Mixtures are constituted by more than one kind of pure form of matter, known as a substance. A substance cannot be separated into other kinds of matter by any physical process. We know that dissolved sodium chloride can be separated from water by the physical process of evaporation. However, sodium chloride is itself a substance and cannot be separated by physical process into its chemical constituents. Similarly, sugar is a substance because it contains only one kind of pure matter and its composition is the same throughout. Soft drink and soil are not single substances. Whatever the source of a substance may be, it will always have the same characteristic properties. Therefore, we can say that a mixture contains more than one substance.
TYPES OF MIXTURES
Depending upon the nature of the components that form a mixture, we can have different types of mixtures.
1. Activity :
• Let us divide the class into groups A, B, C and D.
• Group A takes a beaker containing 50 mL of water and one spatula full of copper sulphate powder. Group B takes 50 mL of water and two spatula full of copper sulphate powder in a beaker.
• Groups C and D can take different amounts of copper sulphate and potassium permanganate or common salt (sodium chloride) and mix the given components to form a mixture.
• Report the observations on the uniformity in colour and texture.
• Groups A and B have obtained a mixture which has a uniform composition throughout. Such mixtures are called homogeneous mixtures or solutions. Some other examples of such mixtures are: (i) saltin water and (ii) sugar in water.
Compare the colour of the solutions of the two groups. Though both the groups have obtained copper sulphate olution but the intensity of colour of the solutions is different. This shows that a homogeneous mixture can have a variable composition.
• Let us again divide the class into four groups – A, B, C and D.
• Distribute the following samples to each group:
− Few crystals of copper sulphate to group A.
− One spatula full of copper sulphate to group B.
− Chalk powder or wheat flour to group C.
− Few drops of milk or ink to group D.
• Each group should add the given sample in water and stir properly using a glass rod. Are the particles in the mixture visible?
• Direct a beam of light from a torch through the beaker containing the mixture and observe from the front. Was the path of the beam of light visible?
• Leave the mixtures undisturbed for a few minutes (and set up the filtration apparatus in the meantime). Is the mixture stable or do the particles begin to settle after some time?
• Filter the mixture. Is there any residue on the filter paper? Discuss the results and form an opinion.
• Groups A and B have got a solution.
• Group C has got a suspension.
• Group D has got a colloidal solution.