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Does Hot Water Weigh More Than Cold Water? The Answer Revealed and Explained in Terms of Molecules

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The answer is no. Cold water has a higher density than and weighs more than hot water. It all boils down to the properties of liquids and their molecules.

Properties of Water

Water is found in 3 different states:

  1. Solid- ice, snow
  2. Liquid- water, rain
  3. Gas- vapor

These different states change as the temperature changes. The changes that morph water into its different states are known as freezing, melting, and evaporating states. The freezing point of water to turn to a solid is about 32 degrees Celsius (depending on your elevation). Likewise, the melting point for water to turn from a sold to a liquid is about 0 degrees Celsius (depending on your elevation). The two states have the same point because there is such a fine line that separates when water can freeze and melt.

When liquid turns to gas the process is called evaporation and the transformation point is called the boiling point. When you boil a pot of water you can see the water vapor rising once it reaches the boiling temperature. The universal boiling point of water is about 100 degrees Celcius, again, depending on your elevation.

How Molecules Work in Different States of Matter

Molecules make up EVERYTHING and are made up of atoms. Even non-living things are packed full of molecules that we can't see.

Molecules are always in motion even in solid objects. Molecules in solids don't move around as much as molecules in liquid and gas phases though. They stay in a more uniform position to each other and kind of vibrate instead of float around.

Molecules in liquids move around more and differently than molecules in a solid. The liquid's molecules move faster and change positions with other molecules inside of the liquid but remain in contact with each other.

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Molecules within gas move much faster than those in a solid and unlike molecules in a liquid, they tend to stay away from each other. The molecules kind of zip through the gas and bump into each other and bounce of in a new direction.

Hot Water vs. Cold Water Molecules


How Molecules Determine Density

As you can see from my picture, molecules in warmer water are separated farther apart than those of the colder water. When the molecules of a substance are closer together it makes the substance have a higher density due to the masses of the molecules. Density is defined as mass per volume in this circumstance. So if you have two cups of water with the same volume, the one with the closer molecules will have more density and will weigh more.

So How Much More Does Cold Water Weigh Than Hot Water?

The weights of hot and cold water are actually very close to each other. This is because they are the same substance made up of the same types of molecules. The only difference is that the colder water will have its molecules closer together and will have a higher density. Molecules hardly weigh anything when looked at as a single component. There are billions of molecules in our bodies and other substances that are too small to see without a very powerful microscope.

The weight of water really depends on how much you have and how cold or hot it is. You're going to have a larger difference of weight and density when the two extremes are a lot colder or hotter than the other. There is no exactly answer due to these variables but I can tell you that the difference between cold and hot water is only going to have a density difference of about .0001 depending on the hot and cold temperatures.


Dale Anderson from The High Seas on October 17, 2012:

Voted up. I had no idea.

Ingenira on September 27, 2012:

Very well explained. Thank you !

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