What is the pH scale, and what is the difference between strong, weak, dilute and concentrated acids?
Acids, bases and the pH scale: hydrogen and hydroxide ions, indicators, the pH scale and hydrogen ion concentration, strong and weak acids, dilute and concentrated, and neutralisation.
A focused answer to Edexcel GCSE Chemistry topic 3, covering acids as sources of hydrogen ions and alkalis as sources of hydroxide ions, indicators, the pH scale and how it relates to hydrogen ion concentration, the difference between strong and weak acids and between dilute and concentrated, and neutralisation.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Edexcel wants you to recall that acids are sources of hydrogen ions and alkalis are sources of hydroxide ions, describe the colours of indicators, explain the pH scale and how it relates to hydrogen ion concentration, distinguish strong from weak acids (degree of dissociation) and dilute from concentrated (amount of substance), and define neutralisation. The factor-of-10 pH relationship is a common Higher mark.
Acids, bases and alkalis
So all alkalis are bases, but not all bases are alkalis (only the soluble ones). Common acids are hydrochloric, sulfuric and nitric acid; common alkalis are sodium hydroxide and potassium hydroxide.
Indicators
An indicator changes colour depending on whether the solution is acidic or alkaline:
Universal indicator gives a range of colours and so an approximate pH, from red (strong acid) through green (neutral) to purple (strong alkali). A pH meter gives an accurate numerical pH.
The pH scale and hydrogen ion concentration
The pH scale runs from 0 to 14 and measures acidity:
- pH below 7: acidic (the lower the pH, the more acidic).
- pH equal to 7: neutral.
- pH above 7: alkaline (the higher the pH, the more alkaline).
The pH depends on the hydrogen ion concentration: the higher the concentration, the lower the pH. As the hydrogen ion concentration increases by a factor of 10, the pH decreases by 1. So a solution of pH 1 has ten times the hydrogen ion concentration of pH 2 and a hundred times that of pH 3.
Strong and weak acids
Hydrochloric, sulfuric and nitric acids are strong; ethanoic, citric and carbonic acids are weak. For the same concentration, a strong acid has a higher hydrogen ion concentration and so a lower pH than a weak acid.
Dilute and concentrated
These terms describe the amount of acid dissolved, not the strength:
- Concentrated means a large amount of acid (more moles) in a given volume.
- Dilute means a small amount of acid in a given volume.
A concentrated weak acid and a dilute strong acid can have similar pH values, so strong/weak and concentrated/dilute are independent ideas.
Neutralisation
The products of neutralisation are a salt and water, and the pH moves towards 7.
Try this
Q1. State the ion that all acids release in solution. [1 mark]
- Cue. The hydrogen ion, .
Q2. Give the colour of phenolphthalein in an alkali. [1 mark]
- Cue. Pink.
Q3. A solution changes from pH 6 to pH 3. By what factor does the hydrogen ion concentration increase? [2 marks]
- Cue. Three pH units, so times.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20193 marksExplain the difference between a strong acid and a weak acid, using the term degree of dissociation. Give an example of each.Show worked answer →
A 3-mark explanation of strong versus weak acids.
A strong acid is fully (completely) dissociated into ions in solution, so all of its molecules release hydrogen ions, for example hydrochloric acid (1 mark for fully dissociated, 1 mark for example). A weak acid is only partially dissociated, so only a small fraction of its molecules release hydrogen ions, for example ethanoic (acetic) acid or citric acid (1 mark).
Markers reward "fully" versus "partially dissociated" and correct examples; do not confuse this with dilute and concentrated.
Edexcel 20214 marksA solution of acid A has a pH of 2 and a solution of acid B has a pH of 5, both at the same concentration. Explain which acid has the higher hydrogen ion concentration, and describe how the hydrogen ion concentration changes as the pH increases by one unit.Show worked answer →
A 4-mark pH and hydrogen-ion-concentration question.
Acid A has the higher hydrogen ion concentration because the lower the pH, the higher the hydrogen ion concentration (1 mark, A is more acidic). As the pH increases by one unit, the hydrogen ion concentration decreases by a factor of (1 mark). So from pH 2 to pH 5 is an increase of three units, a decrease in hydrogen ion concentration by a factor of (1 mark for the factor, 1 mark for the calculation).
Markers reward the factor-of-10-per-unit rule and applying it across the three-unit difference.
Related dot points
- Reactions of acids: the general reactions of acids with metals, metal oxides, metal hydroxides and metal carbonates, the salts produced, and the tests for hydrogen and carbon dioxide.
A focused answer to Edexcel GCSE Chemistry topic 3, covering the general reactions of acids with metals, metal oxides, metal hydroxides and metal carbonates, the salts each acid produces, writing balanced and ionic equations, and the chemical tests for hydrogen and carbon dioxide.
- Preparing salts: making a soluble salt from an insoluble base, the titration method for soluble salts of soluble bases, preparing insoluble salts by precipitation, and the two core practicals.
A focused answer to Edexcel GCSE Chemistry topic 3, covering how to prepare a pure dry soluble salt from an acid and an insoluble base, the acid-alkali titration method for soluble salts, the copper sulfate core practical, preparing insoluble salts by precipitation using solubility rules, and obtaining a pure dry sample.
- Electrolysis: electrolysis of molten ionic compounds and aqueous solutions, predicting the products at the electrodes, writing half-equations, and the electrolysis of copper sulfate core practical.
A focused answer to Edexcel GCSE Chemistry topic 3, covering what electrolysis is, the electrolysis of molten ionic compounds, the rules for predicting products at the cathode and anode in aqueous solutions, writing half-equations, oxidation and reduction at the electrodes, and the copper sulfate electrolysis core practical.
- Chemical formulae and equations: writing formulae from ions, balancing symbol equations, state symbols, ionic equations and half-equations, and the law of conservation of mass.
A focused answer to Edexcel GCSE Chemistry topic 1, covering how to write formulae from ion charges, balance symbol equations, add state symbols, write ionic and half-equations, and apply the law of conservation of mass including why mass appears to change in open systems.
- Calculations in chemistry: relative formula mass, percentage by mass, empirical formulae from masses, reacting masses from balanced equations, concentration in grams per cubic decimetre, and the mole.
A focused answer to Edexcel GCSE Chemistry topic 1, covering relative formula mass, percentage by mass of an element, finding empirical formulae from masses or percentages, the empirical-formula core practical, reacting-mass calculations, concentration in g/dm3, and the definition of the mole.
Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)