Water is a substance with some very unusual properties. Based on a comparison of the water molecule and the structure of ice explain (a) why ice is less dense than water, and (b) why increasing pressure, within a certain temperature range, causes ice to melt. (c) why is it not possible to convert ice to liquid water by applying pressure at very low temperatures?© BrainMass Inc. brainmass.com December 24, 2021, 5:02 pm ad1c9bdddf
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When frozen, a molecule of water is hydrogen bonded to four other water molecules. The two hydrogen atoms that are part of the central water molecule are attracted to oxygen atoms of two other water molecules. Hydrogen from two other water molecules is attracted to the oxygen of the central water molecule. This open crystalline structure occupies a large amount of space.
When ice melts, many, but not all, of the hydrogen bonds are broken. As some of the bonds are broken, the lattice collapses. The water molecules move closer. The same number of molecules now occupy less space. Thus, water is denser than ice
Density = mass/volume, when volume increases for unit mass, the density also increases.
(You can see trapped air bubbles in an ice cube, which are also responsible for this low density)
b) Here the process is reversed; when pressure is applied to ice, it is desirable for the molecules to get closer, and this is achieved by ice changing to the denser liquid water. Or ice occupies more volume than liquid water, so squeezing harder favors the liquid over the ice state.
c) The phase diagram of water is shown here. It shows that at low temperature, (solid) ice is the stable phase. At moderate temperatures and high pressure, (liquid) water is the stable phase, and at high temperature and low pressure, (gas) vapor is the stable phase. Lines separate these phases.
This figure explains why is it not possible to convert ice to liquid water by applying pressure at very low temperatures. To change the phase from ice to liquid, it must cross the line with the negative slope. As you can see, it cannot cross it except under sufficient temperatures.© BrainMass Inc. brainmass.com December 24, 2021, 5:02 pm ad1c9bdddf>