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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 a 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 options given below.

Asked in JEE Main 29th July 1st Shift 2022 · Angle of contact and wetting

• (1) Both (A) and (R) are true and (R) is the correct explanation of (A).
• (2) Both (A) and (R) are true but (R) is not the correct explanation of (A).
• (3) (A) is true but (R) is false.
• (4) (A) is false but (R) is true.

Step-by-step solution

Why the other options are wrong

• (2) comes from accepts both statements but cuts the link; the obtuse contact angle is the whole mechanism — it is why water beads up on grease instead of spreading under it and floating it away.
• (3) comes from rejects (R); water standing on an oily surface makes an angle larger than 90°, which is the textbook signature of a non-wetting pair.
• (4) comes from rejects (A); a plain water wash really does fail on grease, which is exactly why detergents are needed — a surfactant lowers the angle of contact and lets the water creep under the stain.

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