7/31/2019 Water Expands on Freezing

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Water Expands On Frezing

Most liquids have a quite simple behavior when they are cooled (at a fixed pressure): they shrink.

The liquid contracts as it is cooled; because the molecules are moving slower they are less able toovercome the attractive intermolecular forces drawing them closer to each other. Then the freezing

temperature is reached, and the substance solidifies, which causes it to contract some more becausecrystalline solids are usually tightly packed.

Water is one of the few exceptions to this behavior. When liquid water is cooled, it contracts likeone would expect until a temperature of approximately 4 degrees Celsius is reached. After that, it

expands slightly until it reaches the freezing point, and then when it freezes it expands by

approximately 9%.

This unusual behavior has its origin inthe structure of the water molecule. There is a strongtendency to form a network of hydrogen bonds, where each hydrogen atom is in a line between

two oxygen atoms. This hydrogen bonding tendency gets stronger as the temperature gets lower

(because there is less thermal energy to shake the hydrogen bonds out of position). The icestructure is completely hydrogen bonded, and these bonds force the crystalline structure to be very

"open", as shown in the following picture:

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In the following two pictures, the first shows a typical structure of liquid water, while the second isan ice structure; note the extra open space in the ice.

It is this open solid structure that causes ice to be less dense than liquid water. That is why ice

floats on water, for which we should all be thankful because if water behaved "normally" many

bodies of water would freeze solid in the winter, killing all the life within them.

Water's "density maximum" is a product of the same phenomenon. Close to the freezing point, the

water molecules start to arrange locally into ice-like structures. This creates some "openness" in

the liquid water, which tends to decrease its density. This is opposed by the normal tendency forcooling to increase the density; it is at approximately 4 degrees Celsius that these opposing

tendencies are balanced, producing the density maximum.

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7/31/2019 Water Expands on Freezing

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A water molecule consists of two hydrogen atoms and one oxygen atom. The three atoms make an

angle; the H-O-H angle is approximately 104.5 degrees. The center of each hydrogen atom is

approximately 0.0957 nm from the center of the oxygen atom. The structure of a single watermolecule is shown below:

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Because oxygen is more electronegative than hydrogen (in other words, electrons tend to be in the

neighborhood of the oxygen), the hydrogen atoms end up with a partial positive charge and theoxygen atom with a partial negative charge. This separation of charge produces a net dipole

moment on the molecule; for the isolated water molecule this dipole moment is approximately 1.85

Debye units.

This molecular structure leads to hydrogen bonding, which is a stabilized structure in which ahydrogen atom is in a line between the oxygen atom on its own molecule and the oxygen on

another molecule. This picture shows a hydrogen-bonded structure between two water molecules:

These hydrogen bonds, with their extra attractive energy, are the cause of many of the unusualproperties of water, including its large heat of vaporization and itsexpansion upon freezing.

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