What is the primary difference between evaporation and boiling?

Evaporation is a surface phenomenon where liquid molecules with sufficient kinetic energy escape into the gaseous phase at any temperature below the boiling point. It's a slow process. Boiling, on the other hand, is a bulk phenomenon that occurs at a specific temperature (the boiling point) when the liquid's vapor pressure equals the external atmospheric pressure. At boiling point, vapor bubbles form throughout the liquid, leading to rapid vaporization. While both involve liquid turning into gas, their conditions and mechanisms are distinct.

Why does surface tension decrease with an increase in temperature?

Surface tension arises from the net inward cohesive forces experienced by molecules at the liquid surface. As temperature increases, the average kinetic energy of the liquid molecules also increases. This higher thermal energy allows molecules to move more vigorously, weakening the intermolecular forces of attraction between them. With weaker cohesive forces, the net inward pull on surface molecules diminishes, leading to a reduction in surface tension. Essentially, the molecules have more energy to resist being pulled inwards.

How do intermolecular forces affect the viscosity of a liquid?

Viscosity is a measure of a liquid's resistance to flow, which stems from the internal friction between its layers. Stronger intermolecular forces (IMFs) between liquid molecules lead to greater resistance to their relative movement. When IMFs are strong, molecules are more 'sticky' and require more energy to slide past each other, resulting in higher internal friction and thus higher viscosity. Conversely, liquids with weaker IMFs flow more easily and have lower viscosity.

Does increasing the surface area of a liquid increase its equilibrium vapor pressure?

No, increasing the surface area of a liquid does not increase its equilibrium vapor pressure. While a larger surface area will increase the *rate* at which a liquid evaporates, it also increases the rate at which vapor molecules can condense back into the liquid. In a closed system, at equilibrium, the rates of evaporation and condensation become equal, and the vapor pressure achieved is an intrinsic property of the liquid at a given temperature, independent of the surface area. The equilibrium vapor pressure is determined by the nature of the liquid and its temperature.

Why does water rise in a narrow glass tube (capillary action)?

Water rises in a narrow glass tube due to capillary action, which is a result of the interplay between adhesive and cohesive forces. Adhesive forces are the attractive forces between water molecules and the glass wall, which are quite strong (due to hydrogen bonding in water and polar nature of glass). Cohesive forces are the attractive forces between water molecules themselves. In the case of water and glass, the adhesive forces are stronger than the cohesive forces. This causes the water molecules to 'climb' up the glass wall, pulling other water molecules along due to cohesion, until the upward adhesive force is balanced by the downward force of gravity on the column of water.

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Liquid State

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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The liquid state represents an intermediate phase of matter, distinct from both solids and gases, characterized by a delicate balance between intermolecular forces of attraction and the thermal energy possessed by its constituent particles. In liquids, particles are close enough to exert significant attractive forces on each other, preventing them from escaping freely like gas molecules, yet they …

Quick Summary

The liquid state is an intermediate phase of matter where particles are close enough to maintain a definite volume due to significant intermolecular forces (IMFs), but possess enough thermal energy to move past each other, allowing liquids to flow and take the shape of their container.

Key properties include vapor pressure, the pressure exerted by vapor in equilibrium with its liquid, which increases with temperature and depends on the nature of the liquid (weaker IMFs = higher vapor pressure).

Boiling point is the temperature where vapor pressure equals external pressure, increasing with stronger IMFs and higher external pressure. Surface tension is the inward pull on surface molecules, making the surface act like a stretched membrane; it decreases with temperature and is reduced by surfactants.

Viscosity is a liquid's resistance to flow, increasing with stronger IMFs and decreasing with temperature. These properties are crucial for understanding liquid behavior and are frequently tested in NEET.

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Key Concepts

Vapor Pressure and Temperature Relationship

Vapor pressure is a direct consequence of molecules at the liquid surface having enough kinetic energy to…

Surface Tension and its Molecular Basis

Surface tension originates from the imbalance of intermolecular forces at the liquid-air interface. Molecules…

Viscosity and Factors Affecting It

Viscosity is the internal resistance of a fluid to flow. It's essentially a measure of how 'thick' or…

Vapor Pressure ($P_{vap}$): — Pressure of vapor in equilibrium with liquid.

P_{vap} \uparrow

with

T \uparrow

and weaker IMFs.\n- **Boiling Point (

T_b

):**

T

where

P_{vap} = P_{external}

T_b \uparrow

with stronger IMFs and

P_{external} \uparrow

.\n- **Surface Tension (

\gamma

):** Net inward force on surface molecules.

\gamma \downarrow

with

T \uparrow

and addition of surfactants.

W = \gamma \Delta A

.\n- **Viscosity (

\eta

):** Resistance to flow.

\eta \downarrow

with

T \uparrow

and weaker IMFs.

\eta \uparrow

with larger/complex molecules.\n- Capillary Action: Rise if adhesive > cohesive; Fall if cohesive > adhesive.

Very Bright Students Visit Coaching: \n- Vapor Pressure: Increases with Temperature, Decreases with Stronger IMFs. \n- Boiling Point: Increases with Stronger IMFs, Increases with External Pressure.

\n- Surface Tension: Decreases with Temperature, Decreases with Surfactants. \n- Viscosity: Decreases with Temperature, Increases with Stronger IMFs. \n- Capillary Action: Rise if Adhesive > Cohesive, Fall if Cohesive > Adhesive.

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