Water in Mixtures
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.

Distillation
You can obtain pure water from a solution of a solid in water by distilling the mixture.
Figure 1.   Simple distillation.
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Which of the following is a good description of the process of
distillation
Distillation is the process of separating a liquid from a mixture by evaporation followed by condensation.
distillation
?
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What is the temperature on the thermometer when the solution has been boiling for a while?
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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.

Figure 2.   Fractional distillation of a solution of ethanol in water.
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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.

Which two of the following industries do you think use fractional distillation to separate liquids?
  • Oil industry
    Steel industry
    Salt industry
    Alcohol industry
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Water in colloids
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.


Figure 3.   Salad dressing before shaking.
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Figure 4.   Salad dressing after shaking.
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The salad dressing is an example of a colloid.

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


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.

Figure 5.   Emulsion paint.
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Here are some other examples of common colloids:


Type of colloid Disperse phase Continuous phase Examples
emulsion liquid liquid cosmetics, milk, salad cream
sol (suspension) solid liquid toothpaste
liquid foam gas liquid hair mousse, shaving foam

A sample of river water contains tiny particles of clay dispersed through the water. What type of colloid is the river water?
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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).

Exercises
1. How would you separate and collect water from a solution of salt?
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2. If you have a mixture of water and another miscible liquid (boiling point 75 °C), what would be the best way to separate the liquids?
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3. Match the following products to the best description of the mixture present.
  • Milk
    Salad cream
    Hair mousse
    Toothpaste
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4. Emulsion paint is described as a water-based paint. It has oil spread through the water. Match the liquids in emulsion paints to their best description.
  • Water
    Oil
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5. When you look at butter through a microscope, you can see tiny droplets of water spread through the fat. Match the substances in the butter to their best description.
  • Fat
    Water
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Well done!
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