Question

You have read that adding a solute to a solvent can both increase the boiling point and decrease the freezing point. A friend of yours explains it to you like this: “The solute and solvent can be like salt in water. The salt gets in the way of freezing in that it blocks the water molecules from joining together. The salt acts like a strong bond holding the water molecules together so that it is harder to boil.” What do you say to your friend?

Short answer

The changes in boiling and freezing points upon adding a solute to a solvent are classified as colligative properties, which depend on the number of solute particles present in the solution, rather than their specific nature. While it is true that adding salt to water can decrease the freezing point and increase the boiling point, the explanations provided by your friend are not entirely accurate. The freezing point is lowered because the solute particles disrupt the lattice structure formation necessary for freezing, while the boiling point is elevated because the solute particles reduce the vapor pressure of the solvent, requiring a higher temperature for water molecules to enter the vapor phase.

Step-by-step solution

Address the friend's statement about freezing point depression

While it is true that adding salt to water can decrease the freezing point, the explanation provided by the friend is not entirely accurate. Rather than "blocking" water molecules from joining together, the presence of solute particles disrupts the ability of water molecules to efficiently form an ordered lattice structure, which is necessary for freezing. This disruption to the lattice structure results in a lower freezing point.

Address the friend's statement about boiling point elevation

The friend's statement about boiling point elevation is also incorrect. Adding a solute like salt to water does increase the boiling point, but it is not because it acts as a strong bond holding the water molecules together. Instead, the presence of solute particles in the solution causes a reduction in the vapor pressure of the solvent (the water in this case) due to fewer water molecules present at the surface of the solution, and it requires more energy (higher temperature) for these molecules to escape into the vapor phase. This increased energy requirement results in a higher boiling point.

Explain colligative properties

The changes in boiling and freezing points upon adding a solute to a solvent are classified as colligative properties. Colligative properties depend on the number of solute particles present in the solution and not on the specific identity or nature of those particles. Other colligative properties include vapor pressure lowering and osmotic pressure.

Summary

In summary, while your friend's intuition about adding solute to a solvent affecting boiling and freezing points is correct, their explanations are not accurate. Instead, you can explain the phenomenon by talking about how solute particles disrupt the lattice structure formation during freezing and reduce the vapor pressure during boiling. These changes are examples of colligative properties.

Key concepts

Freezing Point Depression

This phenomenon occurs when a solute, such as salt, is added to a solvent, like water, causing the freezing point to lower. While it might sound like the solute gets in the way of freezing by acting as a physically blocking structure, that's not exactly the case. Instead, the solute particles disrupt the orderly structure needed for ice to form. In pure water, molecules align to form a crystalline lattice to transition into solid ice.

However, when solute particles are introduced, they "get in the way," making it harder for water molecules to settle into this lattice. Consequently, the temperature must drop further to achieve freezing, resulting in a lower freezing point. This process is a classic example of a colligative property because it solely depends on the number of particles in the solution rather than their identity.

Remember:

• Freezing Point Depression = Lower freezing temperature.
• Colligative Property = Depends on quantity, not type, of solute.

Boiling Point Elevation

Boiling point elevation happens when adding a solute increases the boiling temperature of a solvent. Some might think that solute particles form strong bonds that hold the water molecules together. In reality, the presence of solute particles results in a reduction in the vapor pressure of the solution.

Vapor pressure is the measure of the tendency of molecules to escape from a liquid phase into a gas phase. Fewer water molecules are available at the surface due to the presence of solute particles in the solution. This means more energy is required for those water molecules to escape into the vapor phase, leading to an increase in the boiling point.

Key Takeaways:

• Boiling Point Elevation = Higher boiling temperature.
• Caused by decreased vapor pressure due to solute presence.

Vapor Pressure Lowering

When a non-volatile solute is added to a solvent, the vapor pressure of the resulting solution is reduced. This is known as vapor pressure lowering. It transpires because solute particles take up space at the surface of the liquid, diminishing the number of solvent molecules that can evaporate.

This reduction in solvent molecules at the surface means decreased escape into the gas phase, thus lowering the vapor pressure. In simpler terms, with fewer solvent molecules able to leave the liquid, the vapor pressure decreases.

Vapor pressure lowering is another example of a colligative property because it relies on the quantity of solute particles rather than their type.

• Vapor Pressure Lowering = Decreased evaporation rate.
• Occurs when solute occupies surface area.