What does the Reactions, Acids and Salts module of O-Level Combined Science cover?

It is the reacting chemistry of the syllabus. You define acids and bases in terms of hydrogen and hydroxide ions, classify oxides, and use the pH scale and indicators to measure acidity. You then learn the characteristic reactions of acids with metals, bases and carbonates, and explain neutralisation. From these reactions you learn to prepare salts, choosing between titration, excess solid and precipitation using the solubility rules. Two physical themes complete the module: energy changes (exothermic and endothermic reactions and bond breaking and forming) and the rate of reaction, explained by collision theory. The thread is that a small set of reaction patterns and ideas explains a wide range of chemistry.

What are the characteristic reactions of acids at O-Level?

Acids show three characteristic reactions. With a reactive metal an acid gives a salt and hydrogen gas: acid + metal gives salt + hydrogen. With a base (a metal oxide or hydroxide) an acid gives a salt and water in a neutralisation: acid + base gives salt + water. With a carbonate an acid gives a salt, water and carbon dioxide: acid + carbonate gives salt + water + carbon dioxide. The salt formed depends on the acid used, for example hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates.

How do you choose a method to prepare a salt?

The choice depends on whether the salt is soluble or insoluble, judged using the solubility rules. A soluble salt of a reactive metal is usually made by adding excess insoluble solid (metal, base or carbonate) to the acid, then filtering off the excess and crystallising. A soluble salt where both reactants are soluble, such as a sodium or potassium salt, is made by titration to find the exact volumes that just neutralise, then crystallising without an indicator. An insoluble salt is made by precipitation: two soluble solutions are mixed, and the insoluble salt is filtered, washed and dried.

How does collision theory explain the rate of reaction?

Collision theory says that for particles to react they must collide with enough energy and in the correct orientation. Anything that makes collisions more frequent or more energetic speeds the reaction up. Increasing concentration (or pressure for gases) packs particles closer so they collide more often; increasing temperature gives particles more energy so more collisions succeed and they occur more frequently; increasing surface area exposes more particles to collide; and a catalyst provides an alternative pathway with a lower activation energy, so more collisions are successful. A catalyst is not used up in the reaction.

How is Combined Science Chemistry assessed by SEAB?

Combined Science is examined under syllabuses 5076 (Physics, Chemistry), 5077 (Physics, Biology) and 5078 (Chemistry, Biology), each combining two sciences into one Singapore-Cambridge GCE O-Level grade. Assessment uses a multiple-choice paper, a written paper of structured and free-response questions in compulsory sections, and a practical assessment, with marks shared between the two chosen sciences rather than a separate Chemistry-only examination.

SingaporeCombined Science

Singapore-Cambridge GCE O-Level Combined Science, Chemistry: Reactions, Acids and Salts, from acids, bases and the pH scale through the reactions of acids and salt preparation to energy changes and the rate of reaction

An O-Level Combined Science module overview for Chemistry: Reactions, Acids and Salts (SEAB 5076/5078). How acids and bases behave on the pH scale, the characteristic reactions of acids, how soluble and insoluble salts are prepared, how reactions release or absorb energy, and what controls the rate of a reaction, with links to every dot point.

Generated by Claude Opus 4.87 min readSEAB-5076Updated 2026-06-13

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

Jump to a section

What this module is about
Acids, bases and the pH scale
The reactions of acids
Preparing salts
Energy changes in reactions
The rate of reaction
How this module is examined
Check your knowledge

What this module is about

Reactions, Acids and Salts is the reacting chemistry of O-Level Combined Science. It takes a small set of patterns, the behaviour of acids and bases, the reactions that make salts, the energy a reaction releases or absorbs, and what controls its speed, and uses them to explain a wide range of chemistry. The connecting ideas are that ions in solution drive acid and base behaviour, that a handful of reaction types let you make almost any salt, and that collisions between particles explain reaction rate. Master these and the descriptive chemistry becomes systematic rather than a list to memorise.

This overview pulls the threads together and links to every dot point page in the module, each with its own worked answers and practice questions.

Acids, bases and the pH scale

The module opens with acids, bases and the pH scale. An acid produces hydrogen ions in water and a base accepts them, with alkalis being soluble bases that produce hydroxide ions. The pH scale runs from below 7 (acidic) through 7 (neutral) to above 7 (alkaline), measured with universal indicator or a pH meter. Oxides are classified as acidic (non-metal oxides), basic (metal oxides) or amphoteric. This vocabulary underpins everything else in the module.

The reactions of acids

Next, reactions of acids and bases sets out the three characteristic reactions of acids: with a metal to give a salt and hydrogen, with a base to give a salt and water (neutralisation), and with a carbonate to give a salt, water and carbon dioxide. Neutralisation is explained at the ionic level as hydrogen ions reacting with hydroxide ions to form water. The acid used fixes the salt type: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates.

