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Materials That Expand When Freezing
Name: Mark F.
Status: educator
Grade: 9
Location: NY
Country: N/A
Date: May 2006
Question:
Water freezes to a lower density, so it floats
on its own liquid.
This occurs between 4 and 0'C.
Is there another liquid that expands when cooled?
Over what temperature range (are they all as small as the 4C' in water)?
Can you add something to water so that it expands more between 4 and >0'C?
Can you add something to water so that it expands over a larger
temperature range than 4 to 0'C?
Replies:
I think you are confusing two phenomena here:
1. The density of water ice is less than liquid water at all
temperatures. So ice floats in water that is at room temperature as
well as water at 0 C. Of course the ice is melting and the
temperature of the ice and the water are different, but the melting
of an ice cube is slow enough to allow one to observe that ice
floats on water that is well above the melting point.
2. The density of liquid water at about 4 C is slightly greater than
temperatures either greater than or less than that temperature.
(Stated another way, the molar volume of ice at 4 C is less than the
molar volume of ice above and below that temperature.) The cause of
this behavior is the presence of lower volume "ice-like" transient
structures in liquid water, even above its melting point. These are
lower density than the surrounding "normal" water, but become less
abundant as the temperature increases. So what you have below 4 C.
is a liquid composed of "normal" water containing "low density"
(higher molar volume) transient configurations of water molecules.
The "normal" part of the liquid increases in volume as the
temperature increases. However, the "low density" ice-like transient
structures begin to fall apart as the temperature increases. This
causes the water to become more dense as these low density (higher
molar volume) transient aggregates disappear. So one factor is
increasing with increasing temperature; the other factor is
decreasing with increasing temperature. This results in a roughly
parabolic dependence of volume (or density) versus temperature.
Regarding how to eliminate this behavior: Most solutes, even at
moderate to low concentrations hydrate, that is, they have clusters
of water molecules attracted to the solute. In almost all cases I
know of this makes the density / temperature behavior of the
solution "normal". That is there is no increase, then decrease in
density as temperature increases; only the "normal" decrease in
density as temperature is increased. I don't know any way to
increases the peculiar behavior of pure water.
A related topic of great practical interest are solids that have
a negative thermal expansion. These include some metal alloys, some
fibers such as Aramid, and a number of ceramics -- ZrW2O8. This
ceramic decreases in volume from very low temperature to very high
temperature (0.3 to 1050 Kelvin's).
In ideal solids the atoms vibrate in such a way that the restoring
force is proportional to the distance the atom moves from its
equilibrium position (Hooke's Law). The atoms of a lattice such as
ZrW2O8 show significant deviations from Hooke's Law -- namely the
restoring force becomes a bit weaker when the atoms are at larger
distances from their equilibrium position. This type of vibration is
called anharmonic vibration, in contrast to Hooke's Law vibrations,
that are called harmonic vibrations. In most solids this deviation
is small and the temperature coefficient of thermal expansion is
positive. But in these ceramics the anharmonic vibrations are 10
times greater than normal solids. These types of ceramic materials
are of technological interest because it is hoped to be able to
combine them with other "normal" ceramics to produce materials that
have zero thermal expansion over a wide temperature range. Such
materials would have wide application in many areas of science and technology.
Vince Calder
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Update: June 2012
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