Liquid State

In the liquid state, molecules are held close together but can execute random motion through the spaces between them. Most physical properties are governed by the strength of intermolecular forces.

Properties of Liquids:

(i) Vapour Pressure

• In a closed vessel, liquid and its vapour establish a dynamic equilibrium; the pressure at equilibrium is called saturated vapour pressure
• Vapour pressure is a kinetic phenomenon — depends on temperature and nature of liquid
• Vapour pressure ∝ Temperature (increases with rise in temperature)
• Vapour pressure ∝ 1 / Intermolecular forces (weaker forces → higher vapour pressure)
• Unit: mm Hg or torr

(ii) Viscosity

• The property that determines the ease with which a fluid flows (resistance to flow)
• Arises due to internal friction between layers of fluid in motion
• Viscosity ∝ 1 / Temperature (viscosity decreases as temperature increases)
• Viscosity ∝ Intermolecular forces (stronger forces → more viscous)
• SI unit of viscosity coefficient: N m⁻² s (pascal-second); CGS unit: poise (g cm⁻¹ s⁻¹)

(iii) Surface Tension

• The force acting along the surface of a liquid at right angles to any line per unit length
• Arises because molecules at the surface experience a net inward pull
• Surface tension ∝ 1 / Temperature (decreases as temperature rises)
• Surface tension ∝ Intermolecular forces (stronger forces → higher surface tension)
• SI unit: N m⁻¹