Any difference in the shape or size of an object is referred to as a change. Some changes can always be reversed, while some can’t. Based on this, you can classify changes into two broad categories (1) Reversible and (2) Irreversible changes- 

(1) Reversible changes are the type of changes that can be undone or reversed. 

• When you can go back to the substance you started with, it is called reversible change. 
• In a majority of cases, a new substance is not formed in a reversible change. For example, you stretch a rubber band, your lungs expand when you breathe, you fold a sheet of paper.
• Sometimes, the physical properties of the material change. For example, water becomes steam when heated.

(2) Irreversible changes are the type of changes that cannot be undone or reversed.

• It’s a type of change you cannot reverse using any chemical or physical means, no matter what you do.  
• A new substance is almost always formed during this change.
• A good example is the burning of coal, a chemical change. It’s irreversible because a new substance is formed, which cannot be changed back to coal.

Apart from reversible and irreversible changes, there is another way to classify changes – Physical and Chemical changes.

(1) Physical changes affect the physical properties of a substance, but the chemical properties remain the same. For example, boiling water, freezing water, melting wax, etc. 

• Usually, physical changes do not involve the production of energy.
• No new substances are formed during such physical changes.
• Unlike chemical changes, the original substance can be recovered after the physical change. For example, water can be converted back into liquid form from steam. 
• Physical change is temporary and not permanent. 

(2) Chemical changes affect both the chemical and physical properties of a substance. For example, digestion of food, burning of coal or paper. 

• One or more new substances always accompany a chemical change. For example, the burning of coal leads to the creation of ashes and gasses like CO2, SO2, NOx.
• Chemical changes are permanent.
• Chemical changes often lead to the production of energy like sound, light, heat, etc.
• Most of the time, chemical changes are irreversible, i.e., the original substance cannot be recovered.

Expansion and contraction

• If you increase the temperature, the particles of a substance expand or loosen. A common example of expansion is the railway tracks during summers, as steel expands when heated. 
• If you decrease the temperature, the particles of a substance contract or become tighter. A common example of this contraction is railway tracks during winters, as steel contracts under low temperatures. 
• This expansion and contraction are different for different states of matter like solid, liquid, and gasses. A good example that covers all this is water as it can transform into liquid (water), solid (ice), and gasses (steam).
• At less than 0°C or 0°C, water will turn into ice. This process is known as freezing. You apply heat to it, and it will turn into its liquid form. This process is called melting and will remain in its liquid form till the temperature reaches 99°C. When the temperature reaches 100°C, water will start to boil and turn into steam. This process is called evaporation.  
• Water can also be converted into its liquid form from steam by a process known as condensation. Condensation is the process of the physical state of matter changing from the gaseous to the liquid phase. This is achieved by lowering the temperature surrounding the matter. 
• Water also expands and contracts like steel, and on reducing the temperature, water contracts. Like most other liquids, it will contract until it reaches approximately 4°C. After that, it does something different from most liquids. It expands slightly till it reaches the freezing point. Other liquids continue to contract at this point. When water freezes, it expands by approximately 9%, which is different from how most liquids behave.

Burning or combustion is an irreversible process where a substance burns to produce a new substance. A good example of this is coal burning; burning coal produces fly ash and various other gasses like CO2, NOx, SO2, and particulate matter that causes pollution.

Many substances you see around you are not in their pure form. They are a mixture of two or maybe more than two substances. You can separate different substances of a mixture to obtain individual components that make up the mixture. A good example is water sedimentation, where you separate impurities like dirt and mud from water to obtain pure water.

It is fascinating to know that every object around you is undergoing some change at all times. Many changes are based purely on heat, and the level and speed of change depend on the heat supplied. Supply enough heat, and you can change the state of any object around you. From simple substances like water to complex ones like coal, when changed, produces results different from a substance like water. 

• What are the changes around us? 

Rotation of earth, rise, and fall of temperature, melting of ice to become water are examples of changes around us. 

• What are the physical changes for Class 6? 

Boiling water, folding paper, and cutting wood. 

• Why do changes occur around us? Explain with examples. 

Changes occur around us all the time; sometimes, these changes happen instantaneously and sometimes slowly. Changes can be classified into 4 types- 

Physical changes: They only change the physical attributes of the material and not the chemical composition. For example, the melting of ice. 

Chemical changes: They change the physical and chemical composition of the material. For example, burning coal. 

Reversible changes: These changes can be reversed. For example, water can be frozen once again when obtained from melting ice. 

Irreversible changes: They cannot be reversed. For example, if you burn paper, you cannot make paper from ashes. 

• What changes take place naturally? 

Rotting of food, rotation of the earth, and movement of the moon.

• What are the two types of changes? 

The two types of changes are reversible and irreversible changes. Reversible changes can be undone or reversed, e.g., melting of ice. Irreversible changes can’t be undone or reversed, e.g., burning of coal. 

• What are physical and chemical changes for class 6?

Physical changes are- melting of ice, cutting paper, folding clothes. 

Chemical changes are- digestion of food, ripening of a mango, burning coal.