Singapore-Cambridge N(A)-Level Combined Science, Chemistry: Reactions, Acids and Salts, from recognising chemical reactions and energy changes to acids, bases, the pH scale and the preparation of salts

What this module is about

The Reactions, Acids and Salts module of N(A)-Level Combined Science Chemistry (SEAB 5105 and 5107) is practical chemistry in action. It teaches you to spot when a reaction has happened, to write balanced equations, to follow the energy that flows in or out, and to use the predictable reactions of acids to make a salt of your choice. The unifying idea is that reactions follow patterns: once you know the pattern, you can predict products and plan a preparation.

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.

Recognising and balancing reactions

The module starts with chemical reactions and energy changes. Signs of a chemical reaction include a colour change, a gas given off, a precipitate forming, or a temperature change. Equations must be balanced so that the same number of each kind of atom appears on both sides. Reactions are exothermic when they give out heat (the temperature rises) and endothermic when they take heat in (the temperature falls).

Acids, bases and the pH scale

Next comes acids, bases and the pH scale. Acids have a pH below 7 and turn blue litmus red; alkalis (soluble bases) have a pH above 7 and turn red litmus blue; a neutral solution has a pH of 7. Universal indicator gives a colour for each pH. Acids react in predictable ways: acid plus metal gives a salt plus hydrogen; acid plus base gives a salt plus water (neutralisation); acid plus carbonate gives a salt plus water plus carbon dioxide.

Salts and their preparation

The module finishes with salts and their preparation. A salt is the compound formed when the hydrogen of an acid is replaced by a metal (or ammonium) ion. Some salts are soluble and some insoluble. A pure soluble salt is often made by reacting an acid with an excess of an insoluble base, filtering off the unreacted excess, then crystallising the salt from the solution.

How this module is examined

• Give a real sign of reaction. Name a colour change, a gas, a precipitate or a temperature change, not just "it reacts".
• Use the three acid patterns. Metal gives a salt and hydrogen; base gives a salt and water; carbonate gives a salt, water and carbon dioxide.
• Explain why a preparation gives a pure salt. The excess insoluble base ensures all the acid reacts, and filtration removes the unreacted excess.

Check your knowledge

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

1. State two signs that a chemical reaction has taken place. (2 marks)
2. Explain how you would tell whether a reaction is exothermic or endothermic. (1 mark)
3. Give the pH range of an acid and of an alkali. (2 marks)
4. State the products when an acid reacts with a metal. (2 marks)
5. State the products when an acid reacts with a carbonate. (2 marks)
6. Describe how to prepare a pure, dry sample of a soluble salt from an acid and an insoluble base. (3 marks)