AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2020: Question 3

✅ Correct Answer

Any one from:

• Measuring cylinder
• Burette
• Pipette (or volumetric pipette)

🧠 Exam Technique

Conical flasks and beakers only have approximate graduation lines. To improve precision or accuracy in any volumetric practical, always suggest a measuring cylinder, burette, or pipette.

❌ Common Errors

Do not write "beaker" or "flask" — these are not calibrated measuring instruments and will receive 0 marks.

✅ Correct Answer

☑ Limewater turns milky (cloudy)

💡 Key Knowledge (Gas Tests Recall)

• Carbon dioxide: Limewater turns milky/cloudy.
• Hydrogen: Burning splint pops with a squeaky sound.
• Oxygen: Glowing splint relights.
• Chlorine: Damp litmus paper bleaches white.

🧠 Chemical Detail

Limewater is an aqueous solution of calcium hydroxide, Ca(OH)₂. It turns cloudy because insoluble calcium carbonate, CaCO₃, forms:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

✅ Correct Plotting & Line

Plot coordinates from Table 2 for large lumps:

• (0, 0), (20, 16), (40, 30), (60, 40), (80, 46), (100, 48)
• Line of best fit: A smooth curve starting at (0,0), levelling off gradually.

🧠 Graph Drawing Rules

• Use small, sharp '×' marks directly on the grid intersections.
• Examiners allow a tolerance of ± ½ small square.
• Draw a single, smooth, continuous curve. Avoid "feathering" (multiple sketchy lines) or connecting points with a ruler.

❌ Common Errors

Connecting the points dot-to-dot with straight ruler lines. Chemical reactions with changing rates produce curved lines of best fit, not jagged polygons!

📐 Step-by-Step Calculation

Step 1: Read value from Figure 4 for small lumps at t = 60 s

Volume at 60 s = 48 cm³

Step 2: Substitute into the given rate equation

Mean rate = 48 cm³ / 60 s

Step 3: Calculate the final value

Mean rate = 0.8 cm³/s

🧠 Exam Technique

• Check which curve: The question specifically asked for small calcium carbonate lumps (Figure 4, not your plotted curve!).
• Look carefully at the grid: At 60 s, the curve passes exactly through 48 cm³.
• If you misread the volume (e.g. read 49 cm³), you can still get 2 method marks if you show 49 / 60 = 0.82 . Always show your working!

❌ Common Errors

Using 50 cm³ (the final volume) instead of reading the volume at 60 seconds. Read the time specified in the question carefully.

✅ Model Answers

Between 0 and 20 seconds:

The volume of gas collected increases (or rises from 0 to 26 cm³ / reaction is fastest).

Between 80 and 100 seconds:

There is no change in the volume of gas collected (it stays constant at 50 cm³ / the reaction has stopped).

🧠 Reading Rate Curves

• A steep upward slope = gas volume is increasing rapidly.
• A flat horizontal plateau = volume stays constant because the limiting reactant has been completely used up.

❌ Common Errors

Stating that the volume "decreases" between 80 and 100 seconds. The rate has decreased to zero, but the total volume stays constant at 50 cm³!

✅ Correct Answer

☑ Systematic error

💡 Key Knowledge: Experimental Errors

• Systematic error: An error that causes readings to differ from the true value by a consistent amount each time (e.g. zero error on a balance).
• Random error: Unpredictable variations caused by human reaction time or slight environmental fluctuations.
• Zero error: A specific type of systematic error where an instrument gives a non-zero reading when the measured quantity is zero.

🧠 Exam Technique

The balance "always read 0.2 g before being used". Because it shifts every measurement in the same direction by the exact same amount (+0.2 g), it is definitely a systematic error.

📐 Step-by-Step Calculation

Dimensions of large cube: 2 mm × 2 mm × 2 mm

Step 1: Calculate the area of one face

Area of 1 face = 2 mm × 2 mm = 4 mm²

Step 2: Multiply by the number of faces (6)

Total surface area = 4 mm² × 6

Step 3: State the final answer

Total surface area = 24 mm²

🧠 Exam Technique

Cubes always have 6 identical square faces. Always structure your calculation clearly in two stages:

1. Area of single square face = length × width
2. Total surface area = single face area × 6

❌ Common Errors

• Calculating volume instead of surface area: 2 × 2 × 2 = 8 mm³ (scores 0).
• Multiplying by 4 sides instead of 6 faces (forgetting top and bottom faces): 4 × 4 = 16 mm².

✅ Correct Answer

The rate of reaction of the eight smaller cubes is faster.

💡 Collision Theory Link

• Smaller cubes have a greater surface area to volume ratio.
• More particles of calcium carbonate are exposed to the acid.
• This leads to a higher frequency of collisions between reacting particles.
• Therefore, the rate of reaction increases (faster).

🧠 Exam Tip

The question provides the word bank: faster , slower , the same . Choose only from the exact words provided in the box.