`Delta H_("fusion")` of a substance is 'x' and `Delta H_("vap")` is 'y', then `Delta H_("sublimation")` will be

Options

A. x + y
B. x - y
C. x/y
D. y/x

Explanation

To find the enthalpy of sublimation (ΔH_sublimation) of a substance given the enthalpy of fusion (ΔH_fusion) as 'x' and the enthalpy of vaporization (ΔH_vap) as 'y', we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Definitions**:
- **Enthalpy of Fusion (ΔH_fusion)**: This is the change in enthalpy when one mole of a solid is converted into a liquid at its melting point. Given as 'x'.
- **Enthalpy of Vaporization (ΔH_vap)**: This is the change in enthalpy when one mole of a liquid is converted into a gas at its boiling point. Given as 'y'.
- **Enthalpy of Sublimation (ΔH_sublimation)**: This is the change in enthalpy when one mole of a solid is converted directly into a gas.

2. **Write the Equations**:
- From the definitions, we can express:
- ΔH_fusion = x (Equation 1)
- ΔH_vap = y (Equation 2)

3. **Combine the Processes**:
- Sublimation can be viewed as a two-step process:
1. Solid to Liquid (ΔH_fusion)
2. Liquid to Gas (ΔH_vap)
- Therefore, the enthalpy of sublimation can be expressed as:
\[ ΔH_{sublimation} = ΔH_{fusion} + ΔH_{vap} \]

4. **Substitute the Values**:
- Substitute the values from Equation 1 and Equation 2 into the equation for sublimation:
\[ ΔH_{sublimation} = x + y \]

5. **Final Result**:
- Thus, the enthalpy of sublimation is:
\[ ΔH_{sublimation} = x + y \]

### Conclusion:
The enthalpy of sublimation (ΔH_sublimation) is equal to the sum of the enthalpy of fusion (ΔH_fusion) and the enthalpy of vaporization (ΔH_vap):
\[ ΔH_{sublimation} = x + y \]