question_answer

  Explain the following giving examples (a) Saturated solution    (b) Pure substance (c) Colloid                            (d) Suspension Or A group of students took an old shoe box and covered it with a black paper from all sides. They fixed a source of light (a torch) at one end of the box by making a hole in it and made another hole on the other side to view the light. They placed a milk sample containedin a beaker/tumbler in the box as shown in the figure. They were amazed to see that milk taken in the tumbler was illuminated. They tried the sameactivity by taking a salt solution but found that light simply passed throughit? (a) Explain why the milk sample was illuminated. Name the phenomenon involved. (b) Same results were not observed witha salt solution. Explain. (c) Can you suggest two more solutions which would show the same effect as shown by the milk solution?

Answer:

  (a) Saturated Solution A solution in which no more of the solid (solute) can be dissolved at agiven temperature is called a saturated solution. Suppose 50 g of a solute is the maximum amount that can be dissolved in 100 g water at 298 K.Then 150 g of solution so obtained is the saturated solution at 298 K.           (1) (b) Pure Substance It consists of a single of matter or particles and cannot be separated into other kind of matter by any physical process. Pure substances always have the same colour, taste and texture at a given temperature and pressure. For example, pure water is always colorless, odorless and tasteless and boils at 373 K at normal atmospheric pressure.                                                                                                                                  (1) (c) Colloids They are heterogeneous mixtures. The particlesize is too small to be seen with a naked eye, but it is big enough to scatter light. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium. Colloids are useful in industry and daily life. The particles of colloid cannot be separated from the mixture by filtration. The process of separation of colloidal particles is known as 'centrifugation'. They do not settle down when left undisturbed. In other words, colloids are quite stable. e.g., smoke, milk, fog, cloud, etc.                                                                                                                                             (2) (d) Suspension It is a heterogeneous mixture in which thesolute particles do not dissolve but remain suspended throughout the bulk of the medium. The particles of suspension can scatter a beam of light passing through it. The particles of a suspension settle down when left undisturbed. The particles of a suspension can be separated from its mixture by filtration.                                                      (1) Or (a) The milk sample was illuminated because milk is a colloidal solution and hence, scatter the light passing through it. The phenomenon observed is called 'Tyndall effect'.                                                                 (1+1) (b) As salt solution is a true solution i.e., solute particle size istoo small to scatter the light, so it does not show "Tyndall effect'.                                                                                                                                                                                (2) (c) Examples of colloid are gold (Au) sol and arsenius sulphide sol.                                                            (1/2+1/2)