Colligative Properties of Solutions


Colligative Properties of Solutions

Depends on concentration of dissolved particles: doesn’t mean if they are small or large or charge molecules, just the number of particles per solution. There are four properties.

1. Vapor Pressure.

For the rate of vaporization and condensation, that’s going to depend on surface area. When you want to measure the vapor pressure of something, you want it in a closed container. When you have a pure liquid, there’s a certain vapor pressure but this pressure is not the same if things are dissolved in it. What is the effect of the rate of vaporization and the effect of condensation if we put a solute in a solution?

If you have a molecule of a solvent that’s trying to escape, that solute is going to be blocking the solvent from evaporating. The effect of condensation, however, is unaffected because it doesn’t matter whether molecules are hitting a solvent or solute. A solute reduces rate of vaporization but has no effect on condensation, therefore the vapor pressure is lower.

2. Boiling point elevation.

What happens during boiling? We get boiling of a liquid if the temperature increases enough so that the vapor pressure exceeds the outside pressure. If a solution has solute, the vapor pressure is lower and we have to get to a higher temperature to get to boiling. So the addition of solute raises the boiling point temperature.

3. Freezing point depression.

Next, let’s consider freezing point. Let’s say we have a liquid in a beaker.  Solids are going to form in the form of ice if we chill it. There will be a definite temperature at which it freezes. This temperature changes however because as things are chilly, solvent molecules leave the liquid and join the solid and they leave behind a smaller volume of liquid in which the solute particles can roam. The solute decreases the rate of crystallization and that leads to a lower freezing point.

This is why people add salt to roads to decrease the freezing point of water. If one uses sucrose (table sugar) instead of sodium chloride, 10 grams (0.35 ounces) in 100 grams (3.53 ounces) of water gives a solution with a freezing point of −0.56°C (31°F). The reason that the salt solution has a lower freezing point than the sugar solution is that there are more particles in 10 grams (0.35 ounces) of sodium chloride than in 10 grams (0.35 ounces) of sucrose.

4. Osmosis.

Imagine a container of water and you add a partition. Initially the water levels are going to be the same on both sides. Then you’re going to put holes in the partition that makes it a semi-permeable membrane which allows particles to go through, but slowly. Initially when you put holes in it, everything is balanced and nothing changes. Imagine we put a solute on the right side though. Assume our liquid is water and the solute is made of large particles. Which side is going to be influenced by the addition of the solute? Our water level on the right, with the solute will increase. The solute slows the rate of right to left. Osmosis is the flow of solvent molecules (of a semipermeable membrane) to the side that has a higher concentration of solute.

Important examples of semipermeable membranes are the cell walls in cells of living things. Osmosis tends to drive solvent molecules through the semipermeable membrane from the low solute concentrations to the high solute concentrations.

Osmotic pressure is pressure required to prevent osmosis.

Osmosis can be prevented by applying pressure to the more concentrated solution equal to the osmotic pressure on the less concentrated side. This can be accomplished either physically, by applying force to one side of the system, or chemically, by modifying a solute concentration so that the two solute concentrations are equal. If one applies a pressure greater than the osmotic pressure to the higher concentration solution, one can force solvent molecules from the concentrated solution to the dilute solution, or pure solvent. This process, known as reverse osmosis, is often used to purify water.

An isotonic solution is one that has the same amount of solute on either side of a physiological fluid. Pretend we have 0.1M glucose and 0.1M NaCl on either side. Would either side act differently? They would not be the same. The definition of the colligative properties depends on the concentration of dissolved particles. There are twice as many particles in NaCl than in glucose because the molarity is based on the moles and not the molecular weight. The size is irrelevant, it’s the number of particles that’s important.

A hospital patient receiving fluids intravenously receives an intravenous (IV) solution that is isotonic with (i.e., at the same solute concentration as) his or her cells. If the IV solution is too concentrated, osmosis will cause the cells to shrivel; too dilute a solution can cause the cells to burst. Similar problems would be experienced by freshwater fish swimming in salt water, or saltwater fish swimming in freshwater. The osmotic pressure, like other colligative properties, does not depend on the identity of the solute, but an electrolyte solute will contribute more particles per formula unit than a nonelectrolyte solute.