Test: Properties of Pure Substances - 2 - Mechanical Engineering MCQ

Correct Answer :- A

Explanation : A substance expands on freezing only if the slope of the fusion line on the pressure temperature chart is negative.

From Gibb's phase rule
C + 2 = P + F
For pure substance C= 1
Phases present, P - 3
1 + 2 = 3 + F
F = 0

For Moiller diagram, condensation takes place at constant pressure which is along vertically inclined.

Example of pure substance:
1. Atmospheric air
2. Steam water mixture
3. Combustion products of a fuel

The critical point of a pure substance has unique characteristics:

• At the critical point, there is no constant temperature vaporization process. This means the substance cannot be separated into distinct liquid and gas phases.
• The critical point is a point of inflection on the phase diagram, where the slope is zero, indicating no change in phase.
• At the critical point, the saturated liquid and saturated vapor states become identical, meaning they have the same properties.

An incorrect statement about the critical point is:

• The substance directly converts from solid to vapor, which is not a characteristic of the critical point.