What is the amount of heat released when 25g of water cools 12.5 degrees c ?

Water can't cool at a single temperature. It must start at a higher temperature, and drop to a lower temperature in order to cool. Unless we know the other temperature, there is no way to calculate the amount of thermal energy released.

In any atom of any element, you will find protons and neutrons in the nucleus of any atom 

Not a  mixture of two common elements that can be separated by physical means.