Introduction


Water is a very good general solvent that forms mixtures easily with many different substances. The solute (substance that is dissolved to form a solution) can be a solid, a liquid, or a gas. Just think of the number of drinks you enjoy that are mixtures! In this unit we will see how to separate the water from a solution, and also look at types of mixtures other than solutions.

You can obtain pure water from a solution of a solid in water by distilling the mixture.

We can separate a mixture of liquids with different boiling points by using fractional distillation. This uses mostly the same apparatus as simple distillation, but a fractionating column is added to get a better separation of the liquids.



The temperature in the flask is kept slightly above the boiling point of the ethanol (78 °C). The ethanol will boil off and pass through the fractionating column to the condenser. Most of the liquid with the higher boiling point, in this case water, will remain in the flask. Some of the water will evaporate but most of this will condense on the glass in the fractionating column and fall back into the flask.

Of course, not all substances dissolve in water. Most people know that oil and water don't mix well. But what happens when you want to put some salad dressing on your lunch? There are two layers in a bottle of salad dressing – one is oil-based and the other is water-based. The oil and water are immiscible (liquids that don't dissolve in each other). Before adding the dressing to your food, you must shake the bottle vigorously. Tiny droplets of oil get spread out through the water. We say that oil becomes dispersed through the water in the mixture. The two liquids then appear as a single cloudy liquid, which will separate back out into its two layers if left to stand.





The salad dressing is an example of a colloid.



In the case of salad dressing, we have two liquids mixed together. This type of

colloid

A colloid is a finely dispersed mixture of two or more substances that do not dissolve in each other.colloid is called an emulsion.

In an emulsion, the liquid that is spread throughout the mixture is called the disperse phase, and the liquid it spreads out into is called the continuous phase.



Here are some other examples of common colloids:

Summary


Water can dissolve many substances to form solutions. We can collect the water from a solution by simple distillation.

If the water is mixed with a miscible liquid (one that dissolves in water) with a similar boiling point, the liquids can usually be separated by fractional distillation.

Substances that don't dissolve in water can form mixtures called colloids. A colloid is a finely dispersed mixture of two or more substances that do not dissolve in each other.

Two immiscible liquids (liquids that don't dissolve in each other) can form a colloid called an emulsion.

The liquid that is spread throughout the emulsion is called the disperse phase, and the liquid it spreads out into is called the continuous phase.

Other types of colloid include sols (solids in liquid) and liquid foams (gas in liquid).