Important Questions Test: Physical & Chemical Changes - Class 7 MCQ

When common salt (sodium chloride) is exposed to air, no chemical reaction occurs under normal conditions because there is no change in its chemical composition. The exposure to air does not alter the chemical structure of common salt, hence no chemical reaction takes place. In contrast, burning coal in air, placing sodium in water, and keeping iron in moist air all result in chemical reactions due to changes in chemical composition.

A physical change is when a substance changes in ways like shape, size, or color but remains the same substance. It can often be reversed, meaning you can change it back to how it was before.

• When you cut a piece of paper into smaller pieces and then try to reassemble them, it is an example of a physical change, not a chemical change.
• Physical changes are changes that do not alter the substance's identity.
• In this case, cutting and reassembling paper does not change the paper into a different substance.
• It is just changing the shape and size of the paper.

When magnesium reacts with oxygen, it forms magnesium oxide. This reaction creates a new substance called magnesium oxide by combining magnesium with oxygen.

When you mix vinegar with baking soda, it creates a fizzy sound and you see bubbles coming out. This is because they react together and make something new. It's like a fun science experiment you can try at home!

• When carbon dioxide is passed through lime water, calcium carbonate is formed. This makes the lime water look milky.
• It's like when you mix milk and water together, they create a milky liquid.
• In this case, mixing carbon dioxide with lime water creates a milky substance called calcium carbonate.

• When something burns, it usually means it's changing into something new. This change is called a chemical change.
• For example, when you burn a piece of paper, it turns into ash. This transformation from paper to ash is a chemical change because a new substance is formed.
• So, the statement is TRUE.

The chemical name of baling soda is: Sodium HydrogenCarbonate (Na₂HCO₃

- When carbon dioxide is mixed with lime water (calcium hydroxide solution), a chemical change occurs.
- This reaction produces calcium carbonate, a white precipitate, and water: