Question

How do the boiling points and melting points of water and carbon tetrachloride vary with pressure? Explain any difference in behavior of these two substances.

Short answer

Overall, the boiling and melting points of both water and carbon tetrachloride increase with pressure. However, due to stronger hydrogen bonds, water is more responsive to pressure changes, thus its boiling and melting points increase more significantly compared to carbon tetrachloride which has weaker van der Waals forces between its molecules.

Step-by-step solution

Understand the Impact of Pressure on Phase Changes

In general, increasing pressure increases the boiling point and melting point of a substance because it makes it harder for the molecules to move into a less dense phase (gas or liquid). Thus, at higher pressures, more energy (higher temperature) is required for the substance to boil or melt.

Describe the Molecule Structure of Water

Water has a polar covalent structure with hydrogen bonding, which means the molecules are strongly attracted to each other. The hydrogen bonding gets stronger under increased pressure, hence increases both the boiling and melting points.

Describe the Molecule Structure of Carbon Tetrachloride

Carbon tetrachloride, on the other hand, has a non-polar molecular structure with weak van der Waals forces acting between the molecules. Pressure increases will compact these loosely held molecules more effectively, thus raising the boiling and melting points, but to a lesser extent compared to water.

Explain the Difference in Behavior

The main difference in behavior lies in the difference in the molecular structure and the resultant intermolecular forces. The stronger hydrogen bonding in water responds to pressure increases more significantly as compared to the weaker van der Waals forces in carbon tetrachloride. Therefore, the boiling and melting points of water tend to increase more significantly with pressure than those of carbon tetrachloride.