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.
Evidence that a chemical change may have occurred
Gas evolution, a persistent colour change, an unexpected temperature change or formation of an insoluble solid can indicate a chemical reaction. However, boiling also creates bubbles and mixing colours can change appearance without producing a new substance, so observations must be interpreted carefully.
Equation audit
For the equation 2H₂ + O₂ → 2H₂O, count atoms on both sides: four hydrogen atoms and two oxygen atoms occur before and after the reaction. Balancing changes coefficients, not the chemical formulas of the substances.
Worked example: balancing a chemical equation
Magnesium reacts with oxygen to form magnesium oxide. Begin with the skeleton equation Mg + O₂ → MgO. There are two oxygen atoms on the left but only one on the right. Put a coefficient of 2 before MgO to make two oxygen atoms on the right. The right side now has two magnesium atoms, so place a 2 before magnesium on the left.
The balanced equation is 2Mg + O₂ → 2MgO. Count atoms to check: two magnesium atoms and two oxygen atoms appear on each side. Coefficients describe quantities of particles; changing a chemical formula's subscripts would describe a different substance and is not a valid balancing method.
| Element | Reactants | Products |
|---|---|---|
| Magnesium | 2 | 2 |
| Oxygen | 2 | 2 |
Classifying reactions using evidence
In a combination reaction, two or more reactants form one product. A decomposition reaction splits one compound into simpler substances. In displacement reactions, a more reactive element can replace a less reactive one from a compound under appropriate conditions. In every case, a visible colour change or bubbles may suggest a reaction, but observations alone do not always prove exactly which reaction occurred.
Practice safely
Balance the equation H₂ + O₂ → H₂O.
2H₂ + O₂ → 2H₂O. Four hydrogen atoms and two oxygen atoms appear on both sides.
Why is rusting described as oxidation?
In rusting, iron reacts with oxygen in the presence of moisture, forming hydrated iron(III) oxides. This involves oxidation of iron. Do not handle chemicals or perform reactions without teacher supervision.
Observation, inference, explanation
Use three columns: what was observed, what it may suggest, and what further evidence would help. “Bubbles appeared” is an observation; “a gas formed” is an interpretation because boiling can also make bubbles.
For equation work, circle each element and count atoms before changing any coefficient. This makes balancing a conservation argument rather than a trial-and-error symbol shuffle.
Reaction audit
Describe evidence carefully, identify the reaction pattern, balance with coefficients, and re-count atoms. Oxidation and reduction describe complementary changes; corrosion is a familiar slow oxidation process, not simply any colour change.
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.