Which property of a fluid determines its suitability as a lubricant?

Step 1: Analyze the question We need the fluid property that makes a substance good for lubrication, meaning it reduces friction between moving surfaces.

Step 2: Explain the concept A lubricant (such as engine oil) forms a thin film between two moving surfaces so they don't rub directly against each other. How well it does this depends on how it flows and resists flow. That property is viscosity, the internal friction of a fluid, or its resistance to flow.

• If the viscosity is too low, the oil is too thin. It squeezes out and doesn't separate the surfaces.
• If the viscosity is too high, the oil is too thick. It resists motion and wastes energy.
• A good lubricant has the right viscosity for its job. This is why engine oils are graded as 5W-30, 10W-40, and so on.

Step 3: Check the other options

• A. Capillarity: the rise or fall of liquids in narrow tubes, as in a wick or in plant roots. It is not relevant to lubrication.
• B. Diffusion: the spreading of particles from high to low concentration. It is not relevant.
• C. Solubility: how well a substance dissolves in a solvent. It is not relevant.
• D. Surface tension: the "skin" effect at a liquid's surface, which causes droplets and lets insects walk on water. It does not determine lubricating ability.
• E. Viscosity: correct, as explained above.

Step 4: Answer The property is viscosity, which is option E.

Exam tip: Link the words by function. Lubricant goes with viscosity (thickness and flow resistance). Capillarity goes with narrow tubes, surface tension with droplets, and diffusion with particle spreading. Many JAMB, WASSCE and NECO questions test this kind of matching.