Which of the following substances lowers the surface tension of water?

Step 1: Analyze the question We are asked which substance lowers the surface tension of water. Surface tension is the tendency of a liquid's surface to contract, caused by the cohesive forces between water molecules.

Step 2: Explain the concept Water molecules attract one another strongly through hydrogen bonds. At the surface, they are pulled inward and sideways, which creates a "skin-like" effect. This is why small insects can walk on water and why a needle can float on it.

Substances called surfactants (surface-active agents) lower surface tension. Their molecules have two ends:

• a hydrophilic (water-loving) head
• a hydrophobic (water-hating) tail

They settle at the water surface and disrupt the cohesive attraction between water molecules, so the surface tension falls. Soaps and detergents are the common examples.

Other ways to lower surface tension include raising the temperature and adding substances like alcohol.

Step 3: Examine the options

• A. Metal: Metals do not dissolve in water or interfere with its molecular attraction. No effect.
• B. Sand: Sand is insoluble and does not alter the surface tension.
• C. Detergent: It is a surfactant. It reduces surface tension, which is why soapy water spreads and wets fabric better, and why it forms bubbles and cleans well.
• D. Paper: It may absorb water, but it does not lower the surface tension.

Step 4: Answer The correct option is C. detergent.

Exam tip: Remember "Soap/detergent = surface tension down". Also remember that temperature rise lowers surface tension, while impurities such as salt slightly raise it. WASSCE often tests these facts, so link detergents with cleaning and wetting.