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Why ice floats in water: A brief explanation

Updated on July 13, 2011
Even massive icebergs can float.
Even massive icebergs can float.

Common sense suggests that, since substances have the highest density when they are solid, ice should be denser than water. However, the case of water is a singular one; in its liquid state, water is denser than it is in its solid state.

This is why lakes freeze over from top to bottom, although that can be a bit inconvenient when it leads to persons falling through the ice. Water’s peculiar properties and the lower density of ice to water are the primary reasons for ice being able to float on it.

Hydrogen bonding is the characteristic of water that contributes to this phenomenon. The molecular structure of water is such that two hydrogen atoms share a covalent bond with one oxygen atom in a single molecule. However, hydrogen bonds, which are weaker, govern the bonding process among the water molecules.

Water has a “partially ordered structure,” where the ratio of hydrogen bonds to covalent bonds is high. At temperatures below four degrees Celsius (39.2 degrees Fahrenheit), this ratio causes a unique expansion process.

In that temperature range, the hydrogen bonds, which are positively charged, adjust in a manner that separates the oxygen atoms, which are negatively charged. The result is an adjustment from the partially ordered structure to a crystal lattice structure. Ice has a more well-structured and defined molecular structure than water does.

The partially ordered molecular structure of water.
The partially ordered molecular structure of water.
The crystal lattice or diamond structure of ice.
The crystal lattice or diamond structure of ice.

A comparison of the relative densities of ice and water shows that ice is about nine percent less dense. This is because one molecule of hydrogen bonds to 3.4 other molecules of water in its liquid state.

In its solid state, one molecule of hydrogen bonds to 4 other water molecules. This means that one liter of water would not produce 1000 cubic centimeters of ice (recall that 1 liter = 1000 cc). It would actually produce nearly 1100 cubic centimeters of ice.

Naturally, in saying that ice floats on water, we’re assuming that their relative densities allow for that. Therefore you can’t expect 1000 cubic centimeters of ice to float in 400 milliliters of water. Large volumes of ice would float in the ocean because the density of the volume of water in the ocean is much greater.

The simple fact is that the bonding properties of water mean that it occupies more space in its solid state than it does in its liquid state. One thousand cubic centimeters of ice is not as dense as one liter of water. That’s the basic reason why ice floats, all other things being equal.


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