Surface phenomena: How does the surface tension of water change as temperature increases (within the liquid range)?

Introduction / Context:
Surface tension reflects the energetic cost of creating new surface area and is governed by intermolecular cohesive forces. In applications such as distillation, bubble formation, atomization, coating, and detergency, knowledge of how temperature affects surface tension is essential for predicting droplet size, foaming, and wetting behavior.

Given Data / Assumptions:

• Pure water over its liquid temperature range well below the critical point.
• Atmospheric pressure; no surfactants or impurities.

Concept / Approach:
As temperature rises, thermal agitation weakens the net cohesive forces at the liquid surface. The free energy difference between molecules at the surface and in the bulk diminishes, reducing the surface tension. Empirically, water’s surface tension decreases monotonically with temperature and approaches zero at the critical point where the distinction between liquid and vapor disappears.

Step-by-Step Solution:

Verification / Alternative check:
Data show ~72 mN/m at 20 °C falling to ~59 mN/m near 100 °C for water, confirming the strong negative slope with temperature.

Why Other Options Are Wrong:

• Increases or increases linearly: contradict empirical data and molecular reasoning.
• Remains constant: not supported; the change is significant across typical process temperatures.
• Oscillates: no such periodic behavior exists for pure water.

Common Pitfalls:
Ignoring contamination or surfactants, which can drastically lower surface tension even at fixed T; the question concerns pure water only.

Final Answer:
decreases