Preparing salts

Building on those reactions, salt preparation and solubility shows how to make a pure, dry salt. The method follows from solubility. A soluble salt is made by adding excess insoluble solid to acid then crystallising, or, when both reactants are soluble, by titration followed by crystallisation. An insoluble salt is made by precipitation: mix two soluble solutions, then filter, wash and dry the precipitate. Choosing the right method from the solubility rules is the skill being tested.

Energy changes in reactions

The first physical theme is energy changes in reactions. Exothermic reactions release energy and warm the surroundings (a temperature rise); endothermic reactions absorb energy and cool them (a temperature fall). At the bond level, breaking bonds takes in energy and forming bonds releases it, and the overall change depends on the balance. Simple energy level diagrams show reactants and products, with an arrow downward for exothermic and upward for endothermic.

The rate of reaction

The module closes with rate of reaction, explained by collision theory: particles must collide with enough energy and the right orientation to react. Increasing concentration or pressure, temperature, or surface area makes collisions more frequent or more energetic and so speeds the reaction up. A catalyst lowers the activation energy by offering an alternative pathway, raising the proportion of successful collisions, and is not used up. These ideas are tested with graphs of product formed against time, where a steeper initial gradient means a faster rate.

How this module is examined

Write the right salt and balance the equation. Match the acid to the salt (chloride, sulfate, nitrate) and remember the extra products: hydrogen with metals, water and carbon dioxide with carbonates.
Justify the salt-preparation method. State whether the salt is soluble or insoluble, then name the matching method and the final crystallisation or filtration step.
Explain rate in terms of collisions. Do not just say "faster"; say collisions are more frequent or more successful, and for a catalyst mention the lower activation energy.

Calculating an average rate of reaction

In a reaction between marble chips and acid, $48\ \text{cm}^3$ of carbon dioxide is collected in $40\ \text{s}$ . Calculate the average rate of reaction, and state one change that would increase it.

step Identify the amount of product and the time

Volume of gas collected $= 48\ \text{cm}^3$ ; time taken $= 40\ \text{s}$ .

step Calculate the average rate

\text{average rate} = \frac{\text{volume of gas}}{\text{time}} = \frac{48\ \text{cm}^3}{40\ \text{s}} = 1.2\ \text{cm}^3\,\text{s}^{-1}

step Suggest a change that increases the rate

Using smaller marble chips (greater surface area) exposes more particles to collide, so collisions are more frequent and the rate increases. Raising the temperature or the acid concentration would also increase it.

Reactions, Acids and Salts is the reacting chemistry of the syllabus, built on a few patterns. Acids produce hydrogen ions and bases accept them; the pH scale (below 7 acidic, 7 neutral, above 7 alkaline) and indicators measure acidity, and oxides are acidic, basic or amphoteric. Acids react with metals (giving salt + hydrogen), bases (salt + water, a neutralisation of hydrogen and hydroxide ions) and carbonates (salt + water + carbon dioxide), with the acid fixing the salt type. Salts are prepared by excess solid, titration or precipitation, chosen from the solubility rules. Exothermic reactions release energy (temperature rises) and endothermic absorb it (temperature falls), with bond breaking taking in and bond forming releasing energy. Collision theory explains rate: concentration, temperature, surface area and catalysts make collisions more frequent or more successful.

Check your knowledge

A mix of recall, reasoning and calculation questions covering the module. Attempt them under timed conditions, then check against the solutions, and use the dot point pages for fuller practice.

State the ion produced by an acid in water and the approximate pH of a strong acid. (2 marks)
Write the word equation for the reaction of an acid with a carbonate. (2 marks)
Explain neutralisation in terms of ions. (1 mark)
State the method you would use to prepare an insoluble salt, and name the key step. (2 marks)
Explain, in terms of energy, why an exothermic reaction causes a temperature rise. (2 marks)
Explain, using collision theory, why increasing the temperature increases the rate of a reaction. (2 marks)

Solutions

Q1: An acid produces hydrogen ions ( $\text{H}^+$ ) in water. A strong acid has a low pH, around pH 1.
Q2: acid + carbonate $\rightarrow$ salt + water + carbon dioxide.
Q3: Neutralisation is the reaction of hydrogen ions from the acid with hydroxide ions from the base (alkali) to form water: $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$ .
Q4: Precipitation: mix two solutions of soluble salts so the insoluble salt forms; the key step is to filter the precipitate, then wash and dry it.
Q5: In an exothermic reaction more energy is released when new bonds form than is taken in to break the old bonds, so energy is given out to the surroundings, raising their temperature.
Q6: At a higher temperature the particles have more kinetic energy, so they move faster and collide more frequently, and a greater proportion of collisions have enough energy to react. Both effects increase the rate.

Sources & how we know this

Singapore-Cambridge GCE O-Level Science (Physics, Chemistry) Syllabus 5076 — Singapore Examinations and Assessment Board (2026)
Cambridge O Level Science - Combined (5129) — Cambridge Assessment International Education (2026)