Surface Tension at Critical Temperature

At What Temperature Does the Surface Tension of a Liquid Become Zero?

Direct Answer: The surface tension of a liquid becomes exactly zero at its critical temperature \(T_c\). At this point, the liquid and vapor phases merge into a single supercritical fluid, causing the meniscus (phase boundary) to disappear entirely. Theoretically, surface tension is also zero in ideal fluids.

Surface tension arises from cohesive forces (intermolecular attractions) acting on the molecules at a liquid's surface. In everyday environments, real liquids always possess measurable intermolecular forces, keeping their surface tension above zero.

Mathematical Relationship: Temperature and Surface Tension

The relationship between surface tension and temperature can be mathematically stated using the modified Eötvös equation (or the Van der Waals-Ferguson equation):

$$\gamma = \gamma_0 \left(1 - \frac{T}{T_c}\right)^n$$

Where \(\gamma\) is the surface tension at temperature \(T\), \(\gamma_0\) is a constant, \(T_c\) is the critical temperature, and \(n\) is an empirical constant (approximately \(1.2\) for most organic liquids). When \(T = T_c\), the term inside the bracket becomes zero, reducing the total surface tension \(\gamma\) to exactly zero.

Conditions Under Which Surface Tension Becomes Zero

There are two primary physical and theoretical scenarios where a fluid exhibits no surface tension:

• At the Critical Temperature \(T_c\): As a liquid is heated, its thermal expansion decreases its liquid density while increasing the density of its vapor. At the critical point, both phases become completely identical. Because the surface interface completely vanishes, the surface tension drops to zero.
• Theoretical Ideal Fluids: In conceptual fluid mechanics, an ideal fluid is assumed to have zero viscosity and zero internal intermolecular attractions. Therefore, an ideal fluid possesses zero surface tension by definition.

Exam Reference: NEET / IIT-JEE & Adv. / CUET Chemistry

NEET/JEE Assertion & Reason (A&R) Assessment

Assertion (A): At critical temperature, the surface tension of liquids becomes zero.

Reason (R): At critical temperature, intermolecular forces for liquids and gases become equal, allowing the fluid to expand without restriction.

• A. Both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
• B. Both Assertion and Reason are true but the Reason is NOT the correct explanation of the Assertion.
• C. Assertion is true but the Reason is false.
• D. Both Assertion and Reason are false.

View Answer & Detailed Explanation

Correct Answer: A (Both A and R are true, and R perfectly explains A).

Assertion Verification: True. Surface tension is caused by net inward cohesive forces acting on surface molecules. As temperature increases, higher molecular kinetic energy overcomes these attractions. At the critical temperature, the boundary between liquid and gas vanishes entirely, dropping surface tension to zero.

Reason Verification: True. At the critical point, physical properties like density and molecular spacing become identical across the fluid. Cohesive forces no longer isolate a distinct liquid state, allowing it to behave like a gas and expand freely without surface boundary layer resistance.

Logic: The Reason provides the fundamental thermodynamic explanation for the Assertion. The structural transition of two phases into a single, uniform fluid phase eliminates the cohesive imbalance needed to generate surface tension parameters.

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Frequently Asked Questions on Temperature and Surface Tension

Why does surface tension decrease with an increase in temperature?

An increase in temperature increases the kinetic energy of the liquid molecules. This thermal energy actively overcomes the intermolecular cohesive forces holding the surface molecules together, causing surface tension to drop consistently as temperature rises.

What happens to the meniscus of a liquid at its critical temperature?

At the critical temperature, the meniscus of a liquid disappears completely because the densities of the liquid phase and vapor phase become completely identical, turning the system into a single phase.