Question 1

Why does temperature remain constant during a pure substance's phase change?

Accepted Answer

During fusion or vaporisation, supplied heat is used to overcome intermolecular attractions and rearrange particles rather than increase their average kinetic energy. Consequently, the temperature stays constant at the relevant transition temperature while latent heat is absorbed.

Question 2

What is the difference between fusion and vaporisation?

Accepted Answer

Fusion is the conversion of a solid into a liquid, while vaporisation is the conversion of a liquid into vapour. Their reverse processes are freezing and condensation, respectively.

Question 3

How is latent heat calculated?

Accepted Answer

The heat involved in a change of state is calculated using Q = mL, where m is the mass of the substance and L is its specific latent heat. No temperature change is included because this heat is associated with the phase transition.

Question 4

Why is the enthalpy of vaporisation of water much larger than its enthalpy of fusion?

Accepted Answer

During vaporisation, particles must become widely separated and overcome many more intermolecular attractions than during melting. The absorbed enthalpy is therefore much larger; for water, the displayed values are about 6.01 and 40.7 kJ per mole.

Question 5

Why does carbon dioxide not form an ordinary liquid in the lab at 1 atm?

Accepted Answer

At 1 atm, carbon dioxide passes directly from the solid state to the vapour state by sublimation. Showing ordinary liquid carbon dioxide at this pressure would not represent its correct phase behaviour.

Phase Change Lab — Fusion, Vapourisation and Transition

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Practice