The Case of Solutions
The solutions are homogeneous substances and heat is also usually evolved or absorbed in the preparation of a solution. Even then a solution is considered a mixture because : (i) it can be separated into its components by physical methods, (ii) it shows the properties of its constituents, (iii) it has a variable composition, and (iv) it does not have a fixed boiling point.
For example, salt-solution is considered a mixture because of the following reasons:
(i) Salt-solution can be separated into salt and water by the physical process of distillation.
(ii) Salt-solution shows the properties of both its constituents, salt as well as water.
(iii) The composition of salt-solution is variable. Different amounts of salt can be dissolved in the same amount of water to get salt solutions having different compositions. The salt solution does not have a definite formula.
(iv) Salt-solution does not have a fixed boiling point.
Suppose we are given two liquids, one a pure compound and the other a solution, and we have to find out which one is compound and which one is solution (or mixture). In order to distinguish between the two, we should evaporate them separately. The liquid which evaporates completely, leaving no residue,is a pure compound. On the other hand, the liquid which leaves behind a residue on evaporation, is a solution or mixture. For example, pure water can be distinguished from aqueous salt-solution by evaporation. Pure water evaporates completely leaving no residue, but salt-solution leaves behind salt as residue on evaporation (Please do not say that salt-solution can be distinguished from pure water "by tasting it"!).
Notes
Matter in Our Surroundings - Notes
1. 11. Metals Usually Have a Silver or Grey Colour
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2. The Case of a Liquid Solute Dissolved in a Liquid Solvent
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8. 7. Non-Metals may be Solid, Liquid or Gases at the Room Temperature.
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10. 3. Non-Metals are Bad Conductors of Heat and Electricity.
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11. 6. Metals are Usually Strong. They Have High Tensile Strength.
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17. Chemical Formula for daily use material
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24. To Study the Properties of a Compound of Iron and Sulphur
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28. 4. Metals are Lustrous (or Shiny), and can be Polished.
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30. To Separate a Mixture of Alcohol and Water
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31. To Study the Properties of a Solution
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34. 7. Metals are Solids at the Room Temperature
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36. To Study the Properties of a Suspension
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39. To Separate the Salt-Water Mixture (or Salt-Solution)
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46. Separation of Mixture Of Two Solids
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48. 6. Non-Metals are Not Strong. They Have Low Tensile Strength.
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