Measurable properties of gases:

Measurable properties of gases:
Pressure
1- Pressure : the force a gas exerts (per unit area) on its surroundings. Gas pressure is caused by gas molecules colliding with the surfaces of the surrounding substances.
Barometer : device that measures atmospheric pressure The weight of the air pushing down on the open dish of mercury supports a column of mercury in a closed tube
A manometer : measures the pressure of a gas trapped in a container.
Pressure is a measure of the force exerted by a gas per unit area. Correspondingly, it has SI units of Newtons per square metre (Nm-2), more commonly referred to as Pascals (Pa). Several other units of pressure are in common usage, and conversions between these units and Pascals are given below:
1 Torr = 1 mmHg = 133.3 Pa
1 bar = 1000 mBar = 100 000 Pa
The gas phase of a substance has the following properties:
1. A gas is a collection of particles in constant, rapid, random motion
(sometimes referred to as ‘Brownian’ motion). The particles in a
gas are constantly undergoing collisions with each other and with
the walls of the container, which change their direction ? hence the
‘random’. If we followed the trajectory of a single particle within a
gas, it might look something like the figure on the right.
2. A gas fills any container it occupies. This is a result of the second law of thermodynamics i.e. gas expanding to fill a container is a spontaneous process due to the accompanying increase in entropy.
3. The effects of intermolecular forces in a gas are generally fairly small. For many gases over a fairly wide range of temperatures and pressures, it is a reasonable approximation to ignore them entirely. This is the basis of the ‘ideal gas’ approximation, of which more later.