Answer:
Scattering of light by particles in a colloid or suspension is known as Tyndall effect. When a strong beam of light is passed through a colloidal solution, the path of the light becomes visible when viewed from a direction at right angle to that of the incident light. It is observed when the particles are larger than the wavelength of visible light. Usually the particles of size in the range of 10-9 m to 10-6 m scatter the visible light. The intensity of Tyndall scattering increases with increase in the size of colloidal particles as well as the concentration.
Starch sol is a lyophilic sol. Hence the sol particles are very small and highly solvated. There is almost no difference in the refractive indices of dispersion medium (water) and dispersed phase (starch). Hence it shows only very weak or almost no Tyndall effect.
Note that Tyndall effect is also not shown by true solutions because the solute particles are of smaller than the wavelength of visible light.
Why starch solution is considered as a lyophilic sol even though starch is a polymer?
Starch solution contains starch molecules dispersed in water. There is strong affinity between starch molecules (dispersed phase) and water (dispersion medium) due to -OH groups present on starch. Recall that starch is a polymer of glucose molecules. These -OH group can interact with water molecules through Hydrogen bonding. Hence it is said to be lyophilic sol.
1) What are colloids?
2) What are lyophilic and lyophobic sols? Give examples.
3) What is the use of Tyndall effect?
4) Give some examples of Tyndall effect.
Author: Aditya vardhan Vutturi