Question

Define specific latent heat of fusion, ${L_f}$.
A.) The specific heat of fusion of a substance is the quantity of heat required to convert a unit mass of the substance from liquid to gaseous state at its boiling point.
B.) The specific heat of fusion of a substance is the quantity of heat required to convert a unit mass of the substance from solid to liquid state at its melting point.
C.) The specific heat of fusion of a substance is the quantity of heat required to raise the temperature of unit mass of the substance by 1$^\circ C$.
D.) The specific heat of fusion of a substance is the quantity of heat required to lower the temperature of unit mass of the substance by 1$^\circ C$.

Answer 1

Hint: When a substance changes its state, some amount of energy is released or absorbed during this process. Fusion is defined as the change of state of material from liquid to solid or vice versa.

Complete step by step solution:
Latent heat of a substance is defined as the amount of energy released or absorbed when the substance changes its state from one to another. This energy is hidden and is either extracted or used during change of state.

Specific latent heat is the amount of heat required to change the state of a substance of unit mass. It is denoted by L and formula for specific heat is

$L = \dfrac{Q}{m}$

where Q is the heat required to change the state and m is the mass of the sample.

We can also calculate latent heat from specific latent heat and the mass of the sample as follows:

$Q = mL$

The change of state from liquid to gas is called vaporization and change from liquid to solid is called fusion. Since latent heat is concerned with the change of state, we can define latent heat for fusion and vaporization separately.

Specific latent heat of fusion is defined as the quantity of heat required to convert a unit mass of the substance from solid to liquid state at its melting point and the specific latent heat of vaporization is defined as the quantity of heat required to convert a unit mass of the substance from liquid to gaseous state at its boiling point.
The correct answer is (B).

Note: Change of state takes place at a constant temperature. All the heat or energy being supplied to the substance during this time is used to change the state of the matter.