Why does vapour pressure increase with temperature?

As the temperature of a liquid increases, the average kinetic energy of its molecules also increases. This means a larger fraction of molecules at the liquid surface will possess enough kinetic energy to overcome the attractive intermolecular forces holding them in the liquid phase and escape into the vapour phase. With more molecules entering the vapour phase per unit time, the concentration of vapour molecules above the liquid increases, leading to more frequent collisions with the container walls and thus a higher equilibrium vapour pressure.

How do detergents reduce surface tension?

Detergents are surfactants, meaning they are surface-active agents. They typically have a long non-polar hydrocarbon tail and a polar head. When added to water, the non-polar tails orient themselves towards the air (or oil phase) and the polar heads remain in the water. By positioning themselves at the water's surface, detergent molecules disrupt the strong hydrogen bonding network between water molecules, effectively reducing the net inward pull on the surface molecules. This lowers the surface tension, allowing water to spread more easily and penetrate fabrics or dissolve grease.

Why is mercury non-wetting with glass, while water is wetting?

This phenomenon is explained by the balance between cohesive and adhesive forces. Cohesive forces are the attractive forces between molecules of the same substance (liquid-liquid), while adhesive forces are between molecules of different substances (liquid-solid). For water and glass, the adhesive forces between water molecules and the polar glass surface (due to hydrogen bonding and dipole interactions) are stronger than the cohesive forces between water molecules. Thus, water spreads and wets the glass. For mercury, the cohesive forces between mercury atoms (metallic bonding) are much stronger than the adhesive forces between mercury and glass. Consequently, mercury minimizes its contact with glass, leading to non-wetting behavior and a convex meniscus.

Does viscosity affect the rate of evaporation?

Viscosity primarily describes a liquid's resistance to flow, which is related to the ease with which molecules can move past each other within the bulk of the liquid. While stronger intermolecular forces generally lead to both higher viscosity and lower vapour pressure (and thus lower evaporation rate), viscosity itself doesn't directly affect the rate of evaporation from the surface. Evaporation rate is more directly influenced by vapour pressure, surface area, temperature, and external air currents. A highly viscous liquid will likely have strong IMFs, which *indirectly* lead to a lower evaporation rate due to lower vapour pressure, but viscosity isn't the direct cause.

What is the relationship between boiling point and intermolecular forces?

The boiling point of a liquid is directly related to the strength of its intermolecular forces (IMFs). To boil, a liquid's molecules must gain enough kinetic energy to completely overcome the attractive forces holding them in the liquid phase and transition into the gaseous phase. If a liquid has strong IMFs (e.g., hydrogen bonding in water), a significant amount of energy (and thus a higher temperature) is required to break these attractions, resulting in a higher boiling point. Conversely, liquids with weak IMFs (e.g., London dispersion forces in noble gases) have low boiling points because less energy is needed for their molecules to escape into the vapour phase.

Properties of Liquids

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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The properties of liquids are macroscopic manifestations of the intermolecular forces acting between their constituent molecules. Unlike gases, where molecules are far apart and interactions are negligible, or solids, where molecules are fixed in a rigid lattice, liquid molecules are in constant motion but remain in close proximity, allowing significant intermolecular forces to influence their beh…

Quick Summary

Liquids are a state of matter characterized by molecules that are close together but still able to move past each other, thanks to a balance between their kinetic energy and intermolecular forces (IMFs).

These IMFs are crucial for understanding the unique properties of liquids. Vapour pressure is the pressure exerted by the vapour in equilibrium with its liquid, indicating its tendency to evaporate; it increases with temperature and decreases with stronger IMFs.

Surface tension is the inward force that causes a liquid surface to contract, making it behave like a stretched membrane; it arises from unbalanced IMFs at the surface and decreases with increasing temperature or the addition of surfactants.

Viscosity is a liquid's resistance to flow, essentially its internal friction; it increases with stronger IMFs and decreases with increasing temperature. Understanding these properties and their dependence on IMFs and temperature is fundamental for NEET, as they govern many physical and chemical phenomena.

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Surface Tension and Spherical Drops

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Viscosity and Flow Rate

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Vapour Pressure ($P_{vap}$) — Pressure of vapour in equilibrium with liquid.

P_{vap} \uparrow

with

T \uparrow

P_{vap} \downarrow

with IMF

\uparrow

Boiling Point ($T_b$) —

T

where

P_{vap} = P_{atm}

T_b \uparrow

with IMF

\uparrow

T_b \downarrow

with

P_{atm} \downarrow

Surface Tension ($\gamma$) — Force/length at surface.

\gamma \downarrow

with

T \uparrow

\gamma \uparrow

with IMF

\uparrow

. Detergents

\downarrow \gamma

Viscosity ($\eta$) — Resistance to flow.

\eta \downarrow

with

T \uparrow

\eta \uparrow

with IMF

\uparrow

. Unit: Pa\cdot s.

IMFs — London Dispersion < Dipole-Dipole < Hydrogen Bonding.

Very Strong Intermolecular Forces Lower Vapour Pressure, Increase Boiling Point, Surface Tension, and Viscosity.