Chemical Reactions
Understand how substances change during chemical reactions, learn to write word equations, explore reaction types, and discover the law of conservation of mass.
What is a Chemical Reaction?
A chemical reaction occurs when substances (called reactants) interact to form new substances (called products) with different chemical properties. During a chemical reaction, the atoms are rearranged — existing bonds break and new bonds form.
General Equation
The arrow means "reacts to form" or "produces"
Indicators of a Chemical Reaction
How do you know a chemical reaction has occurred? Look for these signs:
Temperature Change
Heat released (exothermic) or absorbed (endothermic)
Gas Produced
Bubbles, fizzing, or a popping/squeaky sound
Precipitate Formed
An insoluble solid forms in the solution
Colour Change
A permanent change in the colour of the substance
Light Emitted
A flame or glow (e.g. combustion, chemiluminescence)
Irreversible
Difficult or impossible to reverse (unlike physical changes)
Word Equations
A word equation uses the names of the reactants and products to describe a chemical reaction. The reactants are on the left of the arrow, and the products are on the right.
Rusting of iron:
iron + oxygen + water → hydrated iron oxide (rust)
Burning methane (natural gas):
methane + oxygen → carbon dioxide + water
Acid + metal reaction:
zinc + hydrochloric acid → zinc chloride + hydrogen
Types of Chemical Reactions
Combustion
A substance reacts rapidly with oxygen, producing heat and light (a flame). If the fuel contains carbon and hydrogen, the products are carbon dioxide and water.
fuel + oxygen → carbon dioxide + water (+ energy)
Example: Burning wood, natural gas in a stove, petrol in a car engine
Corrosion (Oxidation)
A slow reaction where a metal reacts with oxygen and moisture in the environment, forming a metal oxide. This gradually deteriorates the metal surface.
iron + oxygen + water → hydrated iron oxide (rust)
Prevention: Painting, galvanising (zinc coating), oiling, using stainless steel (alloy)
Decomposition
A single compound breaks down into two or more simpler substances. This can be caused by heat (thermal decomposition), electricity (electrolysis), or light.
calcium carbonate → calcium oxide + carbon dioxide
Example: Heating limestone, electrolysis of water, rotting food
Conservation of Mass
The law of conservation of mass states that in a chemical reaction, the total mass of the reactants equals the total mass of the products. Matter is neither created nor destroyed — atoms are simply rearranged.
Why does it sometimes look like mass changes? If a gas is produced and escapes to the atmosphere (e.g. CO2 when burning), the measured mass of the remaining products decreases. But the gas is still part of the product — the total mass is still conserved.
Key Vocabulary
Reactant
A starting substance that undergoes change in a chemical reaction. Written on the left of the equation.
Product
A new substance formed as a result of a chemical reaction. Written on the right of the equation.
Exothermic
A reaction that releases heat to the surroundings (e.g. combustion). The temperature of surroundings increases.
Endothermic
A reaction that absorbs heat from the surroundings (e.g. photosynthesis). The temperature of surroundings decreases.
Precipitate
An insoluble solid that forms when two solutions react. It appears as cloudiness or particles settling out.
Catalyst
A substance that speeds up a chemical reaction without being consumed. It lowers the activation energy.
Worked Examples
Write a word equation for burning magnesium in air.
Step 1: Identify the reactants: magnesium (the fuel) and oxygen (from air).
Step 2: When a metal burns in oxygen, it forms a metal oxide.
Answer: magnesium + oxygen → magnesium oxide
12 g of carbon reacts with 32 g of oxygen. What is the mass of carbon dioxide produced?
Step 1: Apply the law of conservation of mass: total mass of reactants = total mass of products.
Step 2: Total mass of reactants = 12 g + 32 g = 44 g
Answer: Mass of carbon dioxide = 44 g
Identify the type of reaction: iron slowly reacts with oxygen and water in the air to form rust.
Step 1: This is a slow reaction between a metal and oxygen/water from the environment.
Step 2: The metal surface gradually deteriorates, forming a metal oxide.
Answer: This is corrosion (specifically rusting). It is a type of oxidation reaction.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
In the reaction methane + oxygen → carbon dioxide + water, what are the products?
Question 2
Which of the following is NOT a sign that a chemical reaction has occurred?
Question 3
20 g of substance A reacts with 15 g of substance B. According to the law of conservation of mass, what is the total mass of the products?
Question 4
Which type of reaction involves a substance breaking down into simpler substances when heated?
Question 5
A reaction that releases heat to its surroundings is called:
Key Concepts Summary
- ●In a chemical reaction, reactants are rearranged to form new products.
- ●Signs include: temperature change, gas produced, precipitate, colour change, light emitted.
- ●Key reaction types: combustion (burning), corrosion (rusting), decomposition (breaking down).
- ●Conservation of mass: total mass of reactants = total mass of products.
- ●Exothermic reactions release heat; endothermic reactions absorb heat.