JEE Main 2022 · 29 July · morning · 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): Clothes containing oil or grease stains cannot be cleaned by water wash.
Reason (R): Because the angle of contact between the oil/grease and water is obtuse.
In the light of the above statements, choose the correct answer from the option 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
Physics sir · 4 steps
Water does not wet grease: an obtuse contact angle means it beads up instead of spreading into the stain.
- What contact angle means. The angle of contact is measured inside the liquid, where its surface meets the solid. An acute angle means the liquid spreads and wets the surface. An obtuse angle means it pulls back into drops.
- Water on grease. Water molecules attract each other more strongly than they attract oil or grease. So the angle of contact between water and grease is obtuse, and R is true.
- Why washing fails. Because water does not wet the greasy patch, it cannot spread over the stain or get into it. The water rolls off and the stain stays, so A is true. Detergents work because they lower this contact angle and let water wet the grease.
- So. Both statements are true, and the obtuse contact angle is exactly why plain water cannot clean the stain. The answer is (a).
The idea to keep. Acute contact angle means the liquid wets the surface. Obtuse means it does not. Anything that must spread or soak in, like washing water, needs an acute angle, which is why we add detergent.
Answer from the NTA final answer key. Solution by PrepWiser.