1. The problem statement, all variables and given/known data If mass-energy equivalence is taken into account, when water is cooled to form ice, the mass of water should increase, decrease or remain unchanged? 2. Relevant equations E = mc2 3. The attempt at a solution Since the internal energy of ice is less than water, it's mass should also be less. This might be a completely wrong interpretation though because the answer says increase. I haven't really studied mass energy equivalence in much detail, I just know the basics from nuclear physics.