AQA GCSE Chemistry Chemistry Paper 1 (Foundation), November 2020: Question 5

✅ Correct Matches

• Conducts electricity ➔ Aluminium has delocalised electrons
• High melting point ➔ Aluminium has strong metallic bonds

💡 Key Knowledge

• Electrical conduction: Requires mobile charge carriers. In metals, these are free-moving delocalised electrons.
• High melting point: Giant metallic lattices require huge amounts of energy to overcome the strong electrostatic attraction between positive metal ions and sea of delocalised electrons (metallic bonds).

❌ Common Errors

• Connecting "high melting point" to intermolecular forces — metals do NOT have molecules!
• Drawing two lines from one property box — doing so negates the mark.

✅ Correct Answer

A mixture of metals (or a mixture of a metal with other elements / carbon).

🧠 Exam Technique

Always specify mixture rather than "combination" or "compound". An alloy is physically blended, not chemically bonded together in fixed proportions.

❌ Common Errors

Calling an alloy a "compound of metals" or "bonded metals". These lose the mark entirely.

✅ Correct Answer

Bauxite contains a variable percentage/proportion of aluminium (or does not have a fixed composition / fixed percentage).

💡 Key Knowledge

• A compound always has a fixed, definite composition by mass (e.g. Al₂O₃ is always 53% Al).
• A mixture has a variable composition depending on the sample (e.g. bauxite is 15% to 25% Al).

❌ Common Errors

Just stating numbers without comparing them, such as writing "because 15% is less than 53%". You must explain that the proportion varies.

✅ Any TWO of the following:

• Danger of dam bursting / lake overflowing
• Leakage of toxic substances from mud into the environment
• Water pollution / contaminating water supplies
• Damage to wildlife habitats
• Visual pollution / eyesore
• Dam blocks light to surrounding houses
• Reduces property/house values
• Unpleasant smell

🧠 Exam Technique

Look closely at the diagram provided (Figure 6). Notice the homes situated directly below the dam wall! This points directly to the threat of structural failure (dam collapse) and local amenity impact.

❌ Common Errors

Vague answers like "it causes pollution" or "bad for the environment" without naming a specific risk like water contamination or habitat loss.

✅ 05.5: Total Number of Atoms in Na₃AlF₆

10

3 (Na) + 1 (Al) + 6 (F) = 10 atoms

✅ 05.6: Reason for Adding Cryolite

☑ To lower the melting point of the mixture

💡 Key Knowledge

Pure aluminium oxide (Al₂O₃) melts at over 2000 °C. Dissolving it in molten cryolite reduces the operating temperature to around 950 °C, massively saving energy and industrial running costs.

❌ Common Errors

• In 05.5: Counting 'Al' as two atoms (A and l) instead of one aluminium atom.
• In 05.6: Confusing lower melting point with "acts as a catalyst" or "increases the amount produced".

✅ Correct Sentence

When the molten aluminium oxide and cryolite mixture is electrolysed the product at the positive electrode is oxygen .

This product reacts with the positive electrode because the positive electrode is made of carbon .

💡 Key Knowledge

• At the anode (+): Oxide ions lose electrons to form oxygen gas: 2O²⁻ ➔ O₂ + 4e⁻
• Electrode corrosion: Anodes are made of graphite (carbon). At high temperatures, carbon reacts with oxygen to form CO₂: C + O₂ ➔ CO₂
• Because of this, the carbon anodes constantly burn away and must be regularly replaced.

🧠 Exam Technique

Negative ions (anions, O²⁻) migrate to the positive electrode (anode). Remember: PANIC (Positive Anode, Negative Is Cathode).

📐 Step-by-Step Calculation

1. Identify percentage & mass:
   Percentage = 25%
   Mass of bauxite = 300 000 kg
2. Calculate mass of aluminium:
   (25 / 100) × 300 000 = 75 000 kg
   (Awards first 2 marks)
3. Convert to standard form:
   Move decimal 4 places to the left:
   75 000 = 7.5 × 10⁴ kg
   (Awards 3rd mark)

🧠 Standard Form Rule

A number in standard form is written as A × 10ⁿ, where 1 ≤ A < 10 and n is an integer.

Writing 75 × 10³ is mathematically equal, but is not valid standard form and will lose the final mark!

❌ Common Errors

• Stopping at 75 000 and forgetting to convert to standard form.
• Writing incorrect power of ten: e.g. 7.5 × 10⁵ or 7.5 × 10⁻⁴ .
• Error carried forward (ECF): If you miscalculate step 2, you can still gain mark 3 for converting your incorrect value correctly into standard form.