Acids, bases and salts: neutralisation and choosing a preparation method
Separate three questions: which particles react, which salt forms, and how that salt can be obtained without excess reactant. A correct equation is only the beginning of a salt-preparation explanation.
Choose the method
Acid + alkali forms salt + water. Acid + carbonate also releases carbon dioxide. The acid supplies the anion of the salt; the base, metal or carbonate supplies its cation.
To make a soluble salt from an acid and an insoluble base, use excess solid so the acid is consumed, then filter off the unreacted solid before concentrating and cooling the solution.
When both reactants are soluble, filtration cannot remove excess alkali. Use titration to determine the reacting volumes. These are revision descriptions for supervised school laboratory work.
Worked examples
Example 1
Write the equation for hydrochloric acid reacting with sodium carbonate. Name a test for the gas.
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂.
The salt is sodium chloride. Carbon dioxide turns limewater milky because an insoluble carbonate forms.
Check the atom count: two sodium atoms and two chlorine atoms appear on each side.
Example 2
25.0 cm³ of 0.100 mol/dm³ HCl neutralises 20.0 cm³ of NaOH. Find the NaOH concentration.
Why is excess copper(II) oxide filtered off before crystallising copper(II) sulfate made with sulfuric acid?
Show the worked answer
Excess insoluble oxide ensures the acid has reacted. The unreacted solid would contaminate the crystals if left in the mixture.
Filtration removes that solid while dissolved copper(II) sulfate passes through in the filtrate.
Common mistakes
Strong and concentrated mean different things: strength concerns ionisation, concentration concerns amount per volume.
Evaporating a solution to dryness is not the same as concentrating it and allowing crystals to form on cooling.
Sources and review
SEAB 2026 syllabus: 2026 Combined Science chemistry component: acids, bases and salts; also useful for Pure Chemistry revision.
Original Rae practice, prepared with AI assistance. Selected numerical results and their displayed working are automatically checked at publication; this does not verify every explanation. Curriculum references checked on 5 September 2026. No teacher review or SEAB endorsement is claimed.
What the syllabus expects
Explain what the terms acid and alkali mean by reference to the ions each releases in aqueous solution and how each affects Universal Indicator.
Describe neutrality and how acidic or alkaline a solution is, using the balance of H+ and OH- ion concentrations, the Universal Indicator colour, and the pH scale. Scope: Working out pH from the concentration of hydrogen ions is not required.
Explain qualitatively how strong and weak acids differ by the degree to which they ionise.
Set out the typical properties of acids as seen when they react with metals, bases and carbonates to give salts.
Describe neutralisation as hydrogen ions combining with hydroxide ions to give water, H+ + OH– → H2O.
Explain why managing soil pH matters and how calcium hydroxide can be used to treat overly acidic soil.
Set out the typical properties of bases as seen in their reactions with acids and with ammonium salts.
Sort oxides into acidic, basic, amphoteric or neutral according to their metallic or non-metallic character.
Describe how salts are prepared, separated and purified, as illustrations of some of the techniques listed in Section 1.2(a). Scope: Preparation methods should cover precipitation and titration together with acids reacting with metals, insoluble bases and insoluble carbonates.
Given appropriate information, propose a way of preparing a particular salt from suitable starting materials.
Explain how ammonia is manufactured using nitrogen from the air and hydrogen obtained by cracking crude oil.
State that some reactions are reversible, such as the manufacture of ammonia.
Interpret data about the industrial conditions chosen for reversible processes, such as making ammonia by the Haber Process. Scope: Le Chatelier's Principle is not examined.
How it's examined
Questions on this topic most often ask you to state, explain. About 10% of the past-paper style questions in Rae's bank for this subject sit in this topic.
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