Lab Separating Mixtures Sm

SEPARATING MIXTURES 1-THE PURIFICATION OF POTASSIUM NITRATE BY RECRYSTALLISATION Introduction A substance is said to be soluble in water when it forms a solution with it (“disappears” in it). Some substances (sugar for example) are very soluble in water: you can add a lot of sugar to a cup of water and it will keep on dissolving (disappearing). We call solubility of a substance the amount of that substance that can be dissolved by 100 g of water. The solubility of sugar then, in water is high. Some other substances are far less soluble than sugar and some others even totally insoluble (sand for instance). Moreover the solubility of any substance depends on temperature: as a general rule the solubility of solid substances increases with temperature, sometimes dramatically. These differences in solubility are used by chemists to purify a substance:  dissolve it in as little hot water as possible. Then  filter hot to get rid of the insoluble dirt and finally  cool down and wait for crystals to appear because in the cold water cannot hold all of it dissolved. Small amounts of soluble impurities remain dissolved. Now  filter the crystals formed,  wash them with cold water to remove the rest of impurities andfinally  dry them letting water evaporate; the sample will be purer than before. This procedure can be repeated over and over again and is called recrystallisation though some material is lost every time you repeat the process. Crystals look very nice both with the naked eye or looking through a microscope. That is because the particles that form the crystals are aligned (as gymnasts and soldiers do) in regular though different patterns. Aims To obtain pure potassium nitrate from a dirty sample. Pieces of Apparatus Funnel, iron ring, retort stand, 50 or 100 ml beaker, 250 ml beaker (ice/water bath), stirring rod, Bunsen burner, tripod, wire gauze, pipette with dispenser, filter paper, watch glass, and a laboratory scale. Procedure 1- Fix the apparatus as shown in Figure 1 2- Weigh some 10 g of impure potassium nitrate and add to the empty beaker. Record the mass in the chart.

3- 10 ml of distilled water. Heat in a boiling water bath while stirring with the glass rod. The solid should be completely dissolved; otherwise add small portions of distilled water until it does. 4- Filter hot to eliminate insoluble grit. Use the apparatus in Figure 2. Below you will find the technique for folding a fluted filter paper. 5- The filtrate is collected in a small beaker (or a 40 ml test tube). 6- Rinse the beaker twice with 2 ml of hot water and pour the washings into the filter. Collect them in the same beaker or test tube. 7- Discard the residue (grit in the paper). 8- Put the beaker or test tube in an ice – water bath and let cool down. Crystals will appear. 9- Filter the crystals cold in a similar apparatus but using plain-folded filter paper. See below how to fold it. 10- Put the filter with the crystals on a watch glass and let them dry for a week. 11- Weigh the crystals, write down their mass in the chart and calculate the % yield

Mass of impure crystals (g)

Mass of pure crystals (g)

% yield

2- SEPARATING COLOURS BY MEANS OF PAPER CHROMATOGRAPHY Introduction Chromatography is the most powerful method for separating substances. It was first developed to separate and study the coloured components of flowers. Nowadays, chromatography has turned into sophisticated methods: coils of capillary tubes more than 300 m long, lined with special polymers (huge molecules) are used instead of paper as a support, and gases or pressurised liquids flow along them. In paper chromatography a sample is spotted on a standardised filter paper and this paper is placed inside a tank dipping in a small amount of a solvent. The solvent climbs up the paper because of capillary action and sweeps the sample up. After some time, if the sample is a mixture, the components will be separated because of their being swept at different rates. These “sweeping rates” are different because the affinity of different substances in the mixture to

both the paper and the solvent vary between wide ranges. The coloured components are easily seen, but in case a colourless substance has to be located, it can be visualised by different methods (UV lamps, iodine vapours, specific reagents). Chromatography can also be used to identify substances just as the melting or the boiling point. If an unknown sample and different standards are spotted one beside the other on a horizontal line and the solvent is allowed to climb. The standards will have climbed up different distances from the starting line and the unknown will lie on the same horizontal line as the standard it is equal to. The figure shows a chromatography tank with a chromatogram with three different mixtures. One mixture is missing the red components, a second one is missing the green component and the third one the blue. Notice that the individual colours are the same for the different mixtures (not two different greens or blues) Aims To study the composition of the colours in different felt-tip markers Pieces of Apparatus Beaker (250 or 300 ml), chromatography paper, glass rod, stapler or clips, pencil, ruler, felt-tip colours. Procedure

1- Cut a strip of chromatographic paper 4 cm wide. Be sure it is not “wedged”. Using a pencil (no ink or ball-pen should be used) draw a horizontal line ½ cm from one the narrow edges of the strip.

2- At 1 cm from the edge and on the pencil line, make a dot with one of the markers chosen. Make a second and a third dot with other markers all of them separated by a distance of about 1 cm. Blacks, browns and violets are most adequate for observing separations because they are usually mixtures.

3- Make a loop at the other end of the paper and staple it so that you can push a glass rod or a pencil through it. (See Figure 3 below). Be sure that the “looped” strip is long enough to dip into the solvent.

4- Put a little solvent (5 or 10 % sodium chloride solution) in a beaker and hang the paper as shown in the diagram. It should dip in the liquid. 5- Three essential precautions to avoid spoiling the experiment

a- Take care that the liquid does not reach the spots b- By no means let the paper touch the walls of the beaker

c- Be sure that the paper is perfectly at right angles to the solvent or the experiment will be spoiled 6- Let the solvent climb up until it reaches near the top of the paper. Then remove and let it dry.

7- Repeat the experiment using 5 % sodium chloride as solvent.

8- Glue the chromatograms to your report.

How to fold a fluted paper

How to fold a plain paper