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Combustion, Rusting and Oxidation Reactions — Answers

Answer sheet with model answers and success criteria for the 9th Grade mixtures & chemical reactions worksheet.

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Success TutoringWorksheetsAnswer sheet9th Grade · Science · Mixtures & Chemical Reactions

Combustion, Rusting and Oxidation Reactions — Answers

Part A: Combustion

Combustion is a fast reaction between a fuel and oxygen that releases heat and light. It is a type of oxidation reaction (a reaction in which a substance combines with oxygen).

Complete combustion happens with plenty of oxygen: a hydrocarbon fuel produces carbon dioxide and water, with a clean blue flame. Incomplete combustion happens when oxygen is limited: it produces carbon monoxide, soot (carbon) and water, and a yellow, smoky flame.

A fire needs fuel, heat and oxygen (the fire triangle). Removing any one puts the fire out.

1.Write a word equation for the complete combustion of methane.

Answer: methane + oxygen → carbon dioxide + water

2.Balance: CH₄ + O₂ → CO₂ + H₂O

Answer: CH₄ + 2O₂ → CO₂ + 2H₂O

  1. Left: C 1, H 4, O 4
  2. Right: C 1, H 4, O 2 + 2 = 4 (balanced)
3.Balance the complete combustion of butane: C₄H₁₀ + O₂ → CO₂ + H₂O

Answer: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O

  1. Balance C: 4CO₂; balance H: 5H₂O. Oxygen needed: 8 + 5 = 13 atoms, an odd number
  2. Double everything: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O
  3. Left: C 8, H 20, O 26. Right: C 8, O 16 + 10 = 26, H 20 (balanced)
4.A gas heater with a blocked air vent produces a yellow, sooty flame. Which dangerous gas is most likely being produced?
  1. AOxygen
  2. BCarbon monoxide
  3. CNitrogen
  4. DHydrogen

Answer: Carbon monoxide

With too little oxygen, incomplete combustion produces carbon monoxide and soot (carbon).

5.Explain why carbon monoxide is so dangerous.

Answer: It is colorless and has no smell, so people cannot detect it. It binds to hemoglobin in red blood cells more strongly than oxygen does, so the blood carries less oxygen, which can cause unconsciousness and death.

6.Name the three parts of the fire triangle, and explain how a fire blanket puts out a small fire.

Answer: Fuel, heat and oxygen. A fire blanket covers the fire and cuts off the oxygen supply, so combustion cannot continue.

Part B: Metals and oxygen

Many metals react with oxygen to form metal oxides: metal + oxygen → metal oxide. For example, magnesium burns with a bright white flame: 2Mg + O₂ → 2MgO.

7.Balance: Cu + O₂ → CuO, and name the product.

Answer: 2Cu + O₂ → 2CuO; copper(II) oxide (copper oxide)

  1. Left: Cu 2, O 2
  2. Right: Cu 2, O 2 (balanced)
8.0.24 g of magnesium ribbon is burned in a crucible and forms 0.40 g of magnesium oxide. What mass of oxygen combined with the magnesium?

Answer: 0.16 g

  1. Mass of oxygen = mass of product − mass of magnesium
  2. 0.40 − 0.24 = 0.16 g
9.By what percentage did the mass increase?

Answer: 66.7%

  1. 0.16 ÷ 0.24 × 100 = 66.67
  2. ≈ 66.7%
10.When a metal burns in air, the solid product weighs more than the metal did, so mass has been created.

Answer: False

No mass is created. The extra mass is oxygen atoms from the air that have combined with the metal.

Part C: Investigating rusting

Rusting is the slow oxidation of iron: iron + oxygen + water → hydrated iron(III) oxide (rust). Rust is flaky, so it falls away and exposes more iron underneath.

A student set up three test tubes, each with an iron nail, and left them for a week. Tube A: tap water open to the air. Tube B: boiled water with a layer of oil on top. Tube C: dry air with a drying agent (calcium chloride) and a stopper. Only the nail in tube A rusted.

11.In which tube or tubes did the nail rust?
  1. ATube A only
  2. BTube B only
  3. CTube C only
  4. DTubes A and B

Answer: Tube A only

Only tube A had both water and oxygen (air).

12.Why was the water in tube B boiled first, and why was a layer of oil added?

Answer: Boiling removes dissolved air (oxygen) from the water; the oil layer stops air dissolving back into it. This makes sure the nail has water but no oxygen.

13.What do the results show are the conditions needed for iron to rust?

Answer: Iron needs both water and oxygen to rust. Without either one, rusting does not happen.

14.Explain why cars rust faster in places near the sea or where salt is spread on icy roads.

Answer: Salt dissolved in water speeds up rusting (corrosion), so salty air or salty water on the car's metal makes it rust faster.

Part D: Preventing corrosion

15.Steel can be protected from rusting by painting, by galvanising (coating with zinc) or by attaching a block of a more reactive metal such as magnesium (sacrificial protection). Explain how each method works and suggest the best method for a steel ship's hull. Justify your choice.[4 marks]

Model answer: Painting forms a barrier that keeps water and oxygen away from the iron, but if the paint is scratched the steel underneath rusts. Galvanising also forms a barrier; in addition, zinc is more reactive than iron, so even if the coating is scratched, the zinc corrodes instead of the iron. Sacrificial protection uses a block of a more reactive metal, such as magnesium or zinc, connected to the steel; it oxidizes in place of the iron and is replaced when it wears away. For a ship's hull, sacrificial blocks (together with paint) are best, because the hull is huge and constantly in salty water, the paint will inevitably be scratched, and the blocks can be replaced easily during maintenance, whereas galvanising a whole ship is impractical.

Success criteria:

  • Painting as a barrier to water and oxygen
  • Galvanising as a barrier plus zinc corroding first
  • Sacrificial protection by a more reactive metal
  • A justified choice for the ship's hull
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