Question

For any substance, why does temperature remain constant during change of state?

Answer 1

Because a condition of `"phase equilibrium"` applies during change of state.

Explanation:

For the thermodynamic processes......

`H_2O(s)rightleftharpoonsH_2O(l)` at `0` `""^@C` and....

`H_2O(l)rightleftharpoonsH_2O(g)` at `100` `""^@C` .....

.........these occur under equilibrium conditions. Energy has to be supplied to break the (strong) bonds between the molecules of the condensed phase; for water, this so-called `"latent heat of fusion"` or `"latent heat of vaporization"` are quite large and reflects the degree of INTERMOLECULAR force between the individual molecules (i.e. a consequence of hydrogen bonding).