AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2020: Question 2

✅ Correct Answer

Acid rain [1 mark]

💡 Key Knowledge

Limestone is primarily calcium carbonate (CaCO₃). Acids react with metal carbonates to produce a salt, water, and carbon dioxide gas:

Acid + Carbonate → Salt + H₂O + CO₂

This neutralisation reaction dissolves and chemically erodes the stone over time.

❌ Common Errors

• Choosing carbon monoxide: CO is toxic to humans, but it does not dissolve rock.
• Choosing ammonia: NH₃ is alkaline, not acidic.

🧠 Exam Technique

Notice the prompt specifically says "eroded where the limestone has reacted". Only an acid reacts readily with carbonate rocks to dissolve them.

✅ Correct Answers

Incomplete combustion happens when there is not enough oxygen. [1 mark]

Incomplete combustion produces particles of carbon. [1 mark]

(Must be in this exact order)

💡 Key Knowledge

• Complete combustion: Fuel + plenty of O₂ → CO₂ + H₂O
• Incomplete combustion: Fuel + limited O₂ → CO (carbon monoxide) and/or solid C (carbon particulates / soot) + H₂O
• Soot is unburnt elemental carbon.

❌ Common Errors

• Swapping the two words around (putting carbon first, then oxygen).
• Writing "methane" or "nitrogen" instead of oxygen. Read the sentence meaning carefully before choosing.

🧠 Exam Technique

Always cross off words from the provided word box once you use them. Make sure the word makes chemical sense in the sentence.

✅ Correct Answer

Particles of soot in the atmosphere cause global dimming. [1 mark]

💡 Key Knowledge

Carbon particulates reflect sunlight back into space and help produce more clouds, which also reflect sunlight. This reduces the amount of sunlight reaching the Earth's surface — a phenomenon called global dimming.

❌ Common Misconceptions

• Writing warming: Greenhouse gases (CO₂, CH₄, H₂O vapour) cause global warming, not particulates.
• Writing cooling: Although it reduces sunlight, the specific syllabus term tested here is dimming.

🧠 Exam Technique

Pair the pollutant with its consequence:

• Soot / particulates → Global dimming & respiratory problems
• CO₂ / CH₄ → Global warming (climate change)
• SO₂ / NOₓ → Acid rain

📐 Step-by-Step Balancing

Equation to balance:
2 CH₄ + 3 O₂ → ___ CO + 4 H₂O

1. Count Carbon (C) atoms on left:
   2 CH₄ has 2 × 1 = 2 Carbon atoms.
2. Balance Carbon on right:
   To get 2 Carbon atoms on the right, the coefficient in front of CO must be 2.
3. Check Oxygen (O) balance:
   Left: 3 O₂ = 6 Oxygen atoms.
   Right: 2 CO (2 O) + 4 H₂O (4 O) = 6 Oxygen atoms. Perfectly balanced!

✅ Correct Answer

Missing number: 2 [1 mark]

2 CH₄ + 3 O₂ → 2 CO + 4 H₂O

❌ Common Errors

• Leaving the space blank (thinking a coefficient of 1 is needed).
• Writing numbers other than 2 because of forgetting to multiply the front coefficient by the subscript in 2 CH₄.

🧠 Exam Technique

Notice that only one blank is provided. All other species already have coefficients. Focus directly on the single element in that molecule (here, Carbon) to find the answer immediately.

✅ Correct Answers

1. The nitrogen in car engines comes from air. [1 mark]

2. At high temperatures, the nitrogen reacts with oxygen. [1 mark]

3. This reaction produces oxides of nitrogen. [1 mark]

(Must be in this exact order)

💡 Key Knowledge

• Nitrogen gas (N₂) is usually unreactive because of its strong triple covalent bond.
• Inside internal combustion engines, extremely high temperatures provide the energy needed for N₂ from the air to react with O₂ from the air.
• The products are various oxides of nitrogen (NO and NO₂), usually written as NOₓ.

❌ Common Errors

• Saying nitrogen comes from petrol or diesel: Hydrocarbon fuels contain hydrogen and carbon (and trace sulfur impurities), but the nitrogen entering the engine comes directly from the sucked-in air.
• Saying the reaction produces sulfur dioxide: Sulfur dioxide is formed from sulfur impurities in the fuel reacting with oxygen, not from nitrogen.

🧠 Exam Technique

Read the clues in the sentence: "nitrogen reacts with [___]" → to make an oxide, it must react with oxygen. Then, combine the names: nitrogen + oxygen = oxides of nitrogen.