What makes a substance acidic or basic, and how is acidity measured and controlled?
The Bronsted-Lowry definitions of acids and bases, strong and weak acids, the pH scale and its relationship to hydrogen ion concentration, neutralisation reactions, and the use of titration to find an unknown concentration.
A CCEA Life and Health Sciences answer on acids, bases and pH: the Bronsted-Lowry definitions, strong and weak acids, the pH scale and hydrogen ion concentration, neutralisation reactions, and using titration to find an unknown concentration.
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What this dot point is asking
CCEA wants you to define acids and bases using the Bronsted-Lowry theory, distinguish strong and weak acids by their dissociation, explain the pH scale and its link to hydrogen ion concentration, describe neutralisation reactions, and use titration to find an unknown concentration. Acids, bases and pH appear throughout the unit and connect to the control of pH in the body and in industry.
Acids and bases
This proton-transfer view explains acid and base behaviour generally. For example, when hydrogen chloride dissolves it donates a proton to water, forming a hydroxonium ion and a chloride ion, so it acts as an acid. Bases such as ammonia accept a proton. The strength of an acid depends on how readily it donates protons, which is described as its degree of dissociation.
Strong and weak acids; the pH scale
Because pH depends on hydrogen ion concentration, a strong acid and a weak acid of the same concentration have different pH values: the strong acid releases far more hydrogen ions, so it has the lower pH. The scale runs from below 0 (very acidic) through 7 (neutral) to above 14 (very alkaline). The relationship is logarithmic, so a solution of pH 2 has ten times the hydrogen ion concentration of a solution of pH 3.
Neutralisation and titration
Neutralisation is the reaction of an acid with a base (or alkali) to form a salt and water; the essential reaction is the hydrogen ion combining with the hydroxide ion to form water. The salt formed depends on the acid and base used (for example hydrochloric acid plus sodium hydroxide gives sodium chloride and water).
A titration finds an unknown concentration accurately. A known volume of one solution is measured into a flask with an indicator; the other solution is added from a burette until the indicator just changes colour at the end-point, showing the reaction is complete. From the volume added, the known concentration and the balanced equation, the unknown concentration is calculated. Careful technique (rinsing apparatus, reading the burette to the nearest half division, repeating to get concordant titres) gives reliable, precise results, drawing on the AS 1 practical skills.
Examples in context
Example 1. Indigestion remedies. Excess hydrochloric acid in the stomach causes indigestion. Antacids contain bases such as magnesium hydroxide or calcium carbonate that neutralise the acid, forming a salt and water and raising the stomach pH towards comfortable levels. This is neutralisation applied to health.
Example 2. Controlling pH in industry. Many industrial processes and waste streams must be kept within a pH range, for example neutralising acidic effluent with lime before discharge to protect rivers. Titration and pH measurement let engineers add exactly enough base, showing why accurate acid and base chemistry matters industrially.
Try this
Q1. Define a Bronsted-Lowry base. [1 mark]
- Cue. A proton (hydrogen ion) acceptor.
Q2. Explain why a strong acid has a lower pH than a weak acid of the same concentration. [2 marks]
- Cue. The strong acid dissociates completely, giving a higher hydrogen ion concentration, and lower pH means higher hydrogen ion concentration.
Q3. State the products of the neutralisation of an acid by an alkali. [1 mark]
- Cue. A salt and water.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA AS 36 marksExplain the difference between a strong acid and a weak acid in terms of dissociation, and explain why a strong acid and a weak acid of the same concentration have different pH values.Show worked answer →
The answer must define strong and weak in terms of dissociation, then link to hydrogen ion concentration and pH.
A strong acid dissociates (ionises) completely in water, releasing all its hydrogen ions, for example hydrochloric acid fully splits into hydrogen ions and chloride ions. A weak acid only dissociates partially; most of its molecules stay intact, with only a small fraction releasing hydrogen ions, for example ethanoic acid sets up an equilibrium with only a few ions formed.
Effect on pH: pH depends on the hydrogen ion concentration. For the same overall concentration, the strong acid releases far more hydrogen ions, so it has a higher hydrogen ion concentration and a lower pH. The weak acid releases fewer hydrogen ions, so it has a lower hydrogen ion concentration and a higher pH.
A linking point: a lower pH means a higher hydrogen ion concentration, since the pH scale is based on the negative logarithm of that concentration.
Markers reward complete versus partial dissociation, the link from hydrogen ion concentration to pH, and the conclusion that the strong acid has the lower pH.
CCEA AS 35 marksIn a titration, 25.0 cubic centimetres of sodium hydroxide solution is exactly neutralised by 20.0 cubic centimetres of 0.100 mol per cubic decimetre hydrochloric acid. Calculate the concentration of the sodium hydroxide solution. (The reaction is 1 to 1.)Show worked answer →
Find the moles of acid, use the 1 to 1 ratio for the moles of alkali, then divide by its volume.
Moles of hydrochloric acid (volume in cubic decimetres times concentration):
The reaction is 1 to 1, so moles of sodium hydroxide also equal 0.00200 mol.
Concentration of sodium hydroxide (moles divided by volume in cubic decimetres):
Markers reward the correct moles of acid, the 1 to 1 transfer to the alkali, and the correct concentration with units.
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Sources & how we know this
- CCEA GCE Life and Health Sciences specification — CCEA (2016)