Question

How does increasing the concentration of a nonvolatile solute in water affect the following properties: (a) vapor pressure, (b) freezing point, (c) boiling point; (d) osmotic pressure?

Short answer

Increasing the concentration of a nonvolatile solute in water: (a) lowers the vapor pressure due to solute molecules occupying the solvent's surface positions; (b) lowers the freezing point because solute particles interfere with the formation of an ordered lattice structure; (c) raises the boiling point as the solution needs to be heated to a higher temperature to reach atmospheric pressure; and (d) increases osmotic pressure due to a stronger osmotic gradient caused by the higher solute concentration.

Step-by-step solution

(a) Vapor Pressure

When a nonvolatile solute is added to water, the vapor pressure of the solution decreases compared to the pure solvent (water). This is because the solute molecules occupy some of the solvent molecules' surface positions, reducing the number of solvent molecules on the surface that can escape into the vapor phase. Hence, the vapor pressure of the solution is lower than that of the pure solvent.

(b) Freezing Point

Adding a nonvolatile solute to water lowers its freezing point. This phenomenon is known as freezing point depression. The reasoning behind this is that solute particles interfere with the formation of an ordered lattice structure, which is required for a substance to freeze. With a higher solute concentration, more particles cause this interference, resulting in a lower freezing point for the solution compared to the pure solvent.

(c) Boiling Point

The presence of a nonvolatile solute in water raises its boiling point. This effect is called boiling point elevation. When a solute is added, the vapor pressure of the solution is lowered compared to the pure solvent. To reach atmospheric pressure (which is required for boiling), the solution needs to be heated to a higher temperature than the pure solvent. Consequently, a higher temperature is needed for the solution to boil, thus raising its boiling point.

(d) Osmotic Pressure

Osmotic pressure is the pressure required to prevent osmosis from occurring when two solutions of different concentrations are separated by a semipermeable membrane. Increasing the concentration of nonvolatile solute in water increases the osmotic pressure of the solution. Higher solute concentration leads to a stronger osmotic gradient, and thus, a greater pressure is required to prevent water from flowing in through osmosis to dilute the solute-rich side.

Key concepts

Vapor Pressure

Vapor pressure refers to the tendency of molecules in a liquid to escape into the gas phase. When a nonvolatile solute is introduced into water, the vapor pressure decreases. Here's why: solvent molecules on the liquid's surface usually jump into the air, creating vapor pressure. However, solute molecules take up space on the surface, leaving fewer solvent molecules free to escape. This reduces vapor pressure because there's less chance for the solvent to vaporize. In essence, adding more solute lowers the likelihood of solvent evaporation, keeping the liquid form stable. Understanding this helps explain why solutions don't evaporate as quickly as pure solvents do.

Freezing Point Depression

Freezing point depression happens when the freezing point of a liquid is lowered by adding a solute. The key to this is the way solute particles interfere with ice formation. For a liquid to freeze, its molecules need to line up in a structured lattice. Solutes get in the way, disrupting this order and making it harder for the solvent to freeze. As more solute is added, more disruption occurs, and the freezing point keeps dropping. This property allows us to see practical applications, like using salt to de-ice roads during winter. The necessary freezing point drops, causing ice to melt even at temperatures below water's normal freezing point.

Boiling Point Elevation

Boiling point elevation is the increase in a liquid's boiling point when a solute is dissolved in it. Normally, a liquid boils when its vapor pressure equals the surrounding atmospheric pressure. When a solute is added, the vapor pressure decreases, meaning the solution needs to boil at a higher temperature for this equality. In simpler terms, you need more heat to make a solution boil than to make a pure solvent boil because the solute disrupts the liquid molecules' chances to escape. That's why salt water has a higher boiling point than pure water. These alterations can be crucial in various industries, such as cooking and chemical manufacturing.

Osmotic Pressure

Osmotic pressure relates to the pressure required to stop the flow of water through a semipermeable membrane separating two solutions with different solute concentrations. By adding more solute, the difference in concentration between the two sides increases. This imbalance creates a force that draws water through the membrane to the solute-rich side to try to balance the concentration on both sides. The more solute added, the greater this force, thus raising the osmotic pressure. It's an important principle in biological and chemical processes, like in our body's cells, where balance is essential for survival.