The pH Scale & NeutralisationOxford AQA IGCSE Chemistry: Revision notes

Acidity and alkalinity are caused by particular ions dissolved in water:

• Hydrogen ions, H⁺(aq), make solutions acidic
• Hydroxide ions, OH⁻(aq), make solutions alkaline

The more H⁺ ions present, the more acidic the solution; the more OH⁻ ions present, the more alkaline it is.

The pH scale is a numerical scale used to measure how acidic or alkaline a solution is.

• The scale runs from 0 to 14
• pH 7 is neutral (e.g. pure water)
• pH below 7 is acidic — the lower the number, the more strongly acidic
• pH above 7 is alkaline — the higher the number, the more strongly alkaline

Universal indicator is a mixture of dyes that changes colour depending on the pH of the solution it is added to. By comparing the resulting colour to a reference chart, the approximate pH of a solution can be estimated.

• Red/orange colours indicate strongly acidic solutions (low pH)
• Green indicates neutral (pH 7)
• Blue/purple colours indicate strongly alkaline solutions (high pH)

Universal indicator only gives an approximate value; a pH meter gives a more precise reading.

Neutralisation is the reaction between an acid and a base (or alkali) that produces a salt and water, cancelling out the acidic and alkaline properties.

At the ionic level, neutralisation is always the same reaction, regardless of which acid or alkali is used:

H⁺(aq) + OH⁻(aq) → H₂O(l)

The hydrogen ions from the acid combine with the hydroxide ions from the alkali to form neutral water.