Question

You make a solution of a nonvolatile solute with a liquid solvent. Indicate whether each of the following statements is true or false. (a) The freezing point of the solution is higher than that of the pure solvent. (b) The freezing point of the solution is lower than that of the pure solvent. (c) The boiling point of the solution is higher than that of the pure solvent. (d) The boiling point of the solution is lower than that of the pure solvent.

Short answer

(a) FALSE - The freezing point of the solution is lower than that of the pure solvent due to freezing point depression. (b) TRUE - The freezing point of the solution with a nonvolatile solute is lower than that of the pure solvent. (c) TRUE - The boiling point of the solution is higher than that of the pure solvent because of boiling point elevation. (d) FALSE - The boiling point of the solution is higher, not lower, than that of the pure solvent.

Step-by-step solution

Statement A - The Freezing Point of the Solution Is Higher than That of the Pure Solvent

For a solution containing a nonvolatile solute in a liquid solvent, the freezing point of the solution is lower than the freezing point of the pure solvent. This phenomenon is known as freezing point depression. Therefore, Statement A is FALSE.

Statement B - The Freezing Point of the Solution Is Lower than That of the Pure Solvent

As previously established, the freezing point depression causes the solution with a nonvolatile solute in a liquid solvent to have a lower freezing point than the pure solvent. Thus, Statement B is TRUE.

Statement C - The Boiling Point of the Solution Is Higher than That of the Pure Solvent

When a nonvolatile solute is dissolved in a liquid solvent, the boiling point of the solution becomes higher than the boiling point of the pure solvent. This effect is known as boiling point elevation. This happens because the presence of a solute decreases the vapor pressure of the solvent, meaning that the solution will require more energy to reach boiling point. Therefore, Statement C is TRUE.

Statement D - The Boiling Point of the Solution Is Lower than That of the Pure Solvent

Since the boiling point elevation causes the boiling point of the solution to be higher than the pure solvent due to the presence of a nonvolatile solute, Statement D contradicts this concept. Hence, Statement D is FALSE. In summary: - Statement A is FALSE - Statement B is TRUE - Statement C is TRUE - Statement D is FALSE

Key concepts

Freezing Point Depression

When you dissolve a solute in a solvent, the freezing point of the solution becomes lower than that of the pure solvent. Freezing point depression occurs because the solute particles interfere with the formation of the solvent's solid structure. This means that the solvent now requires a lower temperature to solidify because the solute particles disrupt the orderly crystalline structure that would form in a pure solvent.

To explain in simple terms, imagine a crowd on a dance floor (the solvent molecules) moving harmoniously. Once several people (solute particles) join the crowd and start moving differently, the original group can no longer dance as easily as before. They need to slow down (cool down) to accommodate the new dancers. The colligative property of freezing point depression is dependent on the number of solute particles present in the solution, rather than the type of particles. This is because each additional solute particle present in the solution further hinders the solvent's ability to form a solid.

Boiling Point Elevation

Another interesting effect of dissolving a solute in a solvent is the boiling point elevation, which is the opposite of freezing point depression. The presence of a nonvolatile solute makes it harder for the solvent to evaporate and convert into a gas. To boil, a liquid must have enough energy for its molecules to break free from the liquid phase and become vapor, but when solute particles are in the way, more energy – and thus a higher temperature – is needed to achieve this.

Sometimes, you might see this like a busy exit in a theatre obstructed by extra obstacles. The people (solvent molecules) need more time and effort to get out (evaporate). The colligative property of boiling point elevation, too, is only affected by the number of solute particles, not what they are. Essentially, more particles mean more obstruction, leading to a higher boiling point for the solution compared to the pure solvent.

Nonvolatile Solute

A nonvolatile solute is one that does not vaporize easily; it has a low propensity to turn into a gas at the given temperature and pressure. When you add such a solute to a solvent, you end up affecting the solution's boiling and freezing points. Because nonvolatile solutes do not readily escape into the gas phase, they remain in the liquid and contribute to the effects like boiling point elevation and freezing point depression.

It's like adding weight to a bag that's already full of items (solvent) – the extra weight (solute) doesn't easily leave the bag, thus altering how the bag behaves when you try to lift it (change its state). This quality keeps the nonvolatile particles in the liquid, ensuring that they fully exert their impact on the solution's colligative properties without evaporating away.

Solution Properties

The solution properties are intrinsic to mixtures formed by solutes and solvents. Not only do they include concepts like freezing point depression and boiling point elevation, but they are also dependent on the concentration of the solute. Colligative properties, a subset of solution properties, are particularly fascinating because they are determined by the quantity of solute particles rather than their identity.

The effect on vapor pressure, osmotic pressure, and the aforementioned boiling and freezing point changes are all part of how the compounds interact within a solution. It's crucial to understand these fundamentals when dealing with solutions in real-life applications, such as antifreeze in cars or salting roads to melt ice. These practical uses all hinge on manipulating the colligative properties of solutions.