The problems are on the combustion worksheet. Try them on your own first, then compare with the answers below.
Quick answer: Combustion is a fast reaction of a fuel with oxygen that releases heat and light. These worked answers correct the 20 problems of the grade 8 science worksheet on combustion, from the fire triangle to comparing fuels: each solution shows the formula, the substitution and the final answer with its unit.
Each answer shows the counting or the calculation first, and then the result in bold. Use the working to find exactly where a mistake began.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-2
Problem 1 The fire triangle
1. A is fuel, B is oxygen and C is heat.2. The fire goes out.
3. The fuel is the wax (and the wick).
Problem 2 Putting it out
1. Blanket: stops air, so it removes oxygen.2. Water: cools the log, so it removes heat.
3. Closing the valve: removes fuel.
Problem 3 Charcoal grill
1. Reactants: carbon and oxygen. Product: carbon dioxide.2. Left: C 1, O 2. Right: C 1, O 2.
3. Balanced.
Problem 4 Limewater test
1. Carbon dioxide made tube P milky.2. The gas in Q is not carbon dioxide because the limewater stayed clear.
3. Calcium carbonate.
Problem 5 Complete the sentences
1. True.2. False: it needs oxygen.
3. True.
4. True. Statements 1, 3 and 4 are true.
Problem 6 Blue or yellow flame
1. The air supply is too low, so combustion is incomplete and makes soot and carbon monoxide.2. Blue.
3. Have the stove checked or cleaned so that enough air reaches the flame; do not keep using it in a closed room.
Problem 7 Methane atoms
1. Left: C 1, H 4, O 2 × 2 = 4.2. Right: C 1, H 2 × 2 = 4, O 2 + 2 = 4.
3. Yes, atoms are conserved.
Problem 8 Burning hydrogen
1. 2 H₂ + O₂ → 2 H₂O (H 4/4, O 2/2).2. Water.
3. Burning hydrogen makes only water, with no carbon dioxide or carbon monoxide.
Problem 9 Camping stove
1. Carbon: 3 CO₂. Hydrogen: 8 H, so 4 H₂O. Oxygen: 6 + 4 = 10, so 5 O₂: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O.2. Left: C 3, H 8, O 10. Right: C 3, H 8, O 6 + 4 = 10.
Problem 10 Mass of carbon dioxide
1. 12 + 32 = 44 g on both sides.2. 3.0 ÷ 12 = 0.25, so 0.25 × 44 = 11 g of CO₂.
3. 0.25 × 32 = 8.0 g of oxygen.
Problem 11 Water from methane
1. 4.0 ÷ 16 = 0.25; 0.25 × 36 = 9.0 g of water.2. 0.25 × 64 = 16 g of oxygen.
3. 0.25 × 44 = 11 g of CO₂. Check: 4.0 + 16 = 20 g; 11 + 9.0 = 20 g.
Problem 12 Incomplete combustion
1. Left: C 2, O 2. Right: C 2, O 2. Balanced.2. Incomplete: 1 O₂ per 2 C, which is 0.5 O₂ per C. Complete: 1 O₂ per C. Complete combustion needs twice as much oxygen.
3. With enough air the carbon burns to CO₂ instead of poisonous CO.
Problem 13 Charcoal indoors
1. Reduce risk: (a), (b) and (d). Blocking the vent (c) increases the risk.2. Carbon monoxide has no smell, colour or taste, so people cannot detect it without an alarm.
Problem 14 How CO harms the body
1. Haemoglobin carries oxygen. If it is carrying CO there is less room for oxygen, so the body gets less oxygen.2. CO cannot be sensed, so breathing seems normal while oxygen delivery to the brain and heart falls.
Problem 15 Limewater equation
1. Left: Ca 1, O 2 + 2 = 4, H 2, C 1. Right: Ca 1, C 1, O 3 + 1 = 4, H 2. Balanced.2. Calcium carbonate.
3. Many gases other than CO₂ also put out a splint, so it does not prove CO₂.
Problem 16 Butane lighter
1. Start with 13/2 O₂ and double all: 2 C₄H₁₀ + 13 O₂ → 8 CO₂ + 10 H₂O.2. Left: C 8, H 20, O 26. Right: C 8, H 20, O 16 + 10 = 26.
3. 2 butane make 8 CO₂, so 1 makes 4: 5 × 4 = 20 molecules.
Problem 17 Extinguisher choice
1. CO₂: it is safe on electrical equipment, and it displaces oxygen around the fire.2. Oil floats on water and the burning oil can spread, and the water turns to steam violently and throws oil upwards.
3. Turn off the heat and cover with a lid or fire blanket to remove oxygen.
Problem 18 Candle in a jar
1. About 22 s.2. 44 ÷ 1000 = 0.044 s per mL; 1250 × 0.044 = 55 s.
3. 0.21 × 500 = 105 mL.
4. The oxygen is used up; the fuel and heat are still present but oxygen is missing.
Problem 19 Steel wool gets heavier
1. 3.10 − 2.50 = 0.60 g.2. 0.60 ÷ 2.50 × 100 = 24 %.
3. The iron combines with oxygen from the air, so the solid gains the mass of that oxygen. Wood gives off CO₂ and water vapour, which escape, so the solid left weighs less.
Problem 20 Comparing fuels
1. Methane 55 × 20 = 1100 kJ; carbon 33 × 20 = 660 kJ; ethanol 30 × 20 = 600 kJ; hydrogen 142 × 20 = 2840 kJ.2. 2200 ÷ 55 = 40 g.
3. Hydrogen: it has no carbon atoms, so it can only make water.
