question_answer

  Classify each of the following as a physical or a chemical change. Give reasons. (i) Drying of a shirt in the sun. (ii) Rising of hot air over a radiator. (iii) Burning of kerosene in a lantern. (iv) Change in the colour of black tea on adding lemon juice to it. (v) Churning of milk cream to get butter.

Answer:

                  (i) 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)                    (ii) As salt solution is a true solution i.e., solute particle size is too small to scatter the light, so it does not show "Tyndall effect". (2)                      (iii) Examples of colloid Gold sol, arsenius sulphide                            (1/2+1/2)