Explain what is observed when
a) The size of the finest gold sol particles increases in the gold sol.
b) Two oppositely charged sols are mixed in almost equal proportions
The vibrant colours show gold nano particles interaction with light. These nano particles are efficient in absorbing specific frequencies and intensely scattering other frequencies. This property depends on size, shape, environment of gold nano particles. Hence individual gold paricles is red as it scatters red light more intensely but when many of nano particles mixed together then it appears blue or purple as it scatters blue or purple light more intensely. As gold nano particles do not scatter frequencies of other colours they will not appear in other colours.
b) Mutual precipitation- When two oppositely charged sols are mixed in equimolar proportions, they mutually neutralise their charge and both get coagulated.