Todd Duncan
University of Chicago
Water is the only compound I know of that expands when it freezes. There may be some others I don't know about, but certainly it's a very rare property. As far as the reason for water having this unusual property, basically it has to do with the structure of the crystal that forms when the water becomes ice. You could use tinker toys to demonstrate this. When the water is liquid, the water molecules (represented by the wooden disks of the tinker toys) are close together, but can slide past each other and move around freely. When it freezes, bonds form which lock the molecules in place in a regular pattern, that stays nearly fixed (molecules can't move around). You can use the sticks of the tinker toys to represent the bonds, holding the disks in place a fixed distance (the length of the sticks) from each other. Whether the compound expands or contracts when it freezes depends on the spacing and structure of these bonds. In most compounds, the molecules end up closer together in this solid state than in the liquid state. But for water, the bonding is such that the molecules end up farther apart in the ice than in the water. By the way, the fact that water expands when it freezes means that ice is less dense than water. For this reason, it floats on the water (look at an ice cube in a glass of water). This property is very important for fish in the winter, because it means the ice forms on top of a pond and insulates the rest of the pond below, preventing it from freezing all the way through.
Darren Galpin
Physics Undergraduate Student, University of Bath, United Kingdom
At room temperature, water consists of molecules which are weakly linked together by Hydrogen bonding. This occurs due to two loan pairs of electrons, electrons which are not involved in the chemical bonding between oxygen and hydrogen in the molecule. The loan pairs float about on the periphery of the oxygen, and are attracted to hydrogen atoms, which are positively charged. This attraction prevents water boiling at a much lower temperature; similar types of compounds (e.g. CO2), are gases at room temperature. However, at room temperature, the bonds are constantly being broken and re-formed. This allows the molecules to slide over each other, making it a liquid. However, at freezing point, the molecules have very little energy, and cannot move about much. The hydrogen bonds therefore become more permanent. However, hydrogen bonds cannot form randomly; the water molecule has to be in the correct orientation for them to form. When the hydrogen bonds are formed, the water molecules are not packed very tightly; there is a lot of space between them, more than when they were liquid. As all molecules form these bonds when they are taken below freezing point, the actual volume has to expand. I don't know of any other molecule that does this, but then I am a physicist, and a chemist could probably explain it better.
Dr. Arnold G. Gill
Astrophysician, Malaspina University-College, British Columbia, Canada
The expansion has to do with the peculiar molecular structure of water. Within the limits imposed by this keyboard, a water molecule looks like
O
/ \
H H
Each of the hydrogen atoms weakly attracts nearby oxygen atoms, so water binds together. When liquid, the binding is weak, but when solid, the bonds become quite strong, and constrain how the molecules may arrange themselves as a crystal. In its liquid state, the molecules are free to roam around wherever they wish. However, in its frozen state, the water molecules form a hexagon (six-sided figure), giving the beautiful six-sided snowflakes that we are all familiar with. In this state, the molecules line up approximately like the sketch below
H2O
/ \
H2O H2O
| |
H2O H2O
\ /
H2O
with the lines showing the molecular bonds holding the molecules in place. Notice that the center is empty! In its liquid form, with the molecules moving about freely, you will find an H2O molecule in the center of such a structure, but not in its solid state. So, a given amount of water will use up more room as ice than as a liquid, because of the empty space that appears as it freezes. Thus, ice is less dense and it floats. (And in case you are wondering, the center is EMPTY; it is NOT filled with air. Air bubbles are extra effects that help to float ice, but air bubbles are not needed to get ice to float.)
You've probably heard that 90% of an iceberg is under water. This suggests that the density of ice is about 10% (1/10) less than that of water. Yet, from the simple diagram above, there are 6 water molecules where before there were 7 - a difference of about 14% (1/7). Why the discrepancy? While frozen water has a unique crystal structure, it also acts like every other substance in the world - it contracts when it cools. That is, the average distance between adjacent water molecules still decreases when water freezes (in the diagram, the lines connecting the molecules get shorter). The net result is that the group of molecules shrinks, but the center remains empty - or 1/7 shrinks to about 1/10.
As far as I know, water is unique. However, there may be more exotic liquids that exhibit similar properties. I doubt that you would find any of them outside of a lab.
Edward Gordon
University of Illinois Medical School
Water is the only liquid which expands when it freezes. Generally, all substances shrink as they cool. Water is no exception, for the most part. However, as water cools, it reaches its maximum density (has the smallest distance between molecules) at 34 degrees Fahrenheit. Between 34 degrees F and 32 degrees F, it actually expands slightly. At 32 degrees F, of course, ice (solid water) forms. There is a sharp expansion as the water freezes. You know this is true, that ice is lighter than water, because you can see ice cubes floating in a glass of water. The reason it expands is that the molecules of water re-arrange at the time of freezing to form the crystal structure of ice. This arrangement takes up a little more space than it did when it was still liquid, and thus,it is larger per unit of mass (less dense), and so the ice floats on the water.
Jim Stringer
Physiology, Palo Alto Medical Clinic, California
One of water's most remarkable properties is that it expands when it freezes to a volume that is always 10% greater than in the liquid state. In other words, 10 cups of water put into the freezer is going to turn into 11 cups of ice when it freezes. This expansion takes place with tremendous force, as anyone knows who has left a full container of water with a tight lid in the freezer. The force is enough to burst the strongest water pipes if the water in them freezes, which is why people in cold climates sometimes leave a little water trickling through the pipes on freezing nights. It is not possible to make a usable pipe strong enough to withstand this force. Freezing water can burst a cast iron pipe over a foot thick. Water expands when it freezes because the molecules of water are actually closer together in the liquid state than they are in ice. Water molecules are polar, which means that they have positive and negative ends like little magnets. When water freezes, the molecules are held together in a rigid crystal pattern called a lattice, lined up with opposite charges next to each other. When the ice melts, the water molecules can tumble over each other and briefly get a little closer than they are in the solid crystal. This expansion is a very important phenomenon in the natural world. The force of freezing water is enough to crack open rocks and speed up the erosion of mountains. If water did not expand, then it would be denser when it froze, and would sink and soon cover the bottom of a lake or ocean. The oceans would fill with frozen water, and life as we know it in the oceans and on land would not be possible.
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