Problem 29 - Chemistry The Science in Context

Question

Explain the term nonvolatile solute.

Step-by-Step Solution

Answer

Answer: A nonvolatile solute is a substance that does not easily evaporate or change from a liquid to a gas when dissolved in a solvent. These solutes are typically larger molecules with strong intermolecular forces, such as salts, sugars, and proteins. Nonvolatile solutes impact the properties of a solution, affecting boiling point, freezing point, and osmotic pressure. Specifically, they raise the boiling point, lower the freezing point, and increase osmotic pressure. These effects are known as colligative properties and depend on the concentration of the nonvolatile solute in the solution.

Step 1: Defining Nonvolatile and Solute

Nonvolatile refers to a substance that does not readily evaporate or change its state from a liquid to a gas under normal conditions, such as room temperature and atmospheric pressure. A solute, on the other hand, is a substance that is dissolved in a solvent, forming a solution.

Step 2: Combining the Definitions

A nonvolatile solute is a solute that does not easily evaporate or change from a liquid to a gas when dissolved in a solvent. This means that the solute remains in the liquid phase and does not contribute to the vapor pressure of the solution.

Step 3: Characteristics and Examples of Nonvolatile Solutes

Nonvolatile solutes are typically larger molecules with a higher molecular weight and have strong intermolecular forces, such as hydrogen bonding, which make them less likely to evaporate. Examples of nonvolatile solutes include salts (e.g., sodium chloride), sugars (e.g., glucose), and large organic molecules (e.g., proteins).

Step 4: Effects of Nonvolatile Solutes on Solution Properties

Nonvolatile solutes have a significant impact on the properties of a solution, including boiling point, freezing point, and osmotic pressure. Specifically, the presence of a nonvolatile solute in a solution will: 1. Raise the boiling point 2. Lower the freezing point 3. Increase the osmotic pressure. These are known as colligative properties and are dependent on the concentration of the nonvolatile solute in the solution.

Key Concepts

Colligative Properties

Colligative properties are fascinating aspects of solutions that are determined by the number of solute particles, rather than the type of particles. When we talk about colligative properties, we are referring to

• Boiling point elevation
• Freezing point depression
• Osmotic pressure

Each of these properties changes as solute particles are added to a solvent. The reason behind this is that the solute particles disrupt the orderly arrangement or interactions of the solvent molecules, altering its physical properties. What’s important to remember here is that it doesn’t matter whether we add salt, sugar, or any other nonvolatile solute. These effects are based entirely on quantity. The more particles, the greater the effect on the boiling point, freezing point, and osmotic pressure.

Boiling Point Elevation

Boiling point elevation occurs when a solute is added to a solvent, such as water. This means the solution will boil at a higher temperature than the pure solvent. But why does this happen? The presence of solute particles in the liquid interferes with the formation of vapor. Hence, more heat (energy) is required to allow the solvent molecules to escape into the gas phase.

Boiling point elevation is directly proportional to the molal concentration of the solute, and can be calculated using the formula: \[ \Delta T_b = i \cdot K_b \cdot m \]where:

• \( \Delta T_b \) is the boiling point elevation
• \( K_b \) is the boiling point elevation constant of the solvent
• \( m \) is the molality of the solution
• \( i \) is the van 't Hoff factor, representing the number of particles produced per formula unit of solute

This principle is widely used in processes like cooking and even in industries for antifreeze solutions!

Freezing Point Depression

Freezing point depression is another intriguing effect of adding a solute to a solvent. This phenomenon results in the solution freezing at a lower temperature than the pure solvent. The science behind it involves the solute particles interfering with the orderly structure necessary for the solvent to become solid.

The relationship for calculating the change in freezing point is similar to boiling point elevation: \[ \Delta T_f = i \cdot K_f \cdot m \]where:

• \( \Delta T_f \) is the freezing point depression
• \( K_f \) is the freezing point depression constant of the solvent
• \( m \) is the molality
• \( i \) is the van 't Hoff factor

One practical application of freezing point depression is in the use of salt to melt ice on roads and sidewalks. By lowering the freezing point, salt makes it harder for the water to refreeze, thereby keeping roads safer during icy conditions.

Osmotic Pressure

Osmotic pressure is a colligative property related to the flow of solvent molecules through a semipermeable membrane. It arises when a solution is separated from pure solvent by a membrane that allows only the solvent, not the solute, to pass through. Osmotic pressure is the pressure required to stop this solvent flow.

As more solute is added, the osmotic pressure increases. The formula to quantify osmotic pressure is given by: \[ \Pi = i \cdot M \cdot R \cdot T \]where:

• \( \Pi \) is the osmotic pressure
• \( M \) is the molarity of the solution
• \( R \) is the ideal gas constant
• \( T \) is the temperature in Kelvin
• \( i \) is the van 't Hoff factor

Osmotic pressure is crucial in biological systems, for example, in maintaining the proper function of cells and in processes like kidney dialysis.