Phase Diagram and Heat

[Susanna Givan 2B]

Why do we have to separate heat released, for example, water to steam, as several different steps?

[Faaizah Arshad 1H]

When you are calculating the heat it takes to change a substance from one phase to another phase, you need to separate it into different steps because of differences in specific heat capacity and due to the presence of phase changes.

For example, if you are heating solid ice from -5 C to + 150 C, you should recognize that the ice is changing from a solid at -5 C to 0 C. Then it will undergo a phase change that involves delta H fusion. When it finishes melting, it will be heated from 0 C to 100 C as a liquid. Then it changes from a liquid to gas that involves delta H vaporization. Once it turns into a gas, it will be heated from 100 C to 150 C. In each of your calculations, you need to consider the fact that water in a solid, liquid, and gas phase has different specific heat capacities and that the delta H fusion and vaporization are involved.

[Shrey Pawar 2A]

The reasoning there are seperate steps is that when a phase change occurs, there is no change in the temperature. On the phase graph you can see it is a straight line. This is where we would use of enthalpy of fusion or vaporization in order to calculate the heat. However, when you are heating the molecules to a certain temperature in order to create a phase change you would use q=M/NC(T2-T1). Hope this helps!