Question

Which of the following will show Tyndall effect?
(I) Salt solution
(II) Milk
(III) Starch solution
A) I, II, III
B) I, II
C) II, III
D) I, III

Answer 1

We know that Tyndall effect is usually shown by colloids and colloids are heterogeneous systems. In colloids insoluble particles are suspended in another substance. The size of colloid particles can range from 1 to 1000 nanometres.

Complete answer:
We know that when two or more pure substances are taken together, we get a mixture. On the basis of the nature of these pure components, mixtures can be categorized as:

Homogenous mixtures: We have a homogeneous mixture if the composition is uniform throughout. Solutions are homogeneous mixtures. In a solution, the size of the particles is which makes it impossible to see them with naked eyes and they are unable to scatter the light passed through the solution. Examples include aqueous solutions of salt or sugar.
Heterogeneous mixtures: we have a heterogeneous mixture if the composition is not uniform throughout. Suspensions and colloids are heterogeneous mixtures. In a suspension, the particles do not get dissolved but are suspended. The size of the particles is enough to make them visible to the naked eye and they can scatter the light passed through the suspension. Chalk powder or wheat flour suspended in water can be some examples of suspension. Another heterogeneous mixture is colloids which might appear to be homogeneous for their particles are smaller than that of suspensions and are not visible to naked eyes but still they are larger than that of solutions and are able to scatter the visible light. Milk and fog are examples of colloids.
So, now we can classify the given mixtures based on the above discussions but what about the Tyndall effect? Let’s try to understand that now.
Tyndall effect comes from scattering of light. As we have seen earlier, particles in a solution do not scatter the light but that of colloidal solutions do. When a beam of visible light is passed through a colloid, it might appear clear if we observe from the direction of light but we can observe the scattered light at right angles. This effect was studied and given by Tyndall and hence, is known as Tyndall effect.
So, now we know that Tyndall effect is a property of colloids only and not solutions. So, let’s classify the given mixtures based on their components. Salt solution is a homogeneous solution but milk and starch are colloidal solutions.

Hence, milk and starch solutions would show Tyndall effect and option C is the correct one.

Note: We have to keep in mind that colloids might appear to be clear solutions but they are not. Colloids are widely useful in food items, medicines and in industries.