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SingaporeCombined Science

Chemistry: Reactions, Acids and Salts

5 dot points across 5 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.

What makes a substance an acid or a base, and how does the pH scale measure how acidic or alkaline it is?

Describe the properties of acids and bases in terms of hydrogen and hydroxide ions, classify oxides, and use the pH scale and indicators to measure acidity and alkalinity
A focused answer to the O-Level Combined Science outcome on acids and bases. Hydrogen and hydroxide ions, strong and weak acids, the pH scale, indicators, and the classification of oxides.
9 min answer →

Why do some reactions warm their surroundings and others cool them, and how do bond changes explain it?

Describe exothermic and endothermic reactions in terms of temperature change and bond breaking and forming, and interpret simple energy level diagrams
A focused answer to the O-Level Combined Science outcome on energy changes. Exothermic and endothermic reactions, energy from bond breaking and forming, activation energy, and reading energy level diagrams.
9 min answer →

What controls how fast a chemical reaction goes, and how do we explain it using colliding particles?

Describe and explain the effects of concentration, temperature, surface area and catalysts on the rate of reaction using collision theory
A focused answer to the O-Level Combined Science outcome on reaction rate. Collision theory, the effects of concentration, temperature, surface area and catalysts, and how rate is measured from graphs of product or mass against time.
9 min answer →

What products form when acids react with metals, carbonates and bases, and how does neutralisation work?

Describe the characteristic reactions of acids with metals, bases and carbonates, and explain neutralisation in terms of hydrogen and hydroxide ions
A focused answer to the O-Level Combined Science outcome on acid reactions. Acids with metals, bases and carbonates, the salt-plus-water and salt-plus-hydrogen patterns, and neutralisation as the reaction of hydrogen and hydroxide ions.
9 min answer →

How do we choose a method to prepare a pure, dry sample of a salt, and how does solubility decide that choice?

Describe the preparation of soluble and insoluble salts by titration, excess solid and precipitation, using the solubility rules to select the method
A focused answer to the O-Level Combined Science outcome on making salts. The solubility rules, preparing soluble salts by excess solid and by titration, making insoluble salts by precipitation, and obtaining a pure dry sample.
10 min answer →