Which of the following liquids has the highest surface tension?

Analyze the question: We need to find which liquid has the highest surface tension, which is the "skin-like" force at a liquid's surface caused by cohesive forces between molecules.

Explain the concept:

• Surface tension comes from the attraction (cohesion) between liquid molecules. The stronger this attraction, the higher the surface tension.
• Temperature: Raising the temperature increases molecular motion and weakens cohesion, so surface tension decreases as temperature rises.
• Impurities: Soap (detergent) and oil are contaminants that lower the surface tension of water. This is why soap helps water spread and wet fabrics.

Step-by-step solution:

1. Soapy water: detergent lowers surface tension, so it is low.
2. Oily water: the oil film lowers surface tension, so it is low.
3. Hot water: higher temperature weakens cohesion, so it is lower than cold water.
4. Cold water: it has no impurities and a low temperature, so cohesion is strongest and surface tension is highest.

Cold water therefore has the highest surface tension.

Answer: B. Cold water

Exam tip: Remember "Heat and Hitchhikers (impurities) hurt surface tension." Anything that adds heat, soap, or oil reduces it, so pure, cold water is the one with the highest value.