Changes of State

The physical state of a substance depends on energy being absorbed and released by the substance. Changes of states are reversible. This process can be explained by using the kinetic theory of particles proposed by Robert Hooke (1635-1703).

Two diagrams below will show the changes of states of a substance X.

Explanation:
A-B : Solid                                              C-D : Liquid                                      D-F : Gas
    B-C : Mixture of solid and liquid             D-E : Mixture of liquid and gas

In point A, the temperature of solid X increases, heat energy is absorbed by the particles in the solid. The heat energy is converted into kinetic energy. The particles start to vibrate faster about their fixed position.

In point B, when the temperature is high enough, the vibration of the particles become sufficient to overcome the attractive forces between them (the melting point of solid X). The particles begin to break away from their fixed positions.  Solid X begins to melt and change to liquid. During the melting process, the temperature of substance X does not rise even though heating continues. The temperature remains constant because all heat energy taken in by the particles is used to overcome the attractive forces holding the particles together.

In point C, all the solid X has melted. The particles are no longer in their fixed positions. The particles slide over one another. The substance is now a liquid. The temperature rises as heating continues. Energy is absorbed by particles in the liquid. The particles gain kinetic energy and start to move faster as the temperature rises.

In point D,  the temperature increases until it reaches 70^oC. This is the boiling point of substance X. The liquid have enough energy to overcome the forces holding them together.  The temperature remains constant during this boiling process until all the substance X has changed into vapour.

In point E, The particles now spread far apart and move rapidly in all direction. The substance is now a gas.

    Explanation:
    G-H : Gas                                            I-J : Liquid                                      K-L : Solid
    H-I : Mixture of gas and liquid            J-K : Mixture of liquid and solid

In point H, When a gas is cooled sufficiently, it changes into a liquid. This process is called condensation. As the temperature drops, the gas particles lose energy and move more slowly. eventually, the movement of the particles becomes slow enough for the gas to change into a liquid. During the condensation process, the temperature of substance X remains constant even though cooling continues. This is because heat energy is released as the particles are attached to each other to form a liquid. The heat energy is completely given out to the surrounding.

In point I, all substance X has condensed. The temperature of liquid X continues to drop as cooling continues until it reach 30^oC

In point J, liquid X starts to freeze and change into a solid. During the freezing process, the temperature of substance X remains constant even though cooling continues. This is because heat energy is released as the particles are attached to each other to form a solid. The heat energy is completely given out to the surrounding.

In point K,  all substance X has solidified. The temperature of solid X  continues to drops as it is allowed to cool.


Reference:

Toon, T Y., Kwong, C L., Sadler, J., & Clare, E., 2007, G. C. E. 'O' Level Chemistry Matters, Singapore, Marshall Cavendish International (Singapore) Private Limited. p. 6-13