Which of the following liquids has the highest surface tension?

Analysis

The question asks us to identify which of the listed liquids possesses the highest surface tension. Surface tension is the property of a liquid surface that allows it to resist an external force, caused by the cohesive forces between liquid molecules.

Concept Explanation

Surface tension depends on the strength of the cohesive forces between the molecules of the liquid.

• Water has relatively high surface tension due to strong hydrogen bonding.
• Soapy water has a lower surface tension than pure water because soap acts as a surfactant, breaking the hydrogen bonds and reducing the cohesive forces.
• Oil generally has lower surface tension than water because its intermolecular forces are weaker (van der Waals forces).
• Mercury is a liquid metal. It has extremely strong metallic bonds between its atoms, which results in very high cohesive forces. This is why mercury forms spherical beads rather than spreading out on surfaces.

Step-by-Step Solution

1. Compare the nature of the liquids: We are comparing a polar liquid (water), a surfactant-treated liquid (soapy water), a non-polar liquid (oil), and a metallic liquid (mercury).
2. Evaluate cohesive forces: • Soapy water is designed to lower surface tension to help with cleaning. • Water has high surface tension, but it is significantly lower than that of liquid metals. • Mercury's metallic bonding is significantly stronger than the hydrogen bonding found in water or the weak forces in oil.
3. Conclusion: Because mercury has the strongest intermolecular (metallic) forces among the options provided, it exhibits the highest surface tension.

Identify the Answer

Comparing the options, Mercury (C) is well-known in physics for having a surface tension value (approx. 0.48 N/m) that is much higher than that of water (approx. 0.072 N/m).

Exam Tip

When you see questions about surface tension, remember: "The stronger the attraction between the molecules, the higher the surface tension." Metals in liquid form (like mercury) will almost always have higher surface tension than common organic liquids or water.