How Everything Works
How Everything Works How Everything Works

Question 1205

If a given volume of water is placed in a container and frozen, will it weigh more, less, or remain the same relative to when it was in a liquid state? — RL, Denver, Colorado
Freezing water has virtually no effect on its weight—as long as the same number of water molecules remain in the container, the overall weight of the container and water/ice won't change significantly. But water does expand as it freezes, so the container will become more full as the ice forms. Water's expansion upon freezing makes ice less dense—less mass per volume—than liquid water. This decrease in density explains why ice floats on water and why pipes often break as the water inside them freezes.

However, you'll notice that I said "freezing the water has virtually no effect on its weight." In reality, the water does lose a tiny fraction of its weight. That's because to freeze the water, you must remove some of the water's energy. As Einstein pointed out with his famous formula E=mc2, energy and mass are related to one another and since mass acquires weight when it's near the earth, so does energy. Because the thermal energy in liquid water has a tiny weight, when you remove some of this thermal energy from the water, the water loses some of its weight. But don't expect to measure this weight loss with a common scale—the weight change is on the order of one part in a trillion, a factor that's presently beyond the precision of even the most advanced research measuring devices.


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