Cambridge IGCSE Chemistry 0620
Paper 4: Theory (Extended)
Topic:
2.5.3 Simple Molecular Properties
Focus:
Weak Intermolecular Forces
Time:
45 minutes
Total Marks:
40 marks
⚠️ EXAMINER TRAP ALERT
The #1 mistake students make:
"Covalent bonds are weak" ❌ WRONG!
Covalent BONDS are STRONG. The INTERMOLECULAR FORCES are weak.
Always specify: "weak intermolecular forces BETWEEN molecules" not "weak bonds"
Section A: True or False
5 questions × 1 mark = 5 marks
Covalent compounds conduct electricity when solid.
Covalent bonds are weak, which is why simple molecular substances have low melting points.
Water has a higher boiling point than methane because water molecules are heavier.
Simple molecular substances are usually gases or liquids at room temperature.
Breaking the O-H bonds in water requires less energy than overcoming the intermolecular forces between water molecules.
Section B: Explanation Questions
5 questions × 3 marks = 15 marks
Methane (CH₄) has a boiling point of -164°C.
Explain why methane has such a low boiling point.
[3 marks]
The table shows boiling points of some simple molecules:
| Molecule | Formula | Boiling Point (°C) |
|---|---|---|
| Oxygen | O₂ | -183 |
| Nitrogen | N₂ | -196 |
| Chlorine | Cl₂ | -34 |
Explain why chlorine has a higher boiling point than oxygen and nitrogen.
[3 marks]
Iodine (I₂) is a simple molecular substance that is solid at room temperature.
Explain why iodine is solid at room temperature, while most simple molecular substances are gases or liquids.
[3 marks]
A student heats some solid iodine. The iodine changes directly from solid to purple vapor without melting.
Explain, in terms of particles and energy, what happens during this process called sublimation.
[3 marks]
Ice melts at 0°C to form water. Water boils at 100°C to form steam.
Compare the energy changes involved in melting ice and boiling water. Explain why boiling requires more energy than melting.
[3 marks]
Section C: Electrical Conductivity
5 questions × 4 marks = 20 marks
Explain why liquid oxygen does not conduct electricity.
[2 marks]
A student tests the electrical conductivity of different substances:
| Substance | Conducts? |
|---|---|
| Sugar solution | No |
| Salt solution | Yes |
| Pure water | No (very poor) |
Explain why sugar solution does not conduct electricity, but salt solution does.
[4 marks]
Ammonia (NH₃) is a simple molecular substance. Predict whether solid ammonia would conduct electricity, and explain your answer.
[3 marks]
Carbon dioxide is a simple molecular substance with the formula CO₂.
Graphite is a form of carbon that conducts electricity.
Explain why graphite conducts electricity but carbon dioxide does not.
[4 marks]
A student investigates whether different liquids conduct electricity. They test:
- Ethanol (C₂H₅OH)
- Hexane (C₆H₁₄)
- Liquid sulfur dioxide (SO₂)
Predict the results of this investigation and explain your reasoning.
[4 marks]
🎯 Key Exam Strategies
The #1 Mistake to AVOID
Never say "covalent bonds are weak"
Always specify: "weak intermolecular forces BETWEEN molecules"
Conductivity Rule
Simple molecular → NEVER conducts
Reason: Neutral molecules, no charged particles
Boiling Point Pattern
Larger molecules → More electrons → Stronger forces
Result: Higher boiling/melting points
Energy Language
Breaking bonds → Chemical change
Overcoming forces → Physical change (melting/boiling)
Cambridge IGCSE Chemistry (0620) • Topic 2.5.3: Simple Molecular Properties
Practice makes perfect - Master the terminology, ace the exam!
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