What is matter? Matter is anything that has mass and occupies space. Generally, matter is classified into 3 different states: solid, liquid, gas.
A substance can be changed from one state to another by the process of heating or cooling. The kinetic theory model explains this. According to the kinetic theory, the particles (atoms, molecules or ions) possess kinetic energy which are in constant motion and constantly colliding with each other. The velocities of the particles in the three different states of matter are different. The kinetic theory also states that the higher the temperature, the higher the kinetic energy, as the velocities increases. At a given temperature, the lighter particles move faster than the heavier particles.
In a solid, the particles are very packed and arranged in a fixed and orderly pattern. When solid is heated, the particles receive heat energy. This makes the kinetic energy of the particles increases as well as making them vibrate faster. The particles vibrate even more at the melting point that they break away from their fixed positions and become liquid. The temperature at which solid changes to liquid is called the melting point. This whole process is called the melting process.
When a liquid is continuously heated, the particles receive more energy and vibrate faster. They collide with each other more often. At the boiling point, the particles receive enough energy to overcome the force of attraction (usually the Van der Waals’ forces) holding them together. The particles in the liquid state break loose to become gaseous state. When liquid is cooled, movement of the particles is slowed down resulting in the formation of stronger forces of attraction between the particles. This process where liquid changes into solid is called solidification. The temperature at which this process occurs is called the freezing point. The value of the melting and freezing point of a substance are the same.
When a gas is cooled, the particles loose kinetic energy. The movement of the particles slows down. Forces of attraction between the particles are formed which hold the particles together in the liquid state. The process of gas changing into liquid is called condensation. The temperature at which the gas condenses to the liquid state is the boiling point.
However, certain substances do not go through the state of liquid when solid is heated. Some substance changes directly from solid to gas and vice versa. This process is called sublimation. Some examples of this are iodine, solid carbon dioxide and naphthalene.
In short, the changes in matter can be summarized as below:
Solid to liquid : Melting process
Liquid to solid : Freezing process
Liquid to gas : Boiling or evaporation
Gas to liquid : Condensation
Solid to gas : Sublimation
Gas to solid : Sublimation