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Class 9, Science, Chapter-1, Lecture-1, Change in State (Notes)

Freezing:

The process in which a matter changes its state from liquid to solid at a fixed temperature without any change in its temperature is termed as freezing.

Freezing Point:

The fixed temperature at which a liquid change to become a solid at atmospheric pressure (760 mm of Hg) is called its freezing point.

Evaporation:

The process in which a matter changes its state from liquid to gaseous at any temperature accompanied by decrease in its temperature is termed as evaporation.

Boiling:

The process in which a matter changes its state from liquid to gaseous at a fixed temperature without any change in its temperature is termed as boiling. 

Boiling Point:

The fixed temperature at which a liquid vaporizes to become a gas at atmospheric pressure (760 mm of Hg) is called its boiling point.

Latent heat of vaporisation:

The amount of heat (energy) required in converting unit mass of a liquid into gas at a given temperature and pressure without any change in temperature of the substance is called the latent heat of fusion of the substance.

  • I. unit is J kg-1.

Evaporation causes cooling:

The particles (molecules) of a liquid collide with each other and energy is transferred from one particle to another.
Some particles at the surface of the liquid acquire enough energy to escape in the form of vapour (gas).
The average kinetic energy of the remaining liquid particles decreases and the temperature of the liquid falls.
Thus, evaporation causes cooling.

Factors affecting rate of evaporation:

  1. Surface Area of the liquid exposed to atmosphere.
  2. Temperature of the liquid
  3. Humidity of the atmosphere
  4. Wind speed
  5. Vapour pressure of the liquid
  6. Nature of vaporizing material.

DIFFERENCES BETWEEN:

Evaporation

Boiling

Takes place at all temperatures

Takes place only at the boiling point of the liquid

Is accompanied by decrease in temperature

No change in temperature takes place

Is slow

Is fast

Takes place only at the surface of a liquid

Takes place in the entire mass of the liquid

Is affected by surface area, temperature, humidity and wind speed

Is not affected by surface area, temperature, humidity and wind speed

Condensation:

The process in which a matter changes its state from gaseous to liquid at a fixed temperature without any change in its temperature is termed as condensation.

Sublimation:

The process in which a matter changes its state from solid to gaseous and vice-versa without changing to liquid state at a fixed temperature without any change in its temperature is termed as sublimation.

Dry Ice is used in cold storage to preserve meat and other perishables.

Reason:
Dry ice is formed by cooling carbon dioxide to a temperature of –78 OC . At normal temperature and pressure, dry ice sublimes to gaseous state without changing to liquid state.
The temperature of the perishable items kept with dry ice lowers due to sublimation and it also remains dry.
The low temperature and the dryness prevents spoilage of the item.

DIFFERENCES BETWEEN:

Property

Solid

Liquid

Gas

 Shape

Fixed shape

No fixed shape

No fixed shape

 Volume

Definite volume

Definite volume

No definite volume

 Density

High density

Lower density

Lowest density

 Rigidity

Rigid

Fluid

Fluid

 Compressibility

Incompressible

Slightly Compressible

Highly Compressible

 Intermolecular force

Very strong

Strong

Very weak

 Melting point

Above room temperature

Below room temperature

Below room temperature

 Boiling point

Above room temperature

Below room temperature

Below room temperature

 Lattice formation

Lattice is formed

No lattice

No lattice