Back to: MBOSE Science Class 7

Atom: An Atom is the smallest particles of a substance which may or may not exist independently.

Molecules: When two or more atoms are chemically bonded together, we call it a molecule.

Pure Substances: Pure substances are made up of elements or compounds. Metals like Gold, Silver, Copper etc and non-metals like Oxygen, Hydrogen etc are pure substances.

Mixtures: Mixtures are not chemically combined. Two or more substances simply mixed together to form a mixture. Air is a mixture of various gases like oxygen, hydrogen, carbon dioxide, nitrogen, neon, argon, water vapour, dust, smoke etc.

Compound: A compound is a substance which is made up of two or more elements combined chemically.

Crystallization: Crystallization is a physical change. It is a process by which pure crystals of a substance are obtained from an impure substance. It is the purest solid form a substance having a fixed geometrical shape. Crystals are formed in nature when liquids cool and stat to harden.

Chemical Change : When two or more substances react in such a way that there is formation of one or more new substances with entirely new properties, then the changes is called chemical change.

Characteristic of Chemical Change: The main characteristic of chemical change:

• A new substance with different properties is formed due to chemical change.
• Chemical change is irreversible and permanent.
• A large amount of heat, light or sound is either evolved or absorbed during a chemical change.

Basic differences between Physical and Chemical Changes

Physical Change	Chemical Change
No new substance are formed	A new substance with different properties is formed.
Physical change is reversible	Chemical Change is irreversible

Rusting of Iron: Air and water are two essential requirements for rusting. Therefore, we can, prevent, rusting by preventing iron articles from coming in contact with air or water or both.  Two ways by which it can be prevented are

• Galvanization
• Alloying

Iron gates are pained to prevent the rusting of iron.

Cutting of potato and frying of potato are considered two separate types of changes. This is because when we cut potato, only its shape and size is changed and no new substance is formed. But, when we fry potatoes, its colour and taste is changed and a new substance is formed.

Why Carbon dioxide turns lime water milky: Carbon dioxide when passed through lime water turns it milky due to the formation of calcium carbonate a new substance. The chemical reaction is

Carbon dioxide + lime water -> Calcium Carbonate + water

CO₂ +Ca(OH)₂ -> CaCO₃+H₂O

What happens when an Iron nail is added to an acidified solution of copper sulphate? When an Iron nail is added to an acidified solution of copper sulphate, we will notice that the colour of the solution changes from blue to green due to the formation of iron sulphate, which is a new substance. A brown coloured layer is also seen on the iron nail.

When baking soda is mixed with vinegar, bubbles are formed. Explain the  type of change and the reactions involved. .

When baking Soda is mixed with Vinegar, we will hear a hissing sound and see bubbles of gas out of the flask. It we pass this gas through freshly prepared lime water we will observe that the lime water turns milky. Carbon dioxide gas is produced when Baking Soda is added to vinegar. So , we can say that this is a chemical change. The reaction involved are :-

Vinegar + Baking Soda -> Carbon Dioxide + Other Substances

CH₃COOH + NaHCO₃ ->CO₂ + other substances