Which of the following statements explain why hot soapy water is more effective in cleaning oil-stained dishes?

Step 1: Analyze the question We need to know why hot, soapy water cleans greasy dishes better than plain cold water. This is about surface tension.

Step 2: The concept

• Surface tension is the tendency of a liquid surface to contract, caused by cohesive forces between molecules.
• Soap is a surfactant (detergent). It lowers the surface tension of water, so the water can spread, wet the surface and mix with the oil. The soap molecules have a water-loving head and an oil-loving tail, so they surround oil droplets and lift them off the dish (emulsification).
• Heating a liquid weakens the cohesive forces between its molecules, so surface tension decreases as temperature rises. Heat also lowers the viscosity of the oil, so it flows and dislodges more easily.

Step 3: Check each option

• A. Wrong. The oil does not repel soap. Soap molecules actually attach to oil with their tails.
• B. Correct. Soap and heat both decrease surface tension, which helps the water spread, penetrate and break up the oil.
• C. Wrong. Soap decreases surface tension and does not increase it.
• D. Wrong. Heat decreases surface tension and does not increase it.

Step 4: Answer Strictly speaking, soap mainly lowers the surface tension of the water, and the oil is emulsified, but B is the only option that is physically correct in its overall idea.

Tip: Remember "Soap and heat reduce surface tension." Whenever you see temperature or detergent in a surface tension question, think of the tension going down. Any option saying "increases" is almost always a distractor.