AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2023: Question 1

✅ Correct Answer

Mars

💡 Key Knowledge

Earth's early atmosphere was mainly carbon dioxide (CO₂) with little or no oxygen (O₂), very similar to the current atmospheres of Mars and Venus today.

✅ Correct Answer

20%

💡 Key Knowledge

• Nitrogen (N₂): ~78% (approx. 80%)
• Oxygen (O₂): ~21% (approx. 20%)
• Other gases (Argon, CO₂, H₂O vapour): ~1%

❌ Common Errors

Confusing nitrogen and oxygen proportions (e.g., ticking 80% instead of 20%). Remember: nitrogen is the most abundant gas, not oxygen!

✅ Correct Answers (Tick Two)

• Algae and plants evolved [1 mark]
• Photosynthesis took place [1 mark]

💡 Key Knowledge

Algae first produced oxygen around 2.7 billion years ago. Over the next billion years, green plants evolved and photosynthesis steadily raised O₂ levels:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

❌ Common Errors

Selecting "Carbonate sediments formed": While this removed CO₂ from the atmosphere, it did not produce or increase oxygen.

✅ Mark Scheme Requirements

• Scale: y-axis labelled at the major grid lines with 5, 10, 15 (with 0 and 20 already present). Also accepted: 4, 8, 12, 16, 20 . [1 mark]
• Bar: Oxygen bar drawn to exactly 16% (±½ small square tolerance). [1 mark]

🔧 Step-by-Step Graph Method

1. Determine the scale: There are 20 large grid divisions between 0 and 20. Thus, each large square represents 1% (or 5 large squares = 5%).
2. Label the grid lines: Count 5 squares up for 5, 10 for 10, and 15 for 15.
3. Draw the bar: Find 16 on your scale (1 square above 15). Use a sharp pencil and ruler to draw a neat bar matching the width of the Carbon Dioxide bar.

🧠 Exam Technique

Always check that numerical scales increase in equal, linear steps. Writing non-linear numbers (e.g., just writing 4 and 16 randomly on the line) scores zero for the scale.

✅ Correct Answer

Test tube A

💡 Why Tube A?

Test tube A displays the greatest number of gas bubbles. A higher temperature gives particles more kinetic energy, leading to more frequent and successful collisions, meaning the rate of decomposition is highest.

🧠 Visual Evidence

Look at Figure 2 carefully:

• Tube A: Lots of bubbles throughout the liquid (fastest reaction → highest temperature).
• Tube B: Very few bubbles (slowest reaction → lowest temperature).
• Tube C: Moderate bubbling.

✅ Correct Answer

A glowing splint

💡 Essential Gas Tests Summary

• Oxygen (O₂): Relights a glowing splint.
• Hydrogen (H₂): Burns with a 'squeaky pop' when tested with a lit splint.
• Chlorine (Cl₂): Bleaches damp litmus paper white.
• Alkenes (not a gas test): Turn orange bromine water colourless.

❌ Common Errors

Students frequently confuse the test for oxygen (glowing splint) with the test for hydrogen (burning/lit splint).

✅ Correct Answer

Manganese dioxide is a catalyst in this reaction.

💡 Definition of a Catalyst

A substance that speeds up a chemical reaction without being chemically changed or used up at the end of the reaction. It works by providing an alternative reaction pathway with a lower activation energy.

❌ Distractor Breakdown

• "Increases activation energy" — False (it lowers it).
• "Is used up during this reaction" — False (catalysts remain unchanged in mass).
• "Reduces the rate" — False (it increases rate).