By which of the following process does a solid substance is directly converted into gas?

The correct answer is Sublimation.

Key Points

• Sublimation: It is the conversion of a solid directly into vapors.
  • Sublimation takes place when the boiling point is less than the melting point.
  • The heat required to change a unit mass of solid directly into vapors at a given temperature is called the heat of sublimation at that temperature.

Additional Information

• Condensation: The process by which water vapor in the air is changed into liquid water is called condensation. This caused the water vapor to condense, or turn into its liquid form.
  • Condensation is crucial to the water cycle because it is responsible for the formation of clouds.
• Evaporation: The vaporization occurring from the free surface of a liquid is called evaporation.
  • Evaporation is the escape of molecules from the surface of a liquid.
  • This process takes place at all temperatures and increases with the increase of temperature.
  • Evaporation leads to cooling because the faster molecules escape and, therefore, the average kinetic energy of the molecules of the liquid (and hence the temperature) decreases.
• Diffusion: It is the movement of a substance from an area of high concentration to an area of low concentration. 
  • Diffusion happens in liquids and gases because their particles move randomly from place to place.