Changing States of Matter

You would have observed changing states of matter when ice cubes melt from solid into liquid water or when water boils into vapour, but have you wondered why substances change form? Changing states of matter occur when matter loses or absorbs energy. When a substance absorbs energy the atoms and molecules move more rapidly and this increased kinetic energy pushes particles far enough, that they change form. This energy is usually heat or thermal energy.

Starting from where we left off in the previous article: change of state; the ice cubes that we took in the beaker and heated have completely melted now. You will remember that as long as there is a phase change occurring there will not be any rise in the temperature. The temperature of the liquid water in the beaker starts to rise once all the ice has melted. So let’s continue heating this and find out what happens. On continued heating, the water first begins to let out all the air dissolved in it and you will see it coming out of the water as bubbles.

Once all the air is out, the bubbling ceases and the water continues to heat. It has been mentioned earlier that the addition of heat to a body increases the vibrational energy of the atoms of the body. If we give it enough energy, the atom will eventually gain enough energy to vibrate out of the body. This is the next change of state. The molecules of water gain enough energy to escape from the surface of the water. This change of state from liquid to gas is called Vaporisation.

Similar to phase change from solid to liquid, it is observed here too that the temperature of the water remains constant until the entire liquid in the beaker is converted to gas. The temperature at which the substance turns from its liquid state to gaseous state is known as its Boiling Point. One of the most commonly known boiling points is the boiling point of water which is at 100oC. We know that pressure affects the melting point of substances as seen in our use of ice skates, but does it affect the boiling point just the same?

The answer is yes. If we were to cover the beaker to prevent any more gaseous water from escaping, we would notice that the boiling in the beaker would stop. As the water vapour generated is not allowed to go out, the accumulation of the gas increases the pressure inside the beaker which causes the boiling point of water to rise. This is what stops the boiling. Now if you removed the cover, the water would start boiling again. The temperature at which a substance boils at standard pressure is known as the normal boiling point of that substance.

Five Changes of State

Melting Freezing Evaporation  Condensation


The process by which a substance changes from the solid phase to the liquid phase is known as melting. The process by which a substance changes from the liquid phase to the solid phase is known as freezing. The process by which a substance changes from the liquid phase to the gaseous phase is known as evaporation. The process by which a substance changes from the gaseous phase to the liquid phase is known as evaporation.

The transition of the gaseous phase to the solid phase without passing the intermediate liquid phase is known as sublimation.


It will interest you know that every object in existence undergoes a state change. It is only a question of the amount of heat supplied to the substance. If you supply enough heat, everything on this planet can be made to change its state. The thing is though not every substance has to follow the solid-liquid-gas path. Some substances can naturally from their solid-state to their gaseous state without entering the liquid state. This phenomenon is known as Sublimation. Examples of sublimation are, the element Iodine, Dry ice (solid CO2) and high-quality coal which at high-temperature burns and sublimates into vapour.

Changing States Of Matter Important Questions

Q1. Which states can be explained using fixed volume?

Ans: Liquid and solid states can be explained using fixed volume.

Q2. What is the boiling point?

Ans: Boiling point is defined as a temperature at which a pure liquid changes into a gas.

Q3. What is the melting point?

Ans: The melting point is defined as the temperature at which the solid starts to melt.

Q4. What is sublimation?

Ans: Sublimation is defined as the process in which the solid-state changes to a gaseous state without changing into a liquid state.

Q5. What is evaporation?

Ans: When the liquid gets converted to gas at all the temperatures, it is known as evaporation.

Q6. What is Boyle’s law equation?

Ans: The mathematical representation of Boyle’s law is:


Q7. What is diffusion?

Ans: Diffusion is defined as the movement of molecules from the higher concentration region to the lower concentration region.

Q8. Who invented Barometer?

Ans: Barometer was invented by Torricelli.

Q9. Express 1 atmospheric pressure in kPa.

Ans: 1atm=101.325 kPa.

Q10. What happens to the volume of any gas with an increase in temperature?

Ans: As the temperature increases, the volume also increases.