03 Mixtures I

MIXTURES I Introduction When different things are all put together, we say that they are mixed or they form a mixture. The same concept holds for chemistry. A mixture is a system (a system is just the part of the Universe we are studying) formed by different things. Sometimes mixtures are coarse mixtures as the parrot’s food shown in the figure below (left). In these cases we can se the parts and separate them just manually or using squeezers. But what in the case we have a mixture of talc and water? We can still see both things (components) in the mixture: the specks swirling around in the liquid. This class of mixture is called a dispersion. The separation of both components is not as easy as for the parrot’s food. And what if we mix a spoonful of salt with a glass of water? Now after some time we won’t even notice if we have one or two substances! The liquid will be transparent (see-through). It will look as if it were just one thing. These homogeneous systems (no different parts seen) are called solutions.

A coarse mixture

Suspensions

Solutions

Some systems seem to be homogeneous but are very fine dispersions and must be seen through a microscope to tell whether they are mixtures or no. Examples of these systems are mayonnaise and milk. In fact, almost any liquid that looks opaque or turbid is a mixture: pure liquids (except for liquid metals and alloys) are transparent (not necessarily colourless). Aims To observe and classify some mixtures. To separate the components of a mixture Materials You will need a test tube rack, test tubes, pipettes, teat pipettes, spatula, funnel, iron ring, stand, filter paper, evaporating dish, Bunsen burner, tripod, wire gauze. Procedure Part one: classifying mixtures 1- Put seven test tubes in a test tube rack. 2- Using a pipette collect some 5 cm3 of water in each of the seven test tubes. 3- Now add to each of the test tubes (using a tip of a spatula measure in case of solids or 5 drops from a teat pipette in case of liquids) one of the following substances:

Sodium chloride Calcium carbonate Copper (II) sulphate Acetone Alcohol Copper (II) carbonate Potassium nitrate 4- After adding the sample, shake the test tube ticking it with your fingers. If the system looks turbid try heating gently and cooling back under tap water stream. 5- Observe the mixtures and complete the table below. What kind of mixture has been formed in each case? Substance

Aspect

Aspect of aqueous mixture

Type of mixture

Sodium chloride Calcium carbonate Copper (II) sulphate Acetone (propanone) Spirits (ethanol) Copper (II) carbonate Potassium nitrate Part Two: Separating the Components of a Suspension by Filtration 1- Fold a filter paper in fourths (the teacher will show you how). 2- Put it on a funnel; three folds on one side the other one opposite. Filter paper will look like a cone. Hold it straight up by means of an iron ring clamped to a retort stand. See that the tip of the funnel dips into a clean test tube. 3- Pour the calcium carbonate suspension all at once into filter and wait it to drain completely. 4- Add 2 cm3 of water to the test tube to rinse it, and pour it on the filter so that the solid is completely transferred to it. The substance in the filter is called the residue. The transparent liquid that passed down the filter to the test tube is called the filtrate. (See diagram)

5- Unfold the filter paper and let it dry. You will have separated calcium carbonate from water

Part Three: Separating the Components of a Solution by Evaporation 1- Collect the sodium chloride solution in a clean evaporating dish. 2- Fix a water bath as shown in the diagram. Use a tripod instead of an iron ring if you prefer 3- Place the evaporating dish on it and heat with a Bunsen burner 4- Evaporate to dryness. 5- Look at the crystals on a slide through the microscope. Draw what you see. 6- Suppose you have mixed water, salt and calcium carbonate. You should see two phases (one liquid and one solid) although there are three substances in the system. Think of a method to get o the three substances separated. 7- Write down a scheme showing the sequence you will follow to get to both pure solids.