Chapter 3: Physical and Chemical Changes

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Q
Physical and Chemical Changes

Question:

The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.

(i) Process – A is a chemical change.

(ii) Process – B is a chemical change.

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change.

Answer:

Physical change: A change in which only the physical properties of a substance like a shape, size, position, or physical state change but the chemical composition remains the same is called a physical change. These changes are mostly reversible and temporary. 

Chemical change: A change that involves a change in composition and chemical properties of the substance that leads to the formation of a new substance is called a chemical change. These changes are irreversible and permanent.

When LPG comes out from the cylinder, its physical state changes from liquid to gas. This means that Change A is a physical change.

When we burn LPG, its chemical composition changes that lead to the formation of new substances along with heat and light. This means that Change B is a chemical change.

Hence, the correct answer is (ii) Process – B is a chemical change.


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