Physical and Chemical Changes – Grade 7 Science Worksheets

Nature of a Physical change –

• Physical changes are mostly reversible
• Physical properties of a substance only changes not its chemical properties
• New substance is not created in this process

Chemical changes are permanent changes which happen to the original substances making it lose its inherent properties. Changes like these are irreversible like – rusting of iron, burning of wood.

    Credit: https://pixabay.com/en/chain-rust-iron-metal-macro-rusty-566778/

Nature of a Chemical change-

• A chemical change cannot be brought to original by physical methods. Chemical properties of substance change
• In a chemical change, new substances are formed
• Chemical changes are not reversible
• The chemical composition and properties of a substance changes during a chemical chnage
• Gas evolves

Results of Chemical Change:

• Change in smell.
• Gas evolves.
• Heat is produced.
• Sound is produced.
• Change in color.

What is an example of a physical change?

Heating of water: When water is heated no new substance is formed. No change in color or smell.

Paper folding:  A paper can be folded or refolded and unfolded again hence it is an example of physical change. Here, no new substance is formed in this. So, it is a physical change.

Molding of clay: Clay when molded. Shape changes, its size changes but no gas evolves, or no new substance is formed and no smell is produced.

Cutting of wood: When we cut a piece of wood we get more number of wood pieces but there is no new substance produced, no gas evolution and no change in the property of wood.

Crushing paper: Here, again no new substance is formed. Can be brought back to its original form. No gas is released.

Mixing salt in water: Water and salt can be easily separated. This is a physical change because no new substance is formed and the chemical properties of substances involved are completely intact.

Chemical Changes

When properties of substance change and new substances are formed is called chemical changes. Sometimes chemical changes are irreversible.  In chemical change  where it can be reversed chemical properties of substance alter and a new substance is formed.

What is an example of a chemical change?

Burning of anything like paper, wood, fuel etc. When a substance burns, many new substances are formed like ash and carbon dioxide is formed.  A burnt substance cannot be turned to the original form by any chance since ash is what remains.

Hence, burning of a substance is an irreversible chemical change.

Wax melting: This is a reversible change because wax solidifies in room temperature. There is no new substance formed. No gas emissions and chemical property of wax does not change.
Cooking vegetables: This involves baking or frying of food which then cannot be brought back to the original self. Smell is produced. It’s a new substance altogether. Digestion is an example of chemical change. New substances are produced. Food is broken down. Energy is released, cannot be converted to food again.
Weathering of rocks: Rock formation involves immense heat and pressure. Running water on them, intense wind and expansion as a result of heat. These changes are irreversible.