Question

For the three states of matter (solid, liquid, gas) there are 6 possible changes of state. What changes of state are exothermic and what changes are endothermic?

Answer 1

Hint: When the phase change occurs from lower energy particle to higher energy particle the change will be endothermic. Conversion of solid to liquid, liquid to gas, and solid to gas requires energy input, therefore they are endothermic. Similarly the phase change of liquid to solid, gas to a liquid, and gas to solid is formed by the release of energy so they are exothermic.

Complete answer:
For conversion of a state of matter in which molecules are close together to the state of matter where the molecules are far apart, an input of energy (heat) is required to the molecules to overcome the intermolecular attractive forces. In phase change from higher energy particles to lower energy particles, energy is released.
Phase changes occurring between solids and liquids are melting and freezing.
Melting: If a cold substance is brought to a warmer area it absorbs energy, which facilitates it to overcome the intermolecular force of attraction; due to this, a change in the state of matter is observed. Melting requires energy so it is endothermic.
Freezing: Phase transition in which liquid turns to solid when its temperature is lowered below its freezing point. This phase is formed by the release of energy so it is exothermic.

Phase changes occurring between liquids and gases are Vaporization and Condensation.
Condensation: Evaporated vapors on coming in contact with a cold substance start losing their energy, so to overwhelm the force of attraction, the cold substance does not have enough energy so they form droplets of liquid. With the decrease of temperature, the vapors change their state (gas-liquid). Condensation is endothermic.
Vaporization: The change of liquid form of water to a gas or vapor phase is vaporization. The change requires energy and is endothermic.

Phase changes occurring between solids and gases are Sublimation and Deposition.
Sublimation: When solid substances change directly to gases then the process is known as sublimation. Energy is absorbed by the solid substance to overcome the solid molecule’s strong intermolecular force of attraction, therefore sublimation is endothermic in nature.
Deposition: Deposition is also known as desublimation. The reverse of sublimation is a deposition and it is exothermic in nature.

Melting, Evaporation, and Sublimation are endothermic whereas Freezing, Condensation, and Deposition are exothermic.

Note:
One of the distinct forms that matter can exist is a state of matter. In everyday life four states of matter are noticeable and they are solid, liquid, gas, and plasma. Liquid crystal is one of the intermediate states known to exist and many other states exist but under extreme conditions.