CHEM...BONDING & STRUCTURE QUESTION....

1.
Critical temperature of a substance is the temperature above which the vapour (gas) of the substance cannot be liquefied by only increasing the pressure.

The critical temperature of oxygen is -118.6oC, below the room. Therefore, at room temperature, oxygen gas cannot be liquefied by only increasing the pressure.

The critical temperature of butane is 152oC, above the room temperature. Therefore, at room temperature, butance gas can be liquefied by only increasing the pressure.

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2.
Water is anomalous in this aspect. The density of ice (solid) is smaller than water (liquid). In ice (solid), each O atom is surrounded tetrahedrally by four H atoms, two by covalent bonds and two by hydrogen bonds. The structure is rather “open”. When ice melts, water (liquid) keeps only part of the “open” structure. Therefore, ice has a greater volume than an equal mass of water, and thus ice floats on water.

Solid hydrogen fluoride does not have such “open” structure as ice. Its molecules are mobile in liquid, while are well arranged in solid. Therefore, like most substances in the world, solid hydrogen fluoride has a smaller volume than an equal mass of its liquid, and thus solid hydrogen fluoride sinks in liquid hydrogen fluoride.

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3.
Refer to Q.2 above. Ice has a greater volume than an equal mass of water. Therefore, ice has a greater surface area than water.

Consider the following phase equilibrium:
(greater ice sce area) ≒ (smallewaterace area)
(greater surface area) (smaller surface area)

When pressure increases, the equilibrium position of the above equilibrium will shift to the right (the water side) in order to decrease the force (due to the air pressure) exerted on the surface of water/ice, because water has a smaller surface area (F = PA). Therefore, more water is formed and it requires a lower temperature to freeze it. In other words, the melting point of ice is decreases with an increase in pressure.
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