# Why does the temperature of water stay steady at 80 degrees Celsius when heated?

A water is being heated in a glass beaker. Although the supply of energy is constant eventually the temperature becomes steady at 80 degrees Celsius. Why is that considering that phase change doesn't happen until 100 degrees Celsius?

$$\frac{dQ}{dT} = -k(T - T_0)$$
where $T$ is the temperature of the water and $T_0$ is the temperature of the environment.