Question

Figure 1. Water at 1 K Figure 2. Argon at 1 K Figure 3. Oxygen at 1 K Figure 4. Neon at 1 K a) As demonstrated in the above figures, explain in detail why water, H2O, expands upon freezing whereas the other substances in these figures contract upon freezing. In your answer ensure that you address the bonding, polarity, density and other relevant factors to describe these phenomena. b) Draw the structure of water when it freezes. c) Link the phenomena indicated from this observation to explain important aspects of the existence of life on our planet Earth. d) Are there any other substances which exhibit similar behaviour? If so, identify these substances by name, chemical symbol/formula, and a brief description – if there are many such substances, identify at least three of these substances.

Answer 1

a) Liquids in general tend to contract upon freezing, This is because as the temperature is lowered the molecular vibration or the molecular motion slows down and attractive forces take over which causes them to contract. Water, however expands upon freezing. This can be attributed to its structure, Water, H2O, is a bent planar molecule where the electronegative 'O' atom carries a partial negative charge while the 'H' atom carries a partial positive charge. This charge separation lends polarity to the O-H bond which inturn results in the formation of a network of intermolecular H-bonds between water molecules. As water freezes the H-bonds become stronger (due to lower molecular vibrations) and arrange themselves into a hexagonal crystalline lattice. The resulting lattice structure is very open with more space inbetween than in liquid water. Since the volume increases upon freezing, ice is known be less dense than water and tends to float on the surface.

c) As water expands upon freezing, its density decreases. This is why ice tends to float on water. The vast majority of the earths surface is covered in water. In the case of oceans, as ice has the tendency to float on the surface, this prevents the water underneath from freezing. The aquatic life forms can thus survive and a balance in the marine ecosystem can be maintained.

d) Besides water, metals like bismuth, antimony, silicon, germanium and gallium are known to exhibit a similar behavior.