AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2022: Question 5

✅ Correct Answer

Tick the box next to: Hydrogen [1 mark]

💡 Key Knowledge

General equation for acid + metal:

metal + acid → salt + hydrogen

Here: Mg + 2HCl → MgCl₂ + H₂

Carbon dioxide is only released if a carbonate is used.

❌ Common Errors

• Ticking Chlorine just because hydrochloric acid was used. Acid-metal reactions release hydrogen gas, never chlorine!
• Ticking Oxygen or Carbon dioxide.

✅ Correct Answer

Length of magnesium ribbon [1 mark]

🧠 Exam Technique

Look at the method: step 8 states "Repeat steps 1 to 7 with different lengths of magnesium ribbon."

The variable the experimenter deliberately changes across comparisons is the independent variable.

❌ Common Errors

• Writing just "magnesium" (too vague – must specify length or mass).
• Confusing independent with dependent variable (writing "volume of gas").

✅ Correct Answer (Any one from)

• Volume of (hydrochloric) acid
• Concentration of (hydrochloric) acid
• Width / thickness of magnesium ribbon
• Temperature of acid / solution

[1 mark]

🧠 Exam Technique

Always specify the property of the substance, not just its name:

• Say "concentration of acid", not just "the acid".
• Say "width/thickness of ribbon", not just "magnesium".

❌ Common Errors

• Writing "amount of acid" – this is too vague. State volume or concentration.
• Stating "length of magnesium ribbon" – that is the variable being changed!

✅ Correct Answer

Trial 1 (or 19 cm³) [1 mark]

💡 Key Knowledge

Look at the four trials:

Trial 1: 19 | Trial 2: 36 | Trial 3: 37 | Trial 4: 32

Trials 2, 3, and 4 cluster closely in the thirties (32–37 cm³). Trial 1 (19 cm³) is substantially lower and does not fit the pattern.

🧠 Exam Technique

An anomalous result is a value that differs significantly from the other repeats under the same conditions. Identifying it simply requires comparing the numbers side by side.

✅ Correct Answer (Any one from)

• Gas escaped before the stopper was put in / stopper was not replaced quickly enough
• Apparatus leaked (e.g. around the delivery tube or stopper)
• Reaction had not finished when reading was taken
• Less than 25 cm³ of acid was used
• Magnesium ribbon was cut too short (less than 10 mm / not long enough)

[1 mark]

🧠 Exam Technique

Notice that Trial 1 gave a smaller volume than expected (19 cm³ vs ~35 cm³). Therefore, your reason must explain why less gas was collected.

The most common and highest-scoring explanation in gas collection experiments is gas escaping before replacing the bung.

❌ Common Errors

• Giving a reason that would cause a higher volume (e.g. "used too much magnesium" or "temperature was too high").
• Saying "human error" or "measured incorrectly" without stating what actually went wrong. Examiner tip: generic "human error" never gets credit!

✅ Required Plotting & Best Fit

• All 6 points plotted correctly: [2 marks]
  (10, 7), (20, 14), (30, 21), (40, 28), (50, 35), (60, 42)
  (Allow 1 mark for 4 or 5 points correctly plotted; tolerance is ± ½ a small square)
• Line of best fit: [1 mark]
  A sharp, single straight line drawn with a ruler passing directly through (0,0) and matching the trend of all points.

📐 Grid Scale Breakdown

X-axis: 10 small squares = 10 mm → 1 small square = 1 mm

Y-axis: 10 small squares = 10 cm³ → 1 small square = 1 cm³

This 1:1 ratio makes counting easy! For example, for 30 mm, count up 21 small squares (one square above 20).

❌ Common Errors

• Plotting large "blobs" instead of neat crosses (×) or small dots with circles (⊙). If the blob covers more than 1 small square, you may lose marks.
• Drawing a sketchy, "furry", or freehand line instead of using a clear, clean ruler.
• Connecting points dot-to-dot instead of drawing a single straight line of best fit.

✅ Correct Answer

"As the length of the magnesium ribbon increases, the mean volume of gas produced..."

increases [1 mark]

💡 Key Knowledge

More magnesium ribbon provides more reactant particles (more moles of Mg), which produce a greater volume of hydrogen gas when reacted with excess acid.

Since the line is straight through the origin, it is actually directly proportional, but "increases" is all that is required to complete the sentence.

🧠 Exam Technique

Always check whether the question asks you to "complete the sentence" or "describe the relationship in detail". Here, a single comparative word like increases fulfills the sentence grammatically and scientifically.