What do all acids and all alkalis release in water?

All acids are sources of hydrogen ions in water, written as the hydrogen ion. All alkalis are soluble bases that release hydroxide ions in water. This single idea explains the whole topic: acids supply hydrogen ions, bases accept them, and when the two meet in neutralisation the hydrogen ion and the hydroxide ion join to form water, leaving a salt behind in solution.

What are the three reactions of acids I must know for N(A)-Level?

Acids react in three characteristic ways. With a reactive metal they give a salt and hydrogen gas, which pops with a lighted splint. With a base or metal oxide they give a salt and water only. With a carbonate they give a salt, water and carbon dioxide, which turns limewater milky. Spotting which of the three is happening from the observations, such as effervescence with a gas test, is a common Paper 4 skill.

How do I choose the right method to prepare a salt?

Start from the solubility rules. If the salt is insoluble, make it by precipitation: mix two soluble solutions that supply the right ions, then filter, wash and dry the precipitate. If the salt is soluble, react an acid with excess of an insoluble base, metal oxide, metal or carbonate (the acid-plus-excess-solid method), filter off the unreacted excess, then evaporate to the point of crystallisation and cool to obtain crystals.

How is Acids, Bases and Salts assessed in N(A)-Level Science 5107?

It is examined in the two Chemistry papers. Paper 3 (multiple choice, 20 marks) tests recall such as indicator colours, the products of the three acid reactions, and the gas tests. Paper 4 (structured, 30 marks) rewards a full ordered salt-preparation description, balanced equations, and recognising reactions from observations. There is no separate practical paper, but practical skills such as filtration and crystallisation are tested in writing.

What does the pH scale tell me?

The pH scale runs from below 0 to 14 and measures how acidic or alkaline a solution is. A pH below 7 is acidic, exactly 7 is neutral, and above 7 is alkaline. A lower pH means a higher concentration of hydrogen ions. Universal indicator gives a colour for the whole range, so it estimates a pH value, while litmus only tells you acid (red) or alkali (blue) with nothing in between.

SingaporeChemistry

Acids, Bases and Salts (Singapore N(A)-Level Science Chemistry 5107): acids as sources of hydrogen ions, the pH scale and indicators, the three reactions of acids, and choosing the right method to prepare a pure salt

A Singapore N(A)-Level Science Chemistry (SEAB 5107) overview of Acids, Bases and Salts. Acids as sources of hydrogen ions and bases that accept them, the pH scale and indicators, the three characteristic reactions of acids, and how to choose between the acid-plus-excess-solid method and precipitation to prepare a pure salt, with links to every dot point.

Generated by Claude Opus 4.88 min readSEAB-5107Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic is really about
Acids, bases and the pH scale
The three reactions of acids
Preparing a pure, dry salt
How this topic is examined
Check your knowledge

What this topic is really about

Acids, Bases and Salts is where N(A) Chemistry stops simply naming substances and starts explaining their behaviour through one idea: ions in water. An acid is a source of hydrogen ions, a base accepts those hydrogen ions, and a salt is what is left when the hydrogen of the acid has been replaced by a metal or ammonium ion. Almost every reaction here, from a fizzing carbonate to a neat salt preparation, is the hydrogen ion at work. This guide ties the three dot points together and links to each one for the worked answers and practice.

The complete set of dot-point pages for this topic lives at /sg-n-level/chemistry/syllabus/acids-bases-and-salts.

Acids, bases and the pH scale

The foundation is the definition. Acids and bases and the pH scale sets up acids as sources of hydrogen ions and bases as substances that accept them, with alkalis being the soluble bases that release hydroxide ions in water. The same page turns acidity into a number on the pH scale from below 0 to 14, where a lower pH means more hydrogen ions, and gives the indicator colours you must recall.

Neutralisation is the reaction between hydrogen ions from an acid and hydroxide ions from an alkali to form water. The ionic equation is the same for any strong acid and any strong alkali.

The three reactions of acids

Reactions of acids is best learnt as a set of three. With a reactive metal, an acid gives a salt and hydrogen. With a base or metal oxide, it gives a salt and water. With a carbonate, it gives a salt, water and carbon dioxide. Each reaction has a gas test that lets you recognise it: hydrogen pops with a lighted splint, and carbon dioxide turns limewater milky.

Preparing a pure, dry salt

Preparation of salts is the practical heart of the topic. The method is chosen from the solubility rules: insoluble salts by precipitation, and soluble salts by the acid-plus-excess-solid route. Each route ends in a purification step, crystallisation for soluble salts or filtration and washing for insoluble ones.

Preparing copper(II) sulfate crystals

step Choose the method

Copper(II) sulfate is a soluble salt and copper(II) oxide is an insoluble base, so use the acid-plus-excess-solid method with dilute sulfuric acid.

step React with excess base

Warm the dilute sulfuric acid and add copper(II) oxide a little at a time, stirring, until no more dissolves. The excess black solid left behind shows all the acid has reacted:

\text{CuO}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{CuSO}_4(aq) + \text{H}_2\text{O}(l)

step Remove the excess

Filter the mixture. The unreacted copper(II) oxide stays on the filter paper; the blue filtrate is copper(II) sulfate solution.

step Crystallise and dry

Heat the filtrate to evaporate water until it is saturated (the point of crystallisation), then leave it to cool so blue crystals form. Filter off the crystals and dry them between sheets of filter paper.

How this topic is examined

Learn the three reactions as a set. Match each to its products and its gas test, and write a balanced equation for at least one example of each.
Describe a salt preparation as an ordered method. Choose the method from solubility, react with excess, filter, then crystallise or wash and dry. Marks are awarded step by step in Paper 4.
Read pH and indicator evidence. Match colours to pH and nature, and remember that lower pH means more hydrogen ions.

Check your knowledge

A mix of recall, reasoning and method questions covering Acids, Bases and Salts. Attempt them under timed conditions, then check against the solutions.

State what all acids release in water, and what all alkalis release in water. (2 marks)
Name the salt and the gas formed when zinc reacts with dilute sulfuric acid, and state the test for the gas. (3 marks)
Write the balanced equation for the reaction of calcium carbonate with dilute hydrochloric acid. (1 mark)
State the colour of universal indicator at pH 1 and at pH 13. (2 marks)
Describe, in order, how you would prepare a pure dry sample of zinc chloride crystals from zinc oxide and dilute hydrochloric acid. (4 marks)

Solutions

Q1: Acids release hydrogen ions, $\text{H}^+(aq)$ , in water. Alkalis release hydroxide ions, $\text{OH}^-(aq)$ , in water.
Q2: The salt is zinc sulfate and the gas is hydrogen. Hydrogen pops with a lighted splint.
Q3: $\text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$ .
Q4: At pH 1 universal indicator is red. At pH 13 it is purple (violet).
Q5: Warm the dilute hydrochloric acid and add zinc oxide a little at a time, stirring, until no more dissolves, so all the acid has reacted. Filter to remove the unreacted excess zinc oxide; the filtrate is zinc chloride solution. Heat the filtrate to the point of crystallisation, then cool to form crystals. Filter off the crystals and dry them between filter paper.

Sources & how we know this

Singapore-Cambridge GCE Normal (Academic) Level Science (Physics, Chemistry) and Science (Chemistry, Biology) (Syllabus 5107) — Singapore Examinations and Assessment Board (2026)
Cambridge Assessment International Education, working with SEAB on the Singapore-Cambridge GCE N(A)-Level — Cambridge Assessment International Education (2026)