Chemistry REVISION

### LIQUIFACTION OF GASES

At 30.98 °C carbon dioxide remains gas up to 73 atmospheric pressure. (Point E in Fig.). At 73 atmospheric pressure, liquid carbon dioxide appears for the first time. The temperature 30.98 °C is called text(critical temperature) (T_C) of carbon dioxide. This is the highest temperature at which liquid carbon dioxide is observed. Above this temperature it is gas. Volume of one mole of the gas at critical temperature is called text(critical volume) (V_C) and pressure at this temperature is called text(critical pressure) (p_C). The critical temperature, pressure and volume are called critical constants.

### SURFACE TENSION

Surface tension of a liquid is defined as the force acting at right angles to the surface along one centimetre length of the surface. Thus, the units of surface tension are dynes per cm.
A molecule in the bulk of liquid experiences equal intermolecular forces from all sides. The molecule, therefore does not experience any net force.
But for the molecule on the surface of liquid, net attractive force is towards the interior of the liquid, due to the molecules below it. Since there are no molecules above it.

### VISCOSITY

Viscosity of liquids decreases as the temperature rises because at high temperature molecules have high kinetic energy and can overcome the intermolecular forces to slip past one another between the layers.