Question 1

What is surface tension?

Accepted Answer

Surface tension is the tendency of a liquid surface to resist external force. It arises from cohesive intermolecular forces. SI unit: N/m (newtons per meter) or equivalently J/m² (energy per area).

Question 2

Why are N/m and J/m² the same unit?

Accepted Answer

Since 1 J = 1 N·m, it follows that 1 J/m² = 1 N·m/m² = 1 N/m. Surface tension can be described as either force per length or energy per area — they are physically and dimensionally equivalent.

Question 3

Is dyn/cm the same as mN/m?

Accepted Answer

Yes. 1 dyn/cm = 1 erg/cm² = 1 mN/m = 0.001 N/m. The CGS unit dyn/cm is widely used in chemistry and colloid science; mN/m is preferred in SI contexts.

Question 4

What liquids have very low surface tension?

Accepted Answer

Organic solvents have lower surface tension than water: ethanol ≈22 mN/m, acetone ≈24 mN/m. Liquid fluorocarbons and silicone oils can be as low as 15–20 mN/m. Mercury, at ≈487 mN/m, has one of the highest values.

Question 5

How does surface tension of water change with temperature?

Accepted Answer

Water surface tension decreases as temperature rises: at 0 °C it is about 75.6 mN/m, at 20 °C about 72.8 mN/m, at 60 °C about 66.2 mN/m, and at 100 °C about 58.9 mN/m. Higher temperatures weaken the intermolecular cohesion.

Question 6

How is surface tension measured experimentally?

Accepted Answer

Common methods include: the Du Noüy ring method (measuring the force to detach a platinum ring from the surface), the Wilhelmy plate method (continuous measurement via a thin plate), and the pendant drop method (analyzing the shape of a hanging droplet). Each method suits different fluid types and precision requirements.

Millinewton per meter to Newton per meter

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