Why Does Cold Water Weigh More Than Hot Water?

Cold water weighs more than hot water.

One liter of hot water weighs less than one liter of cold water. Almost all liquids become heavier when they become colder. This is because, the colder the water, the more dense it is.

Density

When two liquids have the same volume, the weight of the liquids will be determined by density. Density is measured by mass divided by volume, and on Earth, mass equals weight (weight is the force of gravity pulling on a mass--Earth's gravitational force is constant). In water's case, weight is a measurement of how many H2O molecules are present in a given volume. So if you take two, 1-liter bottles and fill one with hot water, and the other with cold, the cold water bottle will be noticeably heavier. In the hot water, the molecules are spaced farther apart, so there are fewer molecules in any given volume, and therefore less mass.

Energy

As the energy in liquid water increases (as its temperature increases) the molecules vibrate at a greater rate, and need to space farther apart from each other. This is what density refers to, and this is why the hotter the water, the lighter it is.

Conducting an Experiment

There is an experiment that can be performed to prove these concepts empirically. First, take two identical 1-liter (or 2-liter) bottles and place them on a weight balance. Zero the scale so that the two water bottles balance each other. Then take cold water (you can put it in a refrigerator to cool it) and fill one water bottle with it. Make sure the water is already cold when you pour it into the bottle. Then take hot water (from a tap or stove top) and fill the other bottle with it. Make sure the water is entirely liquid in both bottles (not ice or steam) and make sure the bottles are filled up entirely, so that the volume of water is the same. Then place the bottles back on the weight balance and the cold water will dip down, proving that it is heavier than the hot water.

Misconceptions

Despite cold water being heavier than hot water, if you take hot water and cool it, it will not become heavier. This is because as the water cools, it's volume will shrink proportionately to the amount of heat lost. This makes sense, since the weight is determined by the amount of H20 molecules present, and by cooling the water you are not adding any extra molecules. Surprisingly, the water will actually get lighter (although not significantly). This is because energy equates to mass (E=MC^2) and therefore energy has weight. By cooling the water you are taking away energy, and therefore, mass.

Ice

Interestingly enough, despite cold water being heavier than hot water, ice is actually lighter. This is why icebergs float. The reason behind this is that the molecular structure of water is arranged in such a way that when it becomes solid, the molecules line up in a crystalline arrangement. Simply put, when water freezes, it expands, therefore becoming less dense. This unique property of water is one reason Earth can sustain life. If ice sank, the oceans would slowly freeze, because instead of ice rising to the surface and slowly melting (by being closer to the sun) the ocean floor would freeze. The ice floor would creep up to the surface (because in the deep ocean there is little sunlight to warm the water) eventually freezing the whole ocean.

References

• Cold Water is Heavier than Hot Water