AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2021: Question 3

✅ Correct Answer

• Limestone [1 mark]
• Sodium carbonate [1 mark]

💡 Key Knowledge

Most everyday glass (windows and bottles) is soda-lime glass. It is made by heating a mixture of:

• Sand (silicon dioxide, SiO₂)
• Sodium carbonate (soda)
• Limestone (calcium carbonate, CaCO₃)

❌ Common Errors

• Selecting Clay — clay is heated to make ceramics/pottery and bricks, not glass.
• Confusing Sodium carbonate with Sodium hydroxide.

🧠 Exam Technique

Pay close attention to the number of boxes: the question explicitly states "Tick (✓) two boxes". Ticking more or fewer will cost marks.

✅ Correct Answers

03.2: Poly(chloroethene) advantage:

• Advantage: Stronger [1 mark]
• Reason: Less easily damaged / resistant to damage [1 mark]

03.3: HDPE advantage:

• Advantage: Lower density [1 mark]
• Reason: Lighter (to install) / easier to install [1 mark]

🧠 Exam Technique: Use the Prompt!

The question prompt gave two essential clues right above the table:

• "resistant to damage" → links to strength (poly(chloroethene) has a relative strength of 72 vs 25).
• "easy to install" → links to density (HDPE density is 0.92 g/cm³ vs 1.4 g/cm³; lower density means lighter parts).

✅ Correct Answer

Tick the 2nd box:

💡 Key Knowledge: Addition Polymerisation

• Addition polymers form when alkene monomers open their double bonds to form a long single-bonded chain.
• To find the monomer from a repeating unit:
  1. Replace the central single C–C bond with a double C=C bond.
  2. Remove the side-extending single bonds and the brackets/ n .
  3. Keep all surrounding atoms ( H , Cl ) on the exact same carbons.

✅ Correct Answer

• Test: (Add damp) litmus paper [1 mark]
• Result: (Litmus paper is) bleached OR turns white [1 mark]

❌ Common Errors & Examiner Commentary

• "Turns red": The mark scheme specifically states ignore (litmus paper) turns red . Chlorine forms an acidic solution when dissolved so it briefly turns damp blue litmus red, but the defining result for chlorine is that it bleaches it white!
• Missing "damp": Always specify damp litmus paper. Dry litmus paper does not react quickly because the gas must dissolve in moisture to act as a bleach.
• Confusing gas tests: Do not confuse with oxygen (relights a glowing splint) or hydrogen (squeaky pop).

✅ Correct Answer

• Advantage of polymers: Last a long time / unreactive / break down slowly / do not rot [1 mark]
• Advantage of wood: Renewable / sustainable / trees can be replanted / aesthetic reasons [1 mark]

🧠 Exam Technique: Steer Clear of "Cost"

The mark scheme strictly notes: ignore references to cost .

Saying "wood is cheaper" or "polymers are cheaper" will earn zero marks. Always base your answers on the bullet points provided in the question prompt:

• "Polymers are unreactive / wood breaks down in wet conditions" → polymers last longer / won't rot.
• "Wood is produced from trees / polymers from crude oil" → wood is renewable; crude oil is finite.

📐 Step-by-Step Calculation

✅ Mark Scheme Breakdown

• Working mark: (5.94 / 6.00) × 100 [1 mark]
• Answer mark: 99 (%) [1 mark]

An answer of 99 with no working scores both marks, but always show working to safeguard against arithmetic slips!

✅ Correct Answer

The alloy is harder (than pure aluminium) OR stronger (than pure aluminium) [1 mark].

💡 Key Knowledge: Why Alloys are Harder

• Pure metals: Atoms are all the same size and arranged in regular layers. These layers can easily slide over each other, making pure metals relatively soft and malleable.
• Alloys: Different elements have different-sized atoms. Adding them distorts the regular layered structure, making it harder for layers to slide over each other.