Change of state of matter

The physical state of matter can be changed in two ways:
  •  By changing the temperature
  •  By changing the pressure
  •  Changing the state of matter by changing the temperature means that the state of matter can be changed by heating it or cooling it.
  •  Changing the state of matter by changing the pressure means increasing the pressure on it (Compressing it) or by decreasing the pressure on it.

  •  Melting (Fusion): The process in which a solid substance changes into a liquid on heating is called melting.

  •  Melting point: The temperature at which a solid substance melts and changes into a liquid at atmospheric pressure is called melting point of a substance.
The melting point of a solid is a measure of force of attraction between its particles. (Atoms or molecules)

  •  Boiling (Vapourisation): The process in which a liquid substance changes rapidly into a gas on heating is called boiling.

  •  Boiling point: The temperature at which a liquid substance boils and changes rapidly into a gas at atmospheric pressure is called boiling point of a liquid.
The boiling point of a liquid is a measure of force of attraction between its particles (atoms or molecules)

  •  Condensation: The process in which a gaseous substance changes into a liquid by cooling is called condensation.
The condensation point of a gas is equal to the boiling point of a liquid.

  •  Freezing (Solidification): The process in which a liquid substance changes into a solid by cooling is called freezing.
The freezing point of a liquid is the same as the melting point of its solid form.
Presence of impurities lowers the freezing point of liquids.

When a solid state is heated, the heat energy makes its particles vibrate more vigorously. At the melting point the particles of a solid have sufficient kinetic energy to overcome the strong forces of attraction between the particles and breaks to form groups of particles. So, the solid melts to form a liquid.

When a liquid substance is heated, the heat energy makes its particles move even faster. At the boiling point, the particles of a liquid have sufficient kinetic energy to overcome the strong force of attraction between the particles and separate into individual particles. So, the liquids boil to form gases.

When a gas is cooled enough, its particles lose so much kinetic energy that they slow down and move closer until they start attracting each other and form liquid.

When a liquid is cooled enough, its particles lose so much kinetic energy they slow down stops moving and vibrate about a fixed position. At this stage the liquid freezes and becomes a solid.