LIMITLESS Chemistry Exam
Topic 1.1.3: Heating Curves
Cambridge IGCSE Chemistry (0620) • Paper 4 Style
Focus: Graph Interpretation & Kinetic Particle Theory
Examiner's Brief
This exam tests your ability to read heating curve graphs and explain state changes using kinetic particle theory. Remember: the examiner wants specific scientific terms like "forces of attraction," "kinetic energy," and "vibrate/slide past/move randomly."
Section A: Graph Reading (5 Questions)
Total: 10 marks
The graph below shows the heating curve for a pure substance X.
(a) What is the melting point of substance X? [1 mark]
(b) What is the boiling point of substance X? [1 mark]
(c) Describe what is happening to the particles during section AB of the graph (the first flat line). [2 marks]
(d) How long does substance X remain at its melting point? [1 mark]
(Hint: Measure the horizontal distance of the first flat section)
(e) State the physical states of substance X at the following temperatures: [3 marks]
(i) 50°C: __________
(ii) 120°C: __________
(iii) 200°C: __________
Section B: Kinetic Particle Theory (5 Questions)
Total: 15 marks
Explain, using kinetic particle theory, why the temperature remains constant during melting even though the substance is still being heated. [3 marks]
Explain, using kinetic particle theory, why the temperature remains constant during boiling. [3 marks]
Compare the arrangement and movement of particles in the three states of matter shown on the heating curve. [6 marks]
SOLID
LIQUID
GAS
What happens to the kinetic energy of particles as the substance is heated in the sloping sections of the graph? [2 marks]
A student says: "During boiling, the particles gain enough energy to break apart into atoms." Explain why this statement is incorrect. [1 mark]
Section C: Purity & Heating Curves (5 Questions)
Total: 10 marks
Below are two heating curves: one for pure substance X and one for impure substance X (containing impurities). [2 marks]
Pure Substance X
Impure Substance X
State two differences between the heating curves of pure and impure substance X.
A student has two samples of ice: pure water ice and salty ice (ice with dissolved salt). Which sample will have a lower melting point? Explain your answer. [2 marks]
How could you use a heating curve experiment to determine if a substance is pure? [3 marks]
Pure water boils at exactly 100°C at standard atmospheric pressure. A student boils salty water and measures the boiling point as 102°C. What can you conclude? [2 marks]
A substance has a melting point range of 78-82°C. Is this substance pure? Explain your reasoning. [1 mark]
Exam Performance Summary
10
Section A Marks
15
Section B Marks
10
Section C Marks
Total: 35 marks
🎯 Examiner's Top Tips:
- • Always use the terms "forces of attraction" NOT "bonds" for state changes
- • Read graph values carefully - melting point is the FIRST flat line
- • Pure substances = sharp melting/boiling points
- • Impurities LOWER melting point but RAISE boiling point
- • During state changes: energy breaks forces, NOT increasing kinetic energy
Keep reading — this lesson marks itself done automatically.