Given two samples of water each weighing 100 g one is at a temperature of 50.0 degrees Celsius, the other at 25.0 degrees Celsius. When mixed together what is the final temperature?

Answer. This is a little more interesting problem. We know the sample of water at the higher temperature will lose heat (We may not have said that, but we know the temperature of the sample of water initially at the higher temperature will fall and the temperature of the sample at the lower temperature will rise.) and the sample of water at the lower temperature gain heat. Since we are mixing the two sample the heat lost by the sample at the higher temperature will be absorbed by the sample at the lower temperature.