Reactions

Acid rain forms when sulfur dioxide and nitrogen oxides from burning fossil fuels dissolve in atmospheric water to make sulfuric and nitric acid. It damages ecosystems, corrodes buildings, and harms aquatic life.

Acids are substances that produce hydrogen ions (H⁺) in water. They taste sour, react with metals to produce hydrogen gas, react with carbonates to produce carbon dioxide, and have pH values below 7.

Bases are substances that neutralise acids. An alkali is a base that dissolves in water, producing hydroxide ions (OH⁻). Alkalis have a pH above 7 and feel slippery to touch.

Acids and alkalis are all around us — in food, cleaning products, our bodies, and the environment. Knowing the pH of a substance and understanding the hazards of strong acids and alkalis helps us use them safely.

Indicators are substances that change colour depending on whether a solution is acidic or alkaline. They are used in chemistry to identify acidic or alkaline conditions and to find the endpoint of a neutralisation reaction.

Neutralisation is the reaction between an acid and a base (or alkali) to produce a salt and water. The pH of the mixture moves towards 7 (neutral) as the reaction occurs.

The pH scale measures how acidic or alkaline a solution is, running from 0 (most acidic) to 14 (most alkaline), with 7 being neutral. It is based on the concentration of hydrogen ions in the solution.

A chemical reaction transforms reactants into entirely new substances called products. Learn to recognise reactions by their signs and understand why matter is always conserved.

Chemical synthesis combines simpler substances to build more complex ones. Discover how this fundamental type of reaction underpins everything from fertilisers to life-saving medicines.

A word equation summarises a chemical reaction by naming the reactants on the left and the products on the right, linked by an arrow. It is the simplest way to describe what happens in a reaction.

Combustion is a chemical reaction in which a fuel reacts rapidly with oxygen, releasing heat and light. Complete combustion of a hydrocarbon fuel produces carbon dioxide and water.

In any chemical reaction, the total mass of the reactants equals the total mass of the products. Atoms are rearranged but never created or destroyed — this is the law of conservation of mass.

A displacement reaction is when a more reactive element takes the place of a less reactive element in a compound. These reactions reveal the reactivity series — the ranking of metals from most to least reactive.

Chemical reactions either release heat energy into the surroundings (exothermic) or absorb heat energy from the surroundings (endothermic).

Chemical reactions always involve energy changes. Exothermic reactions release energy (feel hot), while endothermic reactions absorb energy (feel cold). Understanding these changes is essential for industry, food, and everyday life.

Four key factors control how fast a chemical reaction proceeds — temperature, concentration, surface area, and catalysts. Learn how each one works and why it matters in everyday life.

Every fire-extinguishing method works by removing one side of the fire triangle. Learn how different extinguishers work, which fires they suit, and how to stay safe at home and school.

Fire is not a substance but a chemical reaction called combustion. Learn what fire really is, how the fire triangle works, and what different flame colours tell us.

Careful observation is a fundamental skill in chemistry. Learn what scientists look for when recording a chemical reaction and practise systematic experimental observation.

Oxidation is the reaction of a substance with oxygen. Rusting is the oxidation of iron — a slow reaction that forms iron oxide when iron, water, and oxygen are all present together.

A physical change alters the appearance of a substance but not its chemical identity, while a chemical change produces a new substance with different properties. Knowing the difference is fundamental to understanding chemistry.

A precipitation reaction produces an insoluble solid (precipitate) when two solutions are mixed. The precipitate forms because one of the possible products cannot dissolve. These reactions are used to identify ions and remove pollutants from water.

Reaction rate tells us how fast a chemical reaction produces new substances. Find out how chemists measure speed, what activation energy is, and why some reactions take seconds while others take years.

Thermal decomposition is a reaction in which a single compound breaks down into two or more simpler substances when heated. It is a common type of chemical change and is used in industry to extract metals and make lime.

Study of chemical reactions, energy changes, reaction types, and factors affecting reaction rates.