Question

At what point the liquid changes into solid.
A. Liquefaction point
B. freezing point
C. boiling point
D. sublimation point

Answer 1

Hint: We know that a substance can be classified into three states depending upon the arrangement of its particles which are solid, liquid and gas. The state of a substance can be changed by bringing a change in temperature, pressure and volume of the substance.

Complete answer:
Phase transformation is the most common equilibrium involving physical processes. Since in the question we are asked about the point when liquid changes into solid therefore we will be talking about solid - liquid equilibrium. Let us explain this situation with an example: suppose we take some ice cubes along with water at 273K and normal atmospheric pressure is placed in a thermos flask so that no heat can enter or leave the system, the mass of ice and water is found to remain constant .
However the two opposing processes going on at equilibrium are melting of ice and freezing of water.
The equilibrium rate of melting of ice is equal to the rate of freezing of water. But in the question the process of phase transformation is not under equilibrium. The phase of liquid is transformed to the solid phase, therefore the point at which this phase transformation occurs is freezing point.

Hence the correct option is B.

Note: The temperature at which the solid and liquid form of a pure substance are in equilibrium at the atmospheric pressure is called the normal freezing point or melting point of that substance. Freezing point is usually raised as and when the pressure increases.