The density of cold water at 10 degrees Celsius is 999.7 kg/m3; the density of hot water at 90 degrees Celsius is 965.3 kg/m3.

The temperature range of water is between 0 degrees Celsius and 100 degrees Celsius at one atmosphere of pressure.

Between 4 degrees Celsius and 100 degrees Celsius, cold water is more dense than hot water. This is mainly because the higher the temperature, the greater the distance between the molecules, i.e., the lower the density.

Between 0 degrees Celsius and 4 degrees Celsius, cold water is less dense than hot water, and the lower the temperature, the less dense. The reason for this is that between 0 and 4 degrees Celsius, the microstructure of water gradually converges to ice to the point where it becomes less dense. 4 degrees Celsius is when water is at its densest.

The law of change of density

Generally speaking, no matter what substance, and no matter what state it is in, with the change of temperature and pressure, the volume or density will also change accordingly. Contact temperature T, pressure F and density ρ (or volume) of the three physical quantities of the relationship is called the equation of state. The volume of a gas varies significantly with the pressure to which it is subjected and the temperature at which it is exposed.

If its temperature is constant, the density is proportional to the pressure; if its pressure is constant, the density is inversely proportional to the temperature. For general gases, if the density is not large, the temperature is far away from the liquefaction point, the change in volume with pressure is close to the ideal gas; for high density gases, the above equation of state should also be modified appropriately.