Question

colloidal solutions show tyndall effect but true solutions do not . explain

Answer 1

Tyndall effect is the phenomenon of scattering of light by minute sized particles. As a result of such scattering light spreads in different directions and also becomes visible.
Tyndall effect can be used to distinguish between true solutions and colloids since colloids show this effect but true solutions do not. This difference in behaviour of true solutions and colloids is due to the difference in the size of their particles. In case of true solutions like salt or sugar solutions the size of solute particles is less than 1 nm (10^-9 m) which is too small to scatter a beam of light thus a beam of light passing through such a solution is not scattered whereas in case of colloids e.g. milk, the particle size (due to large protein molecules as in case of milk ) varies between 1-1000 nm thus they are sufficiently large enough to cause scattering of light as light travel through the colloidal solutions.