.

Solutions are described as dilute or concentrated, depending on the ratio of dissolved solute to volume of solution:

• a dilute solution has a low ratio
• a concentrated solution has a high ratio

Take care to use the word ‘dilute’ correctly. It can be used to describe the concentration of a solution (as here), or to describe the process of adding more water to a solution to reduce its concentration.

Strong and weak acids

Acids in solution are a source of hydrogen ions, H⁺. The hydrogen ions are produced when the acid dissociates or breaks down to form ions.

Strong acids

Strong acids completely dissociate in solution. For example, hydrochloric acid is a strong acid. It completely dissociates 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 only partially dissociates 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 reaction, and does not go to completion.

pH and hydrogen ion concentration

The pH 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.

If the hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1.

pH of alkaline solution

The higher the concentration of OH^- ions in an alkaline solution, the higher the pH.