Acids, Bases and the pH Scale

Acids and alkalis are woven through everyday life — the citric acid in a lemon, the hydrochloric acid churning in your stomach, the ammonia in a household cleaner. In chemistry we measure how acidic or alkaline a solution is on the pH scale, and we explain acidity and alkalinity in terms of the ions present. This lesson, part of Topic C3 of OCR Gateway Combined Science, defines acids, bases and alkalis, sets out the pH scale and indicators, links pH to hydrogen and hydroxide ions, and (for Higher tier) tells strong acids apart from weak ones.

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.

This lesson builds AO1 recall of the acid/base/alkali definitions and the pH scale, AO2 application when you write the neutralisation ionic equation and read pH from an indicator, and AO3 analysis when you distinguish strong from weak acids (Higher) and interpret pH data.

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Acids, Bases and Alkalis

These three words are easy to muddle, so define them carefully:

• An acid is a substance that, 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 (such as copper oxide or magnesium hydroxide).
• An alkali is a base that dissolves 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 count as alkalis.

Exam Tip: The distinction examiners test is that an alkali is a soluble base. Copper oxide is a base (it neutralises acid) but 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 (for example, 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 according to 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 handy because it runs 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 join 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 also forms 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 mixing an acid and an alkali moves the pH towards 7: the ions responsible for acidity and alkalinity cancel out 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 it produces 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 — useful 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 only laboratory chemicals — recognising everyday examples helps the ideas stick:

Substance	Acidic, neutral or alkaline?	Why it matters
Lemon juice / vinegar	Acidic (weak acids)	Citric and ethanoic acid give the sharp, sour taste
Stomach acid	Acidic (hydrochloric acid)	Helps digest food and kill microbes
Pure water	Neutral (pH 7)	The reference point of the scale
Soap / oven cleaner	Alkaline	Sodium hydroxide and similar bases break down grease well
Indigestion tablets	Contain a base	Neutralise excess stomach acid to relieve discomfort

Indigestion remedies are a good illustration of neutralisation at work: they contain a base (such as a metal hydroxide or carbonate) that reacts with the excess hydrochloric acid in the stomach, raising the pH back towards neutral and easing the discomfort. This is exactly the $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$H++OH−→H2​O reaction (or, for a carbonate, a reaction that also gives off carbon dioxide) happening in everyday life.

Exam Tip: A favourite application question is why an indigestion tablet relieves acid: it contains a base that neutralises the excess stomach (hydrochloric) acid, raising the pH towards 7.

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Why Controlling pH Matters