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

✅ Correct Answers

• Mass of evaporating dish: (mass) balance [allow: scales] (1)
• Volume of sea water: measuring cylinder [allow: burette / pipette] (1)

❌ Common Errors

• Writing beaker for measuring volume. Beakers have approximate scale markings and are never accepted as measuring instruments.
• Writing weighing machine instead of balance.

📐 Step-by-Step Calculation

1. Step 1: Calculate mass of salt in 50 cm³
   30.49 g - 30.44 g = 0.05 g (1 mark)
2. Step 2: Find scaling factor or volume ratio
   1000 cm³ ÷ 50 cm³ = 20 (1 mark)
3. Step 3: Multiply to find total mass in 1000 cm³
   0.05 g × 20 = 1.0 g (1 mark)
   (Alternative setup: (1000 ÷ 50) × 0.05 = 1.0 g)

❌ Calculation Traps & Tips

• Inverting the ratio: Dividing 50 by 1000 instead of 1000 by 50. The sample is 50 cm³, so 1000 cm³ will contain 20 times more salt.
• Error-Carried-Forward (ECF): If you miscalculate the initial subtraction (e.g. 0.04 g), you can still get the next 2 marks if you correctly multiply your value by 20.

✅ Correct Ticks

• ☑ Heat the evaporating dish and salt again. (1)
• ☑ Measure the mass of the evaporating dish and salt again. (1)

💡 Key Knowledge: "Constant Mass"

To prove any solid is completely dry, you must:

1. Reheat the sample.
2. Reweigh the sample.
3. Repeat until two consecutive mass readings are identical (no more mass lost as steam).

✅ Mark Scheme Requirements

• Point 1 (Escape/Condensation): (More) steam / water vapour escapes OR less steam condenses. (1)
• Point 2 (Cooling efficiency): Because cooling / condensing is less efficient (slower) OR a condenser (Liebig condenser) is not used / cold water is not used for cooling. (1)

🧠 Exam Technique: Two-Sided Contrast

Notice the 2 marks correspond to effect and cause:

• Effect: Steam fails to turn back into liquid and escapes into the air.
• Cause: A delivery tube only has surrounding room air to cool it, whereas a Liebig condenser surrounds the tube with a jacket of circulating cold water.

✅ Accepted Answers

• (Distilled) water is pure. (1)
• OR Microbes / bacteria / pathogens are destroyed / killed by boiling/distillation. (1)

❌ Common Errors

• Writing "it has no salt" — this explains why it is not salty, but sterilisation is about biological microbes, not dissolved salts.
• Writing "it is clean" — too vague. GCSE examiners look for the words pure or microbes/bacteria killed.

✅ Correct Ticks

• ☑ Using chlorine (1)
• ☑ Using ozone (1)

💡 Key Knowledge: Making Potable Water

Treating fresh groundwater involves two essential stages:

• 1. Filtration: Passes through filter beds (sand/gravel) to remove insoluble solids. (Filtration does not kill bacteria!)
• 2. Sterilisation: Kills harmful microbes using chlorine, ozone, or ultraviolet (UV) light.

✅ Correct Answer

pH = 7 (1 mark)

💡 Universal Indicator Colour Scale

• Red / Orange / Yellow: Acidic (pH 1 – 6)
• Green: Neutral (pH 7)
• Blue / Purple: Alkaline (pH 8 – 14)