|Is water, at say 35° F, more dense than water at 80° F?
Yes, water at 35° F is more dense than water at 80° F, so that the hotter water will float on the colder water. But water is special (and almost unique) in that it does expand slightly as it cools to its freezing temperature. Water's density reaches a maximum at 3.98° C (about 39° F) and then actually becomes less dense as you cool it toward 0° C (32° F). That means that 33° F water will float on 39° F water! This bizarre behavior allows ice and very cold water to float above slightly warmer water and keeps ponds from freezing solid. Without it, many animals would perish during the winter.