JEE Main 2025 · 23 January · morning · Single correct · +4 / −1
Given below are two statements:
Statement I: The hot water flows faster than cold water
Statement II: Soap water has higher surface tension as compared to fresh water. In the light above statements, choose the correct answer from the options given below
- (a) Both Statement I and Statement II are false
- (b) Both Statement I and Statement II are true
- (c) Statement I is false but Statement II is true
- (d) Statement I is true but Statement II is false
Answer and solution
Answer (d) Statement I is true but Statement II is false
Solution
Physics sir · 4 steps
Heat lowers viscosity, so hot water flows faster; soap lowers surface tension, it does not raise it.
- Statement I. How fast a liquid flows is set by its viscosity. When water is heated, its viscosity drops sharply, so hot water flows faster than cold water. Statement I is true.
- Statement II. Soap molecules gather at the water surface and weaken the pull between the surface water molecules. This lowers the surface tension, from about 0.072 N m⁻¹ for pure water to roughly a third of that for soap solution. Statement II is false.
- Why soap helps. Lower surface tension lets soap water spread over a surface and wet dirt more easily. That is exactly why soap cleans better.
- So. Statement I is true and Statement II is false, which is option (d).
The idea to keep. Flow speed depends on viscosity, which falls as temperature rises. Soaps and detergents always reduce the surface tension of water, and that is how they help it spread and clean.
Answer from the NTA final answer key. Solution by PrepWiser.