Learning focus
Develop precise chemical language, connect equations and practical observations, interpret periodic trends and apply salt-preparation rules to unfamiliar examples.
Meaning of pH
pH is a measure used to compare hydrogen ion concentration and relative acidity or alkalinity in aqueous solution. Values below 7 are acidic, pH 7 is neutral and values above 7 are alkaline. At O Level, the scale is normally treated from 0 to 14.
Universal indicator
Universal indicator is a mixture of indicators that shows a range of colours. Red and orange indicate strongly or moderately acidic conditions, green indicates neutral conditions, and blue to purple indicates alkaline conditions. The exact pH is estimated by comparison with a colour chart.
Comparing solutions
If solution A has lower pH than solution B, A is more acidic and generally has a greater hydrogen ion concentration. If an alkaline solution has a higher pH, it is more alkaline. Colour descriptions should be linked to the chart supplied rather than recalled vaguely.
Measurement limitations
Indicator paper gives approximate pH because colour judgement is subjective and coloured solutions can interfere. A pH meter gives a numerical value but must be calibrated and rinsed between samples. Paper is suitable for quick comparison; instrumentation is better for precise monitoring.
Practical or data skill
Measure pH of household or laboratory samples using universal indicator paper. Record colour and pH separately and rank samples from most acidic to most alkaline.
Examination tip
pH compares aqueous solutions; state whether a lower value means more acidic or less alkaline.
Review questions and suggested answers
Question 1
What pH is neutral?
Suggested answer
Approximately 7.
Question 2
What does a pH of 2 indicate?
Suggested answer
A strongly acidic solution with relatively high H+ concentration.
Question 3
Why is universal indicator not ideal for an exact pH?
Suggested answer
Colour matching is approximate and subjective.
