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

Water's boiling point increases and melting point decreases with pressure; CCl extsubscript{4}'s boiling and melting points both increase. The difference is due to hydrogen bonds in water.

Step-by-step solution

Understand the Concept

The boiling point of a substance is the temperature where its vapor pressure equals external pressure, while the melting point is the temperature where it changes from solid to liquid. With different substances, changes in pressure can impact these points differently.

Examine Water's Phase Behavior

Water molecules form hydrogen bonds, which makes its structure unique. As pressure increases, the boiling point of water increases. However, the melting point slightly decreases with pressure because water expands when freezing; thus, higher pressure tends to melt ice.

Examine Carbon Tetrachloride's Phase Behavior

Carbon tetrachloride (CCl extsubscript{4}) lacks hydrogen bonds. As pressure increases, both its boiling and melting points increase. This behavior is more typical for non-polar substances like CCl extsubscript{4}, which contract upon freezing.

Compare and Contrast

The difference in behavior under pressure is mainly due to molecular structure and bonding. Water's hydrogen bonds cause unusual behavior like decreasing melting point with pressure. CCl extsubscript{4} behaves more typically, with both points rising as pressure increases.

Conclude Observations

In summary, water's boiling point increases and melting point decreases with pressure, while carbon tetrachloride's boiling and melting points both increase. Differences arise from the presence of hydrogen bonds in water.

Key concepts

Boiling Point

When we discuss the boiling point of a substance, we refer to the temperature at which it transitions from a liquid to a gas. This occurs when the vapor pressure of the substance equals the external pressure exerted on it. For instance, the boiling point of water at standard atmospheric pressure (1 atm) is 100°C. However, this point can change with variations in pressure.

For water, an increase in pressure generally leads to an increase in boiling point. This is because more energy (in the form of heat) is required to match the higher external pressure. Conversely, if the external pressure decreases, the boiling point of water decreases. This explains why water boils at lower temperatures at higher altitudes, where atmospheric pressure is lower.

In contrast, non-polar substances like carbon tetrachloride (CCl_4) don't have hydrogen bonds and might experience slight variations, but their basic principle remains that boiling points generally increase with increasing pressure.

Melting Point

The melting point is the temperature at which a solid becomes a liquid. For many substances, including metals and most ionic compounds, melting means that they absorb enough energy for the solid structure to break down into a liquid form.

Water, however, behaves uniquely when it comes to melting point. Typically, with most substances, pressure increases lead to higher melting points because molecules are pushed closer together, enhancing intermolecular interactions. However, water's melting point actually decreases under pressure. This occurs because as a solid (ice), water has a lower density than liquid water, meaning it expands as it freezes. When pressure is applied, it encourages the transition to the denser liquid form, thus reducing the melting point.

On the other hand, carbon tetrachloride, without the unusual hydrogen bonding seen in water, demonstrates typical behavior where increased pressure leads to increased melting points.

Effect of Pressure on Phase Transition

Pressure plays a crucial role in phase transitions due to its impact on the arrangement and movement of molecules. In general, increased pressure forces molecules closer together, affecting the energy states they prefer to occupy.

In water, increasing pressure raises the boiling point because higher energy is needed for molecules to escape into the vapor phase. However, for the melting point, high pressure reduces it since ice's open cage-like structure collapses into liquid under pressure, preferring the more compact form.

For carbon tetrachloride, increased pressure raises both boiling and melting points, aligning with typical expectations. The space between molecules in solid and liquid forms remains relatively close, so pressure further stabilizes the dense solid structure more, leading to higher phase change temperatures.

Hydrogen Bonding in Water

Hydrogen bonding is a special type of dipole-dipole interaction, particularly strong in molecules such as water (H_2O), due to the polar nature of the molecule. Oxygen is highly electronegative, attracting electrons more than hydrogen, which results in partial charges that promote hydrogen bonding.

This bonding is responsible for many of water’s unique properties, including its high boiling point relative to its molecular weight. The strong bonds require substantial energy to break, elevating water's boiling point compared to substances of similar size and mass that do not form hydrogen bonds.

Moreover, hydrogen bonds give ice a less dense structure than liquid water, causing it to float. This is because these bonds form a hexagonal pattern as water freezes. The structure's expansion under low pressure conditions can be compressed with high pressure, thus melting occurs more readily as ice transitions back to liquid. This behavior highlights how crucial these bonds are in determining water's response to temperature and pressure changes.