Question

What are the three phase changes that require energy to perform?

Answer 1

Hint: Phases are the different types in which any substance is present in different physical forms. The phase changes may be from solid to liquid, liquid to gas, and gas to solid or solid to gas. These all processes have specific names and specific heats that are required for their formation. This heat is the energy to perform the changes.

Complete answer:
The change in phase is the transformation of the physical states of a substance. The three phases of any substance are solid, liquid, and gas. The phase transformation happens in between these phases. The substance requires energy when it changes to a higher energy phase, while it absorbs energy when converted from a higher to lower energy phase. The increasing order of energies of different states is, solid < liquid < gas. This means gases have a higher energy than liquid followed by solids.
The transformation of phase happens when solid changes to liquid, liquid to gas, and solid to gas. They change from the lower energy phases to the higher. The phase changes occur with change in enthalpy, the enthalpies for change from solid to liquid, liquid to gas, and solid to gas are:
-Enthalpy of fusion $\mathrm{\Delta }\underset{fus}{\stackrel{o}{H}}$ is the enthalpy, which is used to convert 1 mole of solid into liquid at its melting point. It is endothermic as energy is needed. Example,$H_{2}O(s)\to H_{2}O(\ell )$, $\mathrm{\Delta }\underset{fus}{\stackrel{o}{H}}$= 6 kJ/mol
-Enthalpy of vaporization$\mathrm{\Delta }\underset{vap}{\stackrel{o}{H}}$is used to convert 1 mole of liquid into gas at its boiling point. It is also endothermic. Example, $H_{2}O(\ell )\to H_{2}O(g)$$\mathrm{\Delta }\underset{vap}{\stackrel{o}{H}}$= 40.79 kJ/mol.
-Enthalpy of sublimation$\mathrm{\Delta }\underset{sub}{\stackrel{o}{H}}$involves conversion of 1 mole of solid into gas at its transition temperature. It is also endothermic. Example, $I_2(s)\to I_2(g)$$\mathrm{\Delta }\underset{sub}{\stackrel{o}{H}}$= 62.39 kJ/mol.
Hence, the three phase changes that require energy to perform are fusion, vaporization and sublimation.

Note:
The example of fusion is ice melting to form water; vaporization is water forming water vapors, while that of sublimation is iodine in solid converting to gas. Also some solids that sublime are naphthalene, camphor and ammonium chloride. The conversion of gas to a solid is called deposition which is a climatic phenomena.