Learning objectives
- Recognise evidence of a chemical reaction.
- Balance chemical equations.
- Classify common reaction types.
- Explain oxidation, reduction and corrosion.
Recognising a chemical change
A chemical reaction forms one or more new substances. Evidence may include a colour change, gas production, precipitate formation, temperature change or emission of light.
A chemical equation uses symbols and formulae to represent reactants and products. Equations must be balanced because atoms are conserved.
Common reaction types
In a combination reaction, substances join to form one product. In decomposition, one compound breaks into simpler substances. Displacement occurs when a more reactive element replaces a less reactive element.
Double displacement involves exchange of ions. Combustion is a rapid reaction with oxygen that releases energy.
Oxidation, reduction and corrosion
Oxidation can involve gain of oxygen or loss of electrons. Reduction can involve loss of oxygen or gain of electrons. Because the two occur together, many reactions are called redox reactions.
Rusting is corrosion of iron in the presence of oxygen and water. Painting, oiling, galvanising and alloying can reduce corrosion.
Practice questions with explanations
Try each question before opening the answer. The explanation shows the reasoning, not only the final response.
Q1. Why must a chemical equation be balanced?
Answer: To obey conservation of atoms.
Explanation: Atoms are rearranged, not created or destroyed.
Q2. What is a combination reaction?
Answer: Two or more reactants form one product.
Explanation: For example, calcium oxide and water form calcium hydroxide.
Q3. What is a decomposition reaction?
Answer: One compound breaks into simpler substances.
Explanation: Energy such as heat, light or electricity may be required.
Q4. What happens in a displacement reaction?
Answer: A more reactive element replaces a less reactive one.
Explanation: Reactivity determines whether displacement occurs.
Q5. What is a precipitate?
Answer: An insoluble solid formed in a solution reaction.
Explanation: It appears when ions combine to form a poorly soluble compound.
Q6. What is oxidation in terms of oxygen?
Answer: Gain of oxygen.
Explanation: For example, magnesium gains oxygen when it burns.
Q7. What is reduction in terms of oxygen?
Answer: Loss of oxygen.
Explanation: Reduction and oxidation occur together in redox reactions.
Q8. What conditions are required for rusting?
Answer: Oxygen and water.
Explanation: Removing either condition slows rust formation.
Q9. How does galvanising protect iron?
Answer: It coats iron with zinc.
Explanation: The zinc layer separates iron from air and moisture.
Q10. Name two signs of a chemical reaction.
Answer: Examples include gas, colour change, precipitate or temperature change.
Explanation: These observations suggest new substances have formed.