Colligative Properties

Now let's go back to a question asked at the very beginning of this lesson, and that is - how are solutions different than pure liquids? One of the ways in which they are different, is that when you add a solute to a liquid both the freezing point and boiling point of the solution change. For example, water's normal freezing and boiling points of 0.0 °C and 100.0 °C are changed by adding solute to the water.

Also, something called the osmotic pressure of the liquid changes and that is related to the process of osmosis. In this section we will look at each of these in turn.

The freezing point depression, boiling point elevation, and osmotic pressure are all related to one another, because the magnitude of the change depends not on the nature or identity of the solute that is dissolved, but simply on the number of solute particles present. Because of this they are all grouped together as a set of properties, and they are called the colligative properties.

Freezing Point Depression

When a solute is dissolved in a solvent, the freezing point of the solvent is lowered. For example, when water has salt dissolved in it, its freezing point drops below 0.0 °C (and, since the freezing point is also the melting point, the melting point is also lowered).

In the Midwest, salting of snowy roads is very common during the winter, and freezing point depression is the reason. Say the temperature out is 28 °F, or -2 °C; if you add enough salt to the snow to bring its freezing/melting point down to -3 °C, the ambient temperature is now above the melting point, meaning the snow will melt.

You may also have encountered freezing point depression if you have ever made homemade ice cream. To make ice cream, rock salt is added to ice packed in around a container, which is churned. The rock salt lowers the freezing point of the mixture, which means the melting ice stays at a lower temperature and keeps the forming ice cream nice and cold as you churn.

The amount of decrease in the freezing point depends on the amount of solute added. Add just a little salt, and the freezing point drops only a tiny bit. Add a lot, and it drops a lot (up to certain limits). This is what it means to be a colligative property.

Boiling Point Elevation

Adding a solute also changes the boiling point of a substance, this time in the other direction. Relative to the pure solvent, a solution has a higher boiling point. Once again, the amount of elevation depends on the concentration of particles, so a highly concentrated solution will have a higher boiling point than a dilute one.

This is another phenomenon with an everyday application. Salting the water used to cook pasta has the effect (besides making the pasta nice and salty) of raisings the temperature at which water boils. This means the pasta cooks hotter and gets fully cooked more quickly.

Osmotic Pressure

You may have heard of the process of osmosis. It refers to a specific way that water moves in response to how much solute is dissolved in it. For this reason, it is a colligative property like boiling point elevation and freezing point depression.

Osmosis takes place when two solutions are separated by a special type of membrane called a semipermeable membrane. A membrane like this allows water molecules to pass through it, but blocks the passage of other molecules and ions. Put another way, solvent can pass through it, but not solute.

If I use a semipermeable membrane to separate two solutions of different concentration, water will move through the membrane from the less concentrated solution to the more concentrated solution, as shown at right. This movement creates a pressure known as the osmotic pressure.

Osmosis in Cells

Osmosis is a very important phenomenon in cell biology because the membranes of cells act as very effective semipermeable membranes. They are good at blocking the flow of ions and molecules dissolved in water, but not the water molecules themselves. This makes it very important for cells to regulate the concentrations of solutes both inside and outside of themselves.

If the solute concentration inside a cell is the same as the concentration outside, then the cell is said to be in isotonic solution. In this case, while molecules move across the cell membrane, they do so equally in both directions, so no osmotic pressure is built up.

However, if the cell (right side of diagram) finds itself in a very concentrated environment (left side) it is said to be in hypertonic solution. In this case, water flows out of the cell into the surroundings, which can cause the cell to shrivel and potentially die.

If the environment is very dilute, known as a hypotonic solution, this can also cause problems. In this case, water flows into the cell, building up pressure inside of it. This can cause cells to burst, killing them. Some plant cells prefer to be in a gently hypotonic solution, because the osmotic pressure helps them keep a rigid structure.

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