Interconversion of States of Matter

The state of matter is interconvertible. They can be interchanged by changing temperature or pressure as

Various terms related to interconversion of states of matter are

  1. Fusion The process of melting, i.e., change of solid state into a liquid state is also known as fusion.
  2. Melting Point The temperature at which a solid starts to melt to become a liquid at the atmospheric pressure is called its melting point. The melting point of a solid is an indication of the strength of the force of attraction between its particles i.e., higher the melting point of solid, higher will be the force of attraction between the particles of solid. Melting point of ice is 0ºC
  3. Sublimation It is the process used for those solids which convert directly into vapors on heating without converting into a liquid phase and the vapors upon cooling give back the solid. Such solids are called sublimates.
  4. Vaporisation The process in which a liquid substance changes into a gas rapidly on heating is called vaporization. The same phenomenon is called evaporation when heating is categorised to be done below the boiling point of the liquid.
  5. Boiling Point The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point. Boiling is a bulk phenomenon and varies from place to place. The boiling point of water at normal pressure is 100ºC.
  6. Condensation It is the process in which gas changes into the liquid state or liquid changes to solid state i.e., solidification.
  7. Latent Heat The word latent means hidden. Thus, latent heat is the amount of heat absorbed or released by a substance undergoing a change of state such as ice changing to water or water to steam at a constant temperature.

Latent heat of fusion is defined as the amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point.

Particles in water at 0ºC have more energy as compared to particles in ice at the same temperature, because of the presence of latent heat of fusion.

Latent heat of vaporization is the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point. Temperature remains constant during boiling due to the latent heat of vaporization.

Effect of Change of Temperature

On heating the solid, kinetic energy of the particles increases. Due to which they start vibrating with greater speed (at their fixed position). The energy supplied by the heat overcomes the forces of attraction between the  particles. Due to reduction in force of attraction, the particles leave their fixed position and start moving freely. Due to this, a stage reached when solid melts and starts to convert into liquid.

Effect of Change of Pressure

By increasing pressure and reducing temperature, we can change a gas into liquid and a liquid into solid and reverse is achieved by decreasing pressure and increasing temperature.

  • Due to the latent heat of vaporization, particles in steam, ie. water vapor at 373 K (100ºC) have more energy than that of water at the same temperature. That’s why steam causes severe burns than that of water at 100ºC.
  • At high altitudes, atmospheric pressure is low, therefore, the vapor pressure of a liquid becomes equal to atmospheric pressure at low temperature, i.e., water boils at a temperature less than 100ºC and hence, food requires more time to cook.
  • Inside the pressure cooker, the pressure is high and hence, water boils at a temperature higher than 100ºC. Thus, less time required to cook the food.
  • In the presence of the impurity, boiling point increases, and freezing point decreases.
  • Solid carbon dioxide is stored under high pressure. It gets converted directly to gaseous state on decreasing pressure to 1 atm without coming into the liquid state. That’s why it is also called ‘dry ice’ or ‘dry cold‘.

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