JEE Main 2022 · 26 July · evening · Single correct · +4 / −1
Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : Finest gold is red in colour, as the size of the particles increases, it appears purple then blue and finally gold.
Reason R : The colour of the colloidal solution depends on the wavelength of light scattered by the dispersed particles.
In the light of the above statements, choose the most appropriate answer from the options given below.
- (a) Both A and R are true and R is the correct explanation of A.
- (b) Both A and R are true but R is NOT the correct explanation of A.
- (c) A is true but R is false.
- (d) A is false but R is true.
Answer and solution
Answer (a) Both A and R are true and R is the correct explanation of A.
Solution
Chemistry ma’am · 4 steps
Particle size decides which wavelength gets scattered, and that decides the colour of the gold sol.
- Check Assertion A. A gold sol made with the smallest particles is red. As the particles grow, the sol turns purple, then blue, and finally golden. So A is true.
- Check Reason R. Colloidal particles are large enough to scatter light. The colour you see depends on which wavelength the dispersed particles scatter. So R is true.
- Link them. The wavelength scattered depends on the size and nature of the particles. Bigger gold particles scatter different wavelengths from smaller ones, so the colour shifts from red towards blue and then golden. R is exactly the reason A happens.
- So. Both statements are true and R correctly explains A, which is option (a).
The idea to keep. In a sol, colour follows particle size: size sets the wavelength scattered, and that sets the colour. Remember the gold sol order: red, purple, blue, golden as the particles grow.
Answer from the NTA final answer key. Solution by PrepWiser.