This is by using the same mass and realizing that the specific heat of ordinary water and hot water is the same. In our procedure, 100 ml of hot water was mixed with 100 ml of plain water; therefore, the masses in equation 3 cancel (the densities of water at different temperatures are not exactly the same, but the difference is negligible). This leads to a change in temperature of the hot water equal to the negative change in temperature in the ordinary water, represented by: (4) Using equation 4, it can be deduced that the initial temperature of the hot water minus the temperature change of the mixture equals the temperature of the cold water plus the temperature change of the mixture (equation 5). This is then rearranged to say that the initial temperature of the hot water is equal to twice the temperature change plus the initial temperature of the regular water. This is shown in the equation 6.