3.6 Solubility can be used to separate mixtures

3.6

Solubility can be used to separate mixtures

In this topic, you will learn that:

• some substances are able to dissolve more easily in solvents than others • chromatography can be used to separate mixtures of substances that have different solubilities.

Video 3.6 Separating mixtures

chromatography a technique used to separate substances according to their differing solubilities

solubility how easily a substance dissolves in a solvent

Solubility

Another property that can differ between substances is solubility. Solubility describes how easily a substance dissolves in a solvent. Some dyes have a higher solubility than others. This can be used to separate them from one another. Many dyes are small particles that are suspended in a solvent. They are usually made from plants or minerals. Ancient Greeks made a mixture of soot and vegetable gum that could be used for writing. One thousand years later, the Chinese made red ink from mercury sulfi de and black ink from iron sulfate mixed with sumac tree sap. Today, many of the inks in textas are made of a mixture of these dyes. We can separate these dye mixtures because the dyes have different solubilities.

Chromatography

Paper chromatography is a common way to separate a mixture. Chromatography works when the end of the absorbent paper is dipped in water, allowing the water to slowly move up the paper. As the water moves past the dye mixture, the most soluble dye dissolves fi rst and starts to move with the water. The other dyes in the mixture take longer to dissolve. Eventually the next dye forms a solution and starts moving towards the top of the paper. Finally, the paper has a series of smudged dye colours running up to the top. The coloured dye that is the most soluble is at the top, whereas the dye that is least soluble is at the bottom.

More complex and sensitive chromatography instruments are used to separate mixtures such as drinks and polluted air. Science laboratories often have chromatography equipment that can be used to detect 1 g of a substance in thousands of litres of solution even if it is mixed with many other substances.

Figure 1 Chromatography is used to separate samples, such as inks and dyes. Figure 2 Performing gas chromatography

Detector response

Ethanol

Methanol Pentanol

Butanol

Propanol Hexanol

0 10 20 30 40 50 60 70 80 90 100 110

Time (s)

120 130 140 150 160 170 180 190 200

Figure 3 A gas chromatogram obtained from performing chromatography shows what is in an alcohol mixture. The higher the peak, the more a substance is present.

Scientists use chromatography to find out what substances are in a mixture. Chromatography works because different substances move through the chromatography equipment at different times, and produces a graph like the one shown in Figure 3. The height of each peak tells the scientist how much of a particular substance is in the mixture.

One of the uses of chromatography today is to identify athletes who use banned substances when they compete by testing their urine. A chromatography machine separates all the substances in the urine, including any illegal drugs that leave the body.

Airport security also tests for illegal drugs. A piece of chromatography paper is wiped over a person or their bag and then inserted into a machine. A gas is pushed through the paper. If the drug is soluble in the gas, it will dissolve and be detected by the sensors.

Figure 4 Airport security uses chromatography to test for illegal drugs.

3.6 Check your learning

Remember and understand

1 Identify the substances that were used to make the first inks.

2 Explain how chromatography can be used to separate inks and dyes. 3 Describe an example of chromatography being used in real life to separate a substance. 4 Identify the solvent that is used in the chromatography for drugs at airports. 5 Define the term ‘solubility’. Apply and analyse

6 Some people think they can disguise drugs at airports by putting them in a strongsmelling substance, such as coffee beans. Explain why this will not work with airport security that uses chromatography.