AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2023: Question 10

✅ Correct Answer

• Independent variable: Mass of ammonium nitrate (added) [1 mark]
• Dependent variable: Lowest temperature reached (by the solution) or change in temperature [1 mark]

💡 Key Knowledge

• Independent Variable: The variable that you deliberately change (the mass added: 4.0 g, 8.0 g, etc.).
• Dependent Variable: The variable that is measured for each change (the temperature).
• Control Variables: Factors kept constant to ensure a fair test (e.g., volume of water = 200 cm³).

🧠 Exam Technique

Look at the table headings! In standard AQA tables, the first column is almost always the independent variable, and the second column is the dependent variable.

❌ Common Errors

• Mixing up the two variables.
• Writing "temperature" without specifying "lowest temperature" or "temperature change".
• Giving control variables like "volume of water".

✅ Correct Answer & Description

• Points plotted:
  (4.0, 18.2) , (8.0, 16.2) , (12.0, 15.2) ,
  (16.0, 13.6) , (20.0, 12.4) , (24.0, 10.6)
• All 6 points plotted within ± ½ small square [2 marks]
  (4 or 5 points correct gives 1 mark).
• Line of Best Fit: A single, straight line drawn with a ruler passing evenly between all points [1 mark].

🧠 Graph Skills Checklist

• Use a sharp pencil and draw small neat crosses ( × ).
• Check axis scale: on the y-axis, 10 small squares = 2 °C, so 1 small square = 0.2 °C.
• On the x-axis, 10 small squares = 5 g, so 2 small squares = 1.0 g.
• Do not "dot-to-dot" the points; use a clear transparent ruler to balance points on either side.

✅ Correct Answer

• Line of best fit extended backwards to cross the y-axis (mass = 0 g) [1 mark].
• Correct temperature read from y-axis intercept: typically 19.6 °C to 20.0 °C (depending on candidate's drawn line, allowed within ± ½ small square) [1 mark].

💡 Why Extrapolate to 0 g?

When the mass of ammonium nitrate added is 0 g, no reaction has occurred. Therefore, the y-intercept represents the initial starting temperature of the pure water before any solute was added.

✅ Correct Answer

The temperature decreased (or temperature went down / solution got colder) [1 mark].

❌ Common Errors

• Giving a theoretical definition instead of referring to results: writing "energy was taken in from the surroundings" does not directly answer how the results show it. (The mark scheme states: ignore correct references to energy transfer).
• Vague statements like "the temperature changed" without stating that it dropped.

✅ What to Write for 2 Marks

• Mark 1: (± 0.3 °C) is the uncertainty.
• Mark 2: Because 0.3 °C is the range about the mean value (or values are a maximum of 0.3 °C either side of the mean / 16.8 = 16.5 + 0.3 and 16.2 = 16.5 − 0.3 ).

❌ Common Errors

• Calling ± 0.3 °C an "error" or "mistake" rather than the scientific term uncertainty.
• Stating it represents the range (0.6 °C is the range; 0.3 °C is half the range or the range around the mean).

✅ Correct Answer

Tick: Random error [1 mark]

💡 Definitions to Remember

• Random error: Causes readings to be spread unpredictably about the mean (e.g. slight timing differences, draughts, stirring variations). Repeating and calculating a mean reduces their effect.
• Systematic error: Causes readings to differ from the true value by a consistent amount each time (e.g. scale miscalibration, uninsulated cup losing heat constantly).
• Zero error: A specific systematic error where a measuring instrument gives a non-zero reading when the true value is zero (e.g. balance not tared).