Revision Notes on States of Matter: Solid, Liquid & Gases

Read below the key-notes on the chemistry topic "The States of Matter". 

Here, we are mainly providing the keynotes the following topics i.e.   Gas laws - Boyle’s law, Charle’s law, Graham’s law of diffusion, Avogadro’s law, Dalton’s law of partial pressure; Ideal gas equation, Kinetic theory of gases (only postulates); Concept of average, root mean square and most probable velocities; Real gases, Van-der-Waals equation, liquefaction of gases, critical constants.

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Before entering into the sub-topics let us get an overview of the properties of the three states of matter:

The 5 important gas laws as given:

1) Boyle's Law: At a constant temperature, the pressure of a fixed amount (i.e. number of moles n) of gas varies inversely with its volume i.e. P ∝ 1/V

2) Charle's Law: The volume of the given mass of a gas increases or decreases by 1/273.15 of its volume at 0°C for each degree rise or fall of temperature at constant pressure. Or, pressure remaining constant, the volume of a fixed mass of a gas is directly proportional to its absolute temperature. V ∝ T (P is constant)

3) Graham's law of diffusion: The rate of diffusion of a gas is inversely proportional to the square root of the mass of its particles.i.e. d ∝ 1/(m)^1/2 

4) Avogadro Law:  It states that equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules. i.e. V ∝ n 

5) Dalton’s law of partial pressure: The total pressure exerted by the mixture of non-reactive gases is equal to the sum of the partial pressures of individual gases i.e. 

p_Total = p₁ + p₂ + p₃ +......(at constant T, V)

Ideal gas equation:

The three main laws when combined together in a single equation which is known as ideal gas equation. i.e. Boyle's Law; Charle's Law and Avogadro Law to get,

                                            V ∝ nT/P  or PV = nRT 

KINETIC MOLECULAR THEORY OF GASES: The postulates of kinetic theory of gas are:

(i) A gas consists of large number of tiny particles called molecules.

(ii) Volume occupied by gas molecules, is negligible as compared to the total volume of gas.

(iii) There is continuous rapid random motion of gas molecules. The molecules collide with each other and against walls of container.

(iv) The molecules are perfect elastic bodies and there is no loss of kinetic energy during collisions.

(v) There are no attractive forces between the molecules of gas.

(vi) The pressure exerted by a gas is due to bombardment of gas molecules against the walls of the container.

(vii) The different molecules possess different velocities and hence different energies. The average K.E. is directly proportional to absolute temperature.

Based upon the postulates of Kinetic theory of gases, the kinetic gas equation is: 

MAXWELL’S DISTRIBUTION OF VELOCITIES :

The molecules present in a given sample of gas move with different velocities in all possible directions. Velocities and directions of

molecules keep on changing due to inter-molecular collisions. Hence it is impossible to find out the individual velocity of each

molecule. It is however possible to predict fraction ( dN/N ) of the total number of molecules having specific velocities at a

particular temperature. As shown by the curve, The gases show ideal behavior at low presence/large volume. Since the volume of molecules can be neglected and at high

temperature since inter-molecular forces decrease.

The different velocities possessed by gas particles are:

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Real Gases:

There is a deviation from ideal behavior to get real gases. It is due to two faulty assumptions of Kinetic theory of gases particularly not valid at high pressure and low temperature.

(i) Volume occupied by the gas molecules is negligible as compared to the total volume of gas.

(ii) There are no attractive forces between the gas molecules.

Thus, the real gases behaviour is ruled by Van-der waals eqaution: 

CRITICAL PHENOMENON AND LIQUEFACTION OF GASES :

The increase of pressure and decrease of temperature tend to cause liquefaction of gases. The effect of temperature is, however more
important.

1. CRITICAL TEMPERATURE (T_c) :
It may be defined as the temperature above which no gas can be liquefied howsoever high the pressure may be, critical temperature of CO₂ is 31.1°C.

2. CRITICAL PRESSURE (P_c) :
At critical temperature, the pressure needed to liquefy a gas is known as critical pressure

3. CRITICAL VOLUME (V_c) :

The volume occupied by one mole of a gas at a critical temperature and critical pressure is known as critical volume.

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