What are acids and bases, and how do they react to form salts?
Acids, bases and alkalis in terms of hydrogen and hydroxide ions, the pH scale and indicators, neutralisation, and the reactions of acids with metals, oxides, hydroxides and carbonates.
A CCEA GCSE Chemistry answer on acids, bases and salts, covering acids and alkalis in terms of hydrogen and hydroxide ions, the pH scale and indicators, neutralisation, and the four reactions of acids with metals, metal oxides, hydroxides and carbonates.
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What this dot point is asking
CCEA wants you to define acids, bases and alkalis using hydrogen and hydroxide ions, use the pH scale and indicators, explain neutralisation, and give the products of acids reacting with metals, metal oxides, metal hydroxides and carbonates.
Acids, bases and alkalis
Common acids are hydrochloric, sulfuric and nitric acid. Common alkalis are sodium hydroxide and potassium hydroxide. Metal oxides and carbonates are bases that may be insoluble.
The pH scale and indicators
Indicators show pH by colour. Universal indicator gives a range of colours (red for strong acid through green at neutral to purple for strong alkali). Litmus turns red in acid and blue in alkali. A pH meter gives a precise numerical value.
Neutralisation
The salt formed depends on the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates. The metal part comes from the base.
The four reactions of acids
Each can be tested in the lab: the metal reaction fizzes and gives a gas that pops with a lit splint; the carbonate reaction fizzes and the gas turns limewater milky.
Worked example
Examples in context
- Example 1. Treating indigestion
- Antacid tablets contain bases such as magnesium hydroxide or calcium carbonate that neutralise excess stomach acid (hydrochloric acid), forming a salt and water (and carbon dioxide from carbonates). This is neutralisation put to medical use.
- Example 2. Correcting acidic soil
- Farmers add lime (calcium oxide or carbonate, both bases) to neutralise acidic soil and raise its pH so crops grow better. The reaction of a base with the acid in the soil is exactly the neutralisation studied here.
- Example 3. Why fizzing reveals a carbonate
- Pouring dilute acid on an unknown white powder and seeing it fizz, with the gas turning limewater milky, is a quick way to show the powder is a carbonate. Builders use this to tell limestone (calcium carbonate) from other pale rocks, a direct field use of the acid-carbonate reaction.
Strong and weak, concentrated and dilute
Two pairs of words are easy to confuse. Strong and weak describe how fully an acid splits into ions in water: a strong acid (hydrochloric, sulfuric, nitric) ionises completely, while a weak acid (such as ethanoic acid in vinegar) only partly ionises. Concentrated and dilute describe how much acid is dissolved per volume of water. So a dilute solution of a strong acid is possible, and so is a concentrated solution of a weak acid. CCEA expects you to keep these ideas separate, because they explain why two acids at the same concentration can have different pH values.
Try this
Q1. State the pH of a neutral solution. [1 mark]
- Cue. pH 7.
Q2. Name the products when hydrochloric acid reacts with zinc. [2 marks]
- Cue. Zinc chloride and hydrogen.
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 20184 marksHydrochloric acid reacts with magnesium and, separately, with copper(II) oxide. Write a word equation for each reaction and name the type of reaction in each case.Show worked answer →
Markers want the correct products for each acid reaction.
Acid plus metal gives a salt plus hydrogen:
hydrochloric acid + magnesium gives magnesium chloride + hydrogen. This is a displacement/redox reaction (the metal displaces hydrogen).
Acid plus metal oxide gives a salt plus water:
hydrochloric acid + copper(II) oxide gives copper(II) chloride + water. This is a neutralisation reaction (the metal oxide is a base).
Markers reward both word equations with the correct salt (a chloride from hydrochloric acid), hydrogen from the metal reaction, water from the oxide reaction, and naming neutralisation for the oxide.
CCEA 20213 marksExplain what is meant by neutralisation, give the ionic equation, and state the pH of a neutral solution.Show worked answer →
The marks are for the definition, the ionic equation and the pH.
Neutralisation is the reaction of an acid with a base (or alkali) to form a salt and water. The hydrogen ions from the acid react with the hydroxide ions from the alkali.
The ionic equation is:
A neutral solution has a pH of 7.
Markers reward acid plus base gives salt plus water, the H+ plus OH- ionic equation, and pH 7.
Related dot points
- The solubility rules for salts, preparing a soluble salt from an acid and an insoluble base, and preparing an insoluble salt by precipitation.
A CCEA GCSE Chemistry answer on preparing salts, covering the general solubility rules, the method for making a soluble salt from an acid and an insoluble base by filtration and crystallisation, and the precipitation method for making an insoluble salt.
- Writing chemical formulae from ions, constructing word and balanced symbol equations with state symbols, and writing simple ionic equations for neutralisation and precipitation.
A CCEA GCSE Chemistry answer on chemical formulae and equations, covering how to build formulae from ion charges, write word equations, balance symbol equations with state symbols, and write simple ionic equations for neutralisation and precipitation reactions.
- Qualitative analysis: flame tests and sodium hydroxide tests for metal ions, tests for halide, sulfate and carbonate ions, and the tests for hydrogen, oxygen, carbon dioxide and chlorine.
A CCEA GCSE Chemistry answer on qualitative analysis, covering flame tests and sodium hydroxide precipitate tests for metal ions, tests for halide, sulfate and carbonate ions, and the laboratory tests for hydrogen, oxygen, carbon dioxide and chlorine gases.
- The reactivity series of metals, the reactions of metals with water and acid, and displacement reactions of metals with metal salt solutions.
A CCEA GCSE Chemistry answer on the reactivity series, covering how metals are ranked by their reactions with water and acid, the order of the common metals, and how a more reactive metal displaces a less reactive one from its salt solution.
- Concentration in g per dm cubed and mol per dm cubed, using titration results to find an unknown concentration, and calculating percentage yield.
A CCEA GCSE Chemistry answer on further calculations, covering concentration in g per dm cubed and mol per dm cubed, how to use titration results and a balanced equation to find an unknown concentration, and how to calculate percentage yield.
Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)