Chemistry Notes of unit 5 9th class

Gases diffuse very rapidly. Diffusion is mixing up of a gas throughout a space or other gases.

Effusion is escaping of a gas molecule through a fine hole into an evacuated space.

Gases exert pressure. The SI unit of pressure is Nm-2 which is also called Pascal.

Standard atmospheric pressure is the pressure exerted by a mercury column of 760 mm height at sea level, it is equivalent to 1 atmosphere

Gases are highly mobile and they can be compressed.

Gases are 1000 times lighter than liquids or solids hence their density is measured in g dm-3.

Boyle’s law states that volume of a given mass of a gas is inversely proportional to the pressure at constant temperature.

Charles’ Law states that volume of a given mass of a gas is directly proportional to the absolute temperature at a constant pressure.

Absolute zero is the temperature at which an ideal gas would have zero volume, it is -273.15 °C.

The conversion of a liquid into vapours at all temperatures is called evaporation. It is a cooling process.

Evaporation depends upon surface area, temperature and intermolecular forces.

Vapour pressure of a liquid is defined as the pressure exerted by the vapours when liquid and vapour states are in dynamic equilibrium with each other.

Boiling point is the temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure or any external pressure.

Boiling point depends upon the nature of liquid, intermolecular forces and external pressure.

Freezing point of a liquid is that temperature at which vapour pressure of liquid phase is equal to the vapour pressure of the solid phase. At this temperature liquid and solid coexist in dynamic equilibrium with one another.

Melting point of solid is the temperature at which solid when heated melts and coexist in dynamic equilibrium with liquid.

Solids are rigid and denser than liquids.

Solids are classified as amorphous and crystalline .

Amorphous solids are shapeless and do not have sharp melting point.

Crystalline solids have definite three dimensional pattern of arrangement of particles .They have sharp melting points.

The existence of a solid in different physical forms is called allotropy.

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