Methods Of Separating Mixtures

Mixtures can be separated into their constituents by using physical methods (i.e. no chemical reaction involved). Separation techniques are physical methods. Which technique to use depends on the different properties of the constituents? That is, different states, solubility, boiling and melting points. Below lists some of the most common separation techniques: •

Chromatography - To separate different coloured dyes. The dyes travel up the chromatography paper at different distances before they cannot remain in solution. The more soluble dyes move further up than the less soluble ones, hence separating from each other.

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Distillation - to separate and collect a liquid from a solution of a soluble solid. The solution is heated in a flask until the liquid boils. The vapor produced passes into the condenser where it is cooled and condenses to a liquid. The pure liquid (distillate) is collected in a beaker.

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Evaporation - This method is suitable to separate a soluble solid from a liquid. If the solution is heated, the liquid evaporates leaving the solid behind.

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Fractional Distillation - This is a special type of distillation used to separate a mixture of liquids. Different liquids boil at different temperatures. When heated, they boil off and condense at different times. The apparatus features a fractionating column, which ensures that only the liquid boils at its boiling point will pass into the condenser. Filtration - To separate an insoluble solid from a liquid. The solid remains in the filter paper and the liquid goes through the paper into the beaker.

Some of the example mixtures that can be separated using the above mentioned techniques: (1) separating dyes in inks, or chlorophyll in plants (ethanol as solvent) - chromatography; (2) separating sand from water - filtration; (3) separating ethanol and water - fractional distillation; (4) separating water from ink - simple distillation; (5) separating salt from water - evaporation

Diagrams below show suggested experiment setting for the various separation techniques.

Filtration

Evaporation

In chromatography technique, only a small amount of mixture samples are required, often as little as a drop. The concept behind this techniques is that different constituent in a mixture have different solubility. As the solvent moves up the paper the less soluble constituent will travel at a slower rate than those of more soluble constituents. Hence the mixture will eventually separate into different colored spots as the solvent moves to the top. Sometimes, more sophisticated medium is used, such as thin plastic sheets coated with very fine pure aluminium oxide.

Diagram on the left shows equipment set-up for fractional distillation. For normal distillation, the set-up is similar, except that the fractionating column is being removed. The curving structure of the fractionating column is to increase the surface area and encourage condensation of liquids with higher boiling points. Sometimes the column consists of a tube tightly packed with glass beads. Some porcelain chips should be added into the conical flask to prevent hot spots and sudden boiling. The thermometer reading ensures that suitable temperature is reached at the boiling point of the desired liquid to condense out. This should be the liquid with a lower boiling point in the mixture.

Below shows an example Experiment Worksheet for separation of water from a salt (Time: about 1 hour).

1. Question (initial assessment prior to experiment) Does salt dissolve in water? (YES) What is the boiling point of water (100°C) Which separation technique is suitable to separate salt and water ? (Distillation) Draw a labelled diagram for the experiment.

2. Set up the experiment, as shown in the diagram drawn. - Make sure that the water going into the condenser is from the lower inlet. - The condenser should be supported by a clamp and stand. - Stop the experiment once you collect a reasonable amount of liquid in the beaker. Observe the progress of the experiment and write down the observation. (a) What do you observe inside the flask at 100°C? (b) What do you observe in the condenser at 100°C? (c) What do you collect in the beaker? (d) What is remained in the flask?

3. Questions and further discussions. These questions can be tricky. They stimulate pupils' interest and give further understanding about the subject.

1. What separation technique do we use if we are to separate water from a solution of an insoluble solid? Can you name one insoluble solid in water? 2. Can we use Evaporation to separate water from the salt solution for this experiment? Why? 3. If the salt solution contains another soluble solid (say sugar), do you think the experiment would be carried out differently in order to get the same result? 4. How do you know that the liquid collected in the beaker is water? 5. What are the common names for the three states of water?

Methods of Separating Mixtures

General Emilio Aguinaldo National High School Palico lV, Imus, Cavite

Module In Science lll Submitted by: Apacible, Criselda Anne Marie A. lll- Deuteronomy Submitted to: Mrs. De Leon Date: October 8, 2007