# Latent Heat III

**Definition of Latent Heat** – The heat-energy released or absorbed by a substance to change its phase by keeping temperature constant is known as Latent Ht. Latent HT equation is given as:

**Q = m ****∆H**

Here Q is the amount of Ht required to change mass m of a substance into another phase. ∆H is specific latent heat of the substance which is the amount of Ht required to change phase of 1 gram of a substance. Unit of Latent ht is joule and of specific latent Ht is J / gm.

For fusion (melting or freezing): **Q = m ****∆H _{fusion}**

∆H_{fusion }is latent Ht of fusion.

For vaporization (for condensation or evaporation): **Q = m ****∆H _{vaporization}**

∆H_{vaporization }is specific latent ht of vaporization.

**Latent Heat of Melting** is the amount of Ht energy absorbed by a solid substance to change into liquid phase without any change in temperature. Latent Ht of melting is equal to latent Ht of fusion. Both are one and the same thing. Don’t get confused with them. Latent ht of fusion is both latent ht of melting and latent ht of freezing. Difference in them is that while melting Latent ht of fusion is absorbed and while freezing latent ht of fusion is released.

**Latent Heat of Fusion Equation** as given above is:

**Q = m ****∆H _{fusion}**

Where Q is the Ht required to change phase of mass m of a substance from liquid to solid or from solid to liquid with constant temperature. Q is actually latent Ht for mass m. and ∆H _{fusion }is latent Ht for 1 gram of the substance known as specific latent Ht. By giving Ht energy equal to** Latent Heat of Fusion Ice** at 0 ^{o}C can be changed into water at 0 ^{o}C and water at 0 ^{o}C can be changed into ice at 0 ^{o}C.

Value of **Latent Heat of Fusion for Water** is 334 joules / gram. This means that 334 joules of energy is released by 1 gram water at at 0 ^{o}C to change into ice at 0 ^{o}C. Also 334 joules of energy is absorbed by 1 gram ice at 0 ^{o}C to change into water at 0 ^{o}C.

We know that sublimation is the process of conversion of solid into gas directly. The ht. absorbed by solid to change into gas directly without changing its temperature is known as **Latent Heat of Sublimation** and for changing 1 gram of solid to gas at constant temperature ht absorbed is called specific Latent Ht of sublimation.