Acids, Bases and the pH Scale

Acids and alkalis are part of everyday life — the citric acid in a lemon, the hydrochloric acid in your stomach, the ammonia in some cleaners. In chemistry we measure how acidic or alkaline a solution is using the pH scale, and we explain acidity and alkalinity in terms of the ions present. This lesson, part of Topic C3 of OCR Gateway Science A, defines acids, bases and alkalis, explains the pH scale and indicators, links pH to hydrogen and hydroxide ions, and (for Higher tier) distinguishes strong from weak acids.

By the end of this lesson you should be able to define an acid, a base and an alkali, describe the pH scale and the use of indicators, explain neutralisation in terms of ions, and (Higher tier) explain the difference between strong and weak acids.

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Acids, Bases and Alkalis

These three terms are easy to mix up, so define them carefully:

• An acid is a substance that, when dissolved in water, produces hydrogen ions ($\text{H}^+$H+). Acids have a pH below 7. Examples: hydrochloric acid, sulfuric acid, nitric acid.
• A base is a substance that neutralises an acid. Many bases are metal oxides or metal hydroxides (e.g. copper oxide, magnesium hydroxide).
• An alkali is a base that is soluble in water. When dissolved, an alkali produces hydroxide ions ($\text{OH}^-$OH−). Alkalis have a pH above 7. Examples: sodium hydroxide, potassium hydroxide.

So all alkalis are bases, but not all bases are alkalis — only the soluble ones are alkalis.

Exam Tip: The distinction examiners test is that an alkali is a soluble base. Copper oxide is a base (it neutralises acid) but it is not an alkali because it does not dissolve in water; sodium hydroxide is a base and an alkali because it does dissolve.

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The pH Scale

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

• pH less than 7 → acidic (the lower the number, the more strongly acidic).
• pH equal to 7 → neutral (e.g. pure water).
• pH greater than 7 → alkaline (the higher the number, the more strongly alkaline).

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Indicators

An indicator is a substance that changes colour depending on the pH, so it can show whether a solution is acidic, neutral or alkaline.

Indicator	In acid	In neutral	In alkali
Litmus	Red	Purple	Blue
Universal indicator	Red/orange (low pH)	Green (pH 7)	Blue/purple (high pH)
Phenolphthalein	Colourless	Colourless	Pink

Universal indicator is especially useful because it goes through a whole range of colours, so its colour can be matched to a pH chart to estimate the actual pH value. A pH meter gives a more precise numerical reading.

Exam Tip: Litmus only tells you acid or alkali (red or blue) — it does not give a pH value. To estimate a pH value you need universal indicator matched to a colour chart, or a pH meter for a precise number. Phenolphthalein is colourless in acid and pink in alkali, which makes it ideal for titrations.

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Acids, Alkalis and Ions

The behaviour of acids and alkalis comes down to the ions they release in water:

• Acids release hydrogen ions, $\text{H}^+$H+, in aqueous solution. It is the $\text{H}^+$H+ ions that make a solution acidic.
• Alkalis release hydroxide ions, $\text{OH}^-$OH−, in aqueous solution. It is the $\text{OH}^-$OH− ions that make a solution alkaline.

Neutralisation

When an acid reacts with an alkali, the hydrogen ions and hydroxide ions combine to form water. This is neutralisation, and its ionic equation is one you must know:

$\text{H}^+_{(aq)} + \text{OH}^-_{(aq)} \rightarrow \text{H}_2\text{O}_{(l)}$H(aq)+​+OH(aq)−​→H2​O(l)​

Because the $\text{H}^+$H+ and $\text{OH}^-$OH− ions are removed (they become water), the solution moves towards pH 7. A salt is also formed from the rest of the acid and alkali, which you will study in the next lesson. Neutralisation is exothermic — the temperature of the mixture rises.

Exam Tip: Learn the neutralisation ionic equation $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$H++OH−→H2​O exactly. It is the reason that mixing an acid and an alkali moves the pH towards 7: the ions responsible for acidity and alkalinity cancel each other by forming water.

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Measuring pH

There are two main ways to find the pH of a solution, and they differ in precision:

• Universal indicator (or pH paper) is added to the solution and the colour produced is matched against a pH colour chart. This is quick and cheap, but it only gives an approximate, whole-number pH because you are judging a colour by eye.
• A pH meter (or pH probe) is dipped into the solution and gives a numerical reading, often to one or two decimal places. This is more accurate and precise than an indicator and does not rely on judging a colour, which is important if the solution is already coloured.

Method	Result	Precision
Universal indicator / pH paper	A colour matched to a chart	Approximate (whole numbers)
pH meter / probe	A number on a display	Precise (decimal places)

Exam Tip: If a question asks for the most precise way to measure pH, the answer is a pH meter (probe), because it gives a numerical reading rather than relying on matching a colour by eye.

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Everyday Acids and Alkalis

Acids and alkalis are not just laboratory chemicals — recognising everyday examples helps the ideas stick: