Intermolecular forces are stronger in the liquid state than in the gaseous state. Liquids have a definite volume because molecules do not separate from each other. However, molecules of liquids can move fast freely from one another therefore, liquids can flow and can assume the shape of the container in which these are stored. Some physical properties of liquids are discussed below.
(I) Vapour Pressure
At equilibrium state, the pressure exerted by the vapour phase is called vapour pressure at a specific temperature.
- The magnitude of vapour pressure depends upon the following two factors.
- The liquids having weak intermolecular attraction have greater tendency of escaping from the liquid surface in comparison to liquids having stronger forces of intermolecular attraction. For example, the vapour pressure of ether and acetone is more than that of water or acetic acid at any specific temperature.
- The vapour pressure of liquid increases with the increase in temperature.
- The vapour pressures of a given liquid at two different temperatures may be compared using Clausius Clapeyron equation
where P1 and P2 are the vapour pressures at temperature T1 and T2, ∆H is the heat of vapourisation and R is molar gas constant.
- The temperature at which the vapour pressure of the liquid becomes equal to atmospheric pressure (or the external pressure) is termed as the boiling point of the liquid. For example at 1 atmospheric pressure acetone boils at 56 °C, benzene at 80 °C, ethyl alcohol at 78.4°C and water at 100 °C.
- The temperature at which the solid-state and the liquid form of a substance are in equilibrium at one atmospheric pressure is known as the freezing point
(II) Surface Tension
- The surface tension is defined as the force per cm acting perpendicular to the tangential line on the surface of the liquid which tends to compress the surface area.
- The units of surface tension are force per unit length i.e. dynes cm-1 or Nm-1 (in SI units)
- Stronger the forces of intermolecular attraction (cohesive forces) greater is the surface tension.
- The surface tension decreases with rising in temperature or surface tension is inversely proportional to temperature. Surface tension proportional 1/Temperature
Measurement of surface tension of a liquid by the drop number method is the most convenient method.
It is a measure of resistance to flow which arises due to internal
friction between layers of fluid as they slip fast one another as the liquid flows. Strong intermolecular forces between molecules hold them together and resist movement of layers.
A force is required to maintain the flow of layers. This force is proportional to the area of contact of layers and velocity gradient,
n ->proportionality constant called coefficient of Viscosity. Its SI unit is N m2 and in C.GS. its unit is poise. (1 Poise = 1g cm-1 s-1).
- Greater the viscosity, more slowly the liquid flows. H-bonding and van der Waal’s forces are strong enough to cause high viscosity.
- The viscosity of liquids decreases as the temperature rises because, at high temperature, molecules have high K.E. and can overcome the intermolecular forces to slip past one another between the layers.