Draw a cooling graph of water as it passes from 20°c to - 20°c

The graph will begin at 20 °C and have temperature on the y-axis and time on the x-axis. The line will be a downwards sloping one, until the value of temperature of the graph reaches 0 °C. There, it will become horizontal for a small amount of time, and then continue to slope downwards until the graph reaches - 20 °C.

The horizontal region is where the freezing of water will take place, and there is no temperature change that occurs when a state change is taking place. This is because the energy involved then is latent energy.