Describe the pH scale as a measure of acidity and alkalinity, relate pH to hydrogen ion concentration, and use indicators and universal indicator to determine pH

What this dot point is asking

SEAB wants you to describe the pH scale as a measure of how acidic or alkaline a solution is, relate pH to the concentration of hydrogen ions, and use indicators (including litmus and universal indicator) to find the pH of a solution. This is a short but high-frequency topic: pH appears in neutralisation, in salt preparation and in the practical paper.

The answer

The pH scale

The pH scale runs from about $0$ to $14$ and measures how acidic or alkaline a solution is:

• pH below 7: acidic. The lower the number, the more acidic (pH $1$ is strongly acidic).
• pH equal to 7: neutral (pure water and neutral solutions).
• pH above 7: alkaline. The higher the number, the more alkaline (pH $14$ is strongly alkaline).

pH and hydrogen ion concentration

The pH depends on the concentration of hydrogen ions ($\text{H}^+$) in the solution:

The higher the concentration of hydrogen ions, the lower the pH.

So a strongly acidic solution has many hydrogen ions and a low pH, while an alkaline solution has very few hydrogen ions (and many hydroxide ions) and a high pH. This links pH back to the definition of an acid as a source of hydrogen ions.

Indicators

An indicator is a substance that changes colour depending on whether it is in acid or alkali:

• Litmus is red in acid and blue in alkali. It gives a simple acidic-or-alkaline answer.
• Methyl orange is red in acid and yellow in alkali.
• Phenolphthalein (thymolphthalein style) is colourless in acid and pink in alkali.

These single-colour-change indicators are useful for titrations, where you only need to spot the point at which the solution turns.

Universal indicator

Universal indicator is a mixture of indicators that shows a range of colours across the pH scale, so it gives an approximate pH value rather than just acid or alkali:

• red or orange at low pH (strong acid),
• yellow at slightly acidic pH,
• green at pH $7$ (neutral),
• blue at slightly alkaline pH,
• purple at high pH (strong alkali).

Matching the colour to a chart gives the pH. This makes universal indicator ideal for following how acidity changes, for example during a neutralisation.

Examples in context

Example 1. Testing soil for crops. Gardeners test soil pH with universal indicator because different crops grow best in slightly acidic or slightly alkaline soil. If the soil is too acidic, lime (a base) is added to raise the pH, applying neutralisation guided by an indicator reading.

Example 2. Choosing an indicator for a titration. In a titration only the moment of neutralisation matters, so a single-colour-change indicator such as methyl orange is used; it switches sharply at the end point. Universal indicator would be unhelpful here because its gradual range makes the exact end point harder to see.

Try this

Q1. State the pH of a neutral solution and the colour of universal indicator at this pH. [1 mark]

• Cue. pH $7$; universal indicator is green.

Q2. A solution turns universal indicator red. State its approximate pH and the nature of the solution. [2 marks]

• Cue. pH about $1$ to $2$; strongly acidic.

Q3. Explain how the hydrogen ion concentration changes as pH increases from $2$ to $6$. [2 marks]

• Cue. As pH increases, the hydrogen ion concentration decreases; the solution becomes less acidic with fewer hydrogen ions.