Simple molecular structures
3 marks · NormalHexane is a simple molecular compound. It melts at –95°C and boils at 69°C. Calculate the temperature range (in °C) over which hexane is a liquid and explain why this range exists for simple molecular substances.
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164 °C (±0)
Must show: subtract melting point from boiling point to get the liquid range; explanation of weak intermolecular forces
Also accept: Melting point to boiling point calculation; –95°C to 69°C is a liquid
Common mistakes: confusing melting/boiling points; incorrect subtraction
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Marking points
M1 · 1 mark · Correctly identify that the temperature range is boiling point minus melting point.
A1 · 1 mark · Correct subtraction: 69 – (–95).
B1 · 1 mark · Correct explanation: exists due to weak intermolecular forces; simple molecular substances can exist as liquids over a range because of low energy needed to overcome forces between molecules.
Worked solution
Temperature range = boiling point – melting point
So, hexane is a liquid between –95°C and 69°C, a range of 164°C. This wide range exists for simple molecular substances because the forces between molecules are weak, so they can easily change state over a large temperature range.