AQA GCSE Chemistry Chemistry Paper 1 (Foundation), November 2020: Question 6

✅ Correct Answer

Complete the missing subscripts:

C₆H₈O₇

Fill in: H₈ and O₇.

🧠 Exam Technique: Systematic Counting

• Carbon (C): 6 already given in the prompt.
• Hydrogen (H): Count around the structure methodically from left to right:
  Left group: 1 (on O) + 2 (on CH₂) = 3
  Middle group: 1 (on OH) + 1 (on COOH) = 2
  Right group: 2 (on CH₂) + 1 (on O) = 3
  Total = 3 + 2 + 3 = 8.
• Oxygen (O): 2 (left carboxyl) + 1 (central hydroxyl) + 2 (central carboxyl) + 2 (right carboxyl) = 7.

✅ Correct Answer

Tick the box next to:

[✓] Covalent

💡 Key Knowledge

Citric acid consists entirely of non-metal atoms (carbon, hydrogen, and oxygen). When non-metal atoms bond, they share pairs of electrons, forming covalent bonds. The solid lines in Figure 7 directly represent shared electron pairs (single and double covalent bonds).

✅ Correct Answer (Any two of the following)

• Shows which atoms are which element (uses chemical element symbols: C, H, O).
• Shows single and double bonds clearly (shows double bonds between C and O as =).

💡 Key Knowledge: Comparing Models

• Representation A (Displayed formula): Clear 2D representation; explicitly shows element symbols (C, H, O) and bond order (single vs double bonds), but does not show 3D shape or bond angles.
• Representation B (Ball-and-stick): Shows 3D arrangement and spatial orientation, but atoms are only shaded spheres without chemical labels, and double bonds look like single sticks.

✅ Correct Answer

The temperature decreases (the solution gets colder / temperature drops as citric acid is added).

💡 Key Knowledge

• Endothermic reaction: Takes in thermal energy from the surroundings, resulting in a fall in temperature of the reaction mixture.
• Exothermic reaction: Releases thermal energy to the surroundings, causing a rise in temperature.

✅ Graph Requirements

1. Plotting 6 points from Table 4:
   (0.00, 22.6), (0.25, 22.2), (0.50, 21.8), (0.75, 21.4), (1.00, 21.0), (1.25, 20.6)
   [2 marks for all 6 correct to within ± ½ small square; 1 mark for 4 or 5 points]
2. Line of best fit: Draw a straight line using a ruler that passes through all plotted points [1 mark].
3. Extrapolation: Extend this straight line downwards until it intersects the second line already drawn on Figure 9 [1 mark].

🧠 Visualising the Graph

What your completed graph must look like:

• On the y-axis, 1 large square = 1.0 °C (so 1 small square = 0.1 °C).
• On the x-axis, 1 large square = 0.50 g (so 1 small square = 0.05 g).
• Plot small neat crosses ( × ).
• Use a sharp pencil and a ruler to draw a continuous straight line going down from (0.00, 22.6) down through (1.25, 20.6), continuing until it meets the printed line around (1.50, 20.2).

📐 Step-by-Step Calculation

1. Step 1: Read the initial temperature (at 0.00 g)
   Initial temperature = 22.6 °C
2. Step 2: Read the minimum temperature from the graph intersection
   Where the two lines cross = 20.2 °C
3. Step 3: Calculate the difference
   Overall temperature change = 22.6 – 20.2 = 2.4 °C

🧠 Exam Technique & ECF

• Error Carried Forward (ECF): The mark scheme allows ECF from your own graph in 06.5. Always show your working clearly!
• Sign convention: The question asks for the temperature change, so you can write either 2.4 °C or –2.4 °C (the examiner ignores the sign).
• If you made an error reading the intersection point but wrote 2.2 °C (e.g. from the printed point at 1.50 g: 22.6 – 20.4), you still gain 1 compensation mark.

✅ Correct Answer

Tick the box next to:

[✓] Temperature of solution

💡 Key Knowledge: Experimental Variables

• Independent variable: What you change deliberately (here, the mass of citric acid added).
• Dependent variable: What you measure as the outcome (here, the temperature of the solution).
• Control variable: What you keep constant to ensure a valid test (here, the volume of sodium hydrogencarbonate solution, 25 cm³).