In general, density can be changed by changing either the pressure or the temperature. Increasing the pressure always increases the density of a material. Increasing the temperature usually lowers the density, but there are exceptions. For example, the density of water increases slightly between its melting point at 0 °C and 4 °C. When water freezes, it expands by about 9% in volume, making ice that is less dense than liquid water. Water expands as it drops below 4 °C.