With reference to the thermal properties of substances, consider the following statements:
- (i) The temperature at which the solid and the liquid states of a substance are in thermal equilibrium with each other is called its melting point.
- (ii) The temperature at which the liquid and the vapour states of a substance coexist is called its boiling point.
- (iii) The change of state from solid to liquid is called melting, and from liquid to solid is called fusion.
Which of the above statements are correct?
- AOnly (i) and (iii)
- BOnly (ii) and (iii)
- COnly (i) and (ii)
- DAll three
Show answer and explanation
Correct answer: (c) Only (i) and (ii)
Explanation
- (i) is correct: at the melting point, the solid and the liquid are in equilibrium.
- (ii) is correct: at the boiling point, the liquid and its vapour exist together; the vapour pressure equals the outside pressure.
- (iii) is wrong: the change from solid to liquid is called melting or fusion. The change from liquid to solid is freezing (solidification), not fusion.
So only (i) and (ii) are correct.