Chemical Reactions: Word Equations and Balancing Equations — Answers
Answer sheet with model answers and success criteria for the Year 10 mixtures & chemical reactions worksheet.
Success TutoringWorksheetsAnswer sheetYear 10 · Science · Mixtures & Chemical ReactionsChemical Reactions: Word Equations and Balancing Equations — Answers
Part A: Reactions and equations
In a chemical reaction, reactants are changed into new substances called products. The atoms are rearranged into new combinations, but no atoms are created or destroyed.
A word equation names the substances: hydrogen + oxygen → water. A balanced formula equation shows the formulas and has the same number of each type of atom on both sides: 2H₂ + O₂ → 2H₂O. Check: on the left there are 2 × 2 = 4 hydrogen atoms and 2 oxygen atoms; on the right there are 2 × 2 = 4 hydrogen atoms and 2 × 1 = 2 oxygen atoms.
To balance an equation, change only the coefficients (the large numbers in front of formulas). Count each type of atom, balance one element at a time, and leave elements that appear on their own (such as O₂) until last.
Answer: Reactants: methane and oxygen. Products: carbon dioxide and water.
Answer: hydrochloric acid + sodium hydroxide → sodium chloride + water
Answer: Any three of: a gas is produced (bubbles), a colour change, a precipitate (solid) forms, a temperature change (heat given out or taken in), light is given out, a smell is produced.
- AIt would change the substance into a different one
- BIt would make the equation too long
- CSubscripts only apply to gases
- DIt would change the mass of the container
Answer: It would change the substance into a different one
Subscripts are part of the formula. H₂O is water, but H₂O₂ is hydrogen peroxide, a different substance. Balance only by changing the large numbers in front (coefficients).
Part B: Balance the equations
Balance each equation by writing coefficients in front of the formulas. Check by counting each type of atom on both sides.
Answer: H₂ + Cl₂ → 2HCl
- Left: H 2, Cl 2
- Right with 2HCl: H 2, Cl 2 (balanced)
Answer: 2Mg + O₂ → 2MgO
- Left: Mg 2, O 2
- Right: Mg 2, O 2 (balanced)
Answer: 2Na + Cl₂ → 2NaCl
- Left: Na 2, Cl 2
- Right: Na 2, Cl 2 (balanced)
Answer: N₂ + 3H₂ → 2NH₃
- Left: N 2, H 6
- Right: N 2, H 6 (balanced)
Answer: CH₄ + 2O₂ → CO₂ + 2H₂O
- Left: C 1, H 4, O 4
- Right: C 1, H 4, O 2 + 2 = 4 (balanced)
Answer: Zn + 2HCl → ZnCl₂ + H₂
- Left: Zn 1, H 2, Cl 2
- Right: Zn 1, Cl 2, H 2 (balanced)
Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
- Balance C first: 3CO₂. Then H: 8 H atoms means 4H₂O.
- Oxygen on the right: 3 × 2 + 4 × 1 = 10, so 5O₂ on the left.
- Left: C 3, H 8, O 10. Right: C 3, H 8, O 10 (balanced)
Answer: 4Fe + 3O₂ → 2Fe₂O₃
- Left: Fe 4, O 6
- Right: Fe 2 × 2 = 4, O 2 × 3 = 6 (balanced)
Part C: 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, because atoms are only rearranged.
Answer: 1.6 g
- Mass of reactants = mass of products
- 2.4 g + mass of oxygen = 4.0 g
- Mass of oxygen = 4.0 − 2.4 = 1.6 g
Answer: 4.4 g
- 10.0 g = 5.6 g + mass of carbon dioxide
- Mass of carbon dioxide = 10.0 − 5.6 = 4.4 g
Answer: No. One of the products is carbon dioxide gas, which escapes into the air, so it is no longer weighed. If the reaction were done in a sealed container, the total mass would stay the same.
Part D: Types of reaction
Classify each reaction, then answer the questions.
Combustion: a substance reacts with oxygen, releasing energy as heat and light. When a hydrocarbon fuel burns completely: fuel + oxygen → carbon dioxide + water.
Neutralisation: an acid reacts with a base: acid + base → salt + water. The salt is named from the metal in the base and the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.
Precipitation: two solutions react to form an insoluble solid, called a precipitate, which makes the mixture go cloudy.
Safety: acids and burning fuels are used only in the school laboratory, with eye protection and your teacher's supervision.
- Acombustion
- Bneutralisation
- Cprecipitation
Answer: combustion
A fuel (methane) reacts with oxygen to produce carbon dioxide and water.
- Acombustion
- Bneutralisation
- Cprecipitation
Answer: neutralisation
A base (potassium hydroxide) reacts with an acid (nitric acid) to make a salt (potassium nitrate) and water. Check: K 1, O 4, H 2, N 1 on each side.
- Acombustion
- Bneutralisation
- Cprecipitation
Answer: precipitation
Two solutions react to form silver chloride, an insoluble white solid (precipitate). Check: Ag 1, N 1, O 3, Na 1, Cl 1 on each side.
- Acombustion
- Bneutralisation
- Cprecipitation
Answer: combustion
Ethanol burns in oxygen to produce carbon dioxide and water. Check: C 2, H 6, O 7 on each side.
Answer: Potassium sulfate (with water).
Answer: Sodium nitrate (with water).
Answer: Magnesium chloride (with water).
Answer: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
- Balance Na first: 2NaOH
- That gives 4 H on the left, so 2H₂O on the right
- Left: H 4, S 1, O 6, Na 2. Right: Na 2, S 1, O 6, H 4 (balanced)
Part E: Challenge
Answer: False
The number of atoms of each element must be the same, but the number of molecules can change. In 2H₂ + O₂ → 2H₂O there are 3 molecules on the left and 2 on the right.
Model answer: (a) A precipitation reaction. The yellow solid is the precipitate lead(II) iodide, PbI₂, which is insoluble in water. (b) Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃. Left: Pb 1, N 2, O 6, K 2, I 2. Right: Pb 1, I 2, K 2, N 2, O 6. Every element balances.
Success criteria:
- Identifies a precipitation reaction
- Names lead(II) iodide as the precipitate
- Correct coefficients: 1, 2, 1, 2
- Shows a correct atom count for every element on both sides
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.
Great work. Here's what to try next: