AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2021: Question 1

✅ Correct Answer

4.6 billion years ago

💡 Key Knowledge

Evidence from radioactive dating of rocks and meteorites suggests the Earth was formed roughly 4.6 billion (4,600,000,000) years ago.

❌ Common Errors

Confusing million and billion. Remember: geologic and planetary formation timescales are measured in billions of years.

🧠 Exam Technique

Read the question carefully: "Tick (✓) one box." Double-ticking immediately invalidates the mark.

📐 Step-by-Step Calculation

1. Total atmosphere percentage = 100%
2. Sum the known gases in Earth's atmosphere:
   0.04 + 78.09 + 20.95 = 99.08%
3. Subtract from 100 to find X:
   X = 100 - 99.08 = 0.92%

✅ Correct Answer

X = 0.92 (%)

❌ Common Errors

Adding the numbers incorrectly under timed conditions or accidentally calculating for Mars instead of Earth. Always double-check which column is requested!

🧠 Exam Technique

The "%" sign is already written on the answer line, so simply enter the numeric value 0.92 .

✅ Correct Matches

• Nitrogen ➔ connects to Increased by about 40 times
• Oxygen ➔ connects to Increased by about 21%

💡 The Logic Behind the Numbers

• Nitrogen: Rose from 1.89% to 78.09%.
  78.09 ÷ 1.89 ≈ 41.3 (approx. 40 times multiplier).
• Oxygen: Rose from 0.15% to 20.95%.
  20.95% - 0.15% = 20.8% (an absolute increase of about 21 percentage points).

❌ Common Errors

• Drawing multiple lines from one box on the left (the mark scheme specifically states: "do not accept more than one line from a box on the left").
• Confusing a percentage point difference with a multiplication factor ("times").

🧠 Exam Technique

Use a ruler to draw straight, clear lines. If you change your mind, cross out the incorrect line completely rather than leaving ambiguous lines.

✅ Correct Choices (Tick 2)

• ☑ Carbon dioxide dissolving in sea water
• ☑ Formation of sedimentary rocks

💡 Key Knowledge

As the Earth cooled, water vapour condensed into oceans. CO₂ dissolved into the water, forming carbonate precipitates that settled to produce sedimentary rocks (like limestone, CaCO₃). Marine organisms also used dissolved carbonates to form shells and skeletons.

❌ Common Errors

• Selecting combustion or volcanoes: both of these release CO₂ rather than decreasing it.
• Selecting farming of animals: this increases greenhouse gases like methane (CH₄) and CO₂ in modern times, not early Earth.

🧠 Exam Technique

Notice the prompt explicitly states: "Which two processes caused this decrease?". Ticking more than two boxes will lose marks.

✅ Correct Answer

carbon dioxide + water → glucose + oxygen

💡 Key Knowledge

• Algae first produced oxygen about 2.7 billion years ago.
• Over the next billion years, green plants evolved and photosynthesis further increased the percentage of O₂ and decreased CO₂.

🧠 Exam Technique & Allowed Alternatives

The mark scheme accepts correct chemical formulae: CO₂ and O₂ . However, writing out the standard chemical names guarantees you avoid making subscript errors!

❌ Common Errors

• Confusing photosynthesis with respiration (writing water and glucose on the left).
• Writing "energy" or "light" in the blank spaces instead of the chemical substances.

✅ Correct Answer

...because there is a lack of evidence / proof.

💡 Key Knowledge

The Earth formed 4.6 billion years ago, which means there are very few surviving rocks or direct samples from that time period. Theories about the early atmosphere are scientific deductions based on other planets (like Mars and Venus) rather than direct measurements.

🧠 Exam Technique

Keep the completion concise. A single word like evidence fits the sentence grammatically and matches the specification wording directly.

❌ Common Errors

Writing vague statements like "nobody was alive to see it" or "technology wasn't good back then". Always use scientific terms: lack of evidence or lack of data/proof.