Chapter Exercises Solutions

Questions and Answers

1. Reaction A + B -> C

Question: A and B cannot be broken down simpler. Which statement is correct?

• A and B are Elements (since they can’t be broken down).
• C is formed from A and B chemically, so C is a Compound.
• Compounds have a fixed composition. Answer: (iv) A and B are elements, C is a compound, and has a fixed composition.

2. Assertion-Reason: Air is a Mixture

• Assertion: Air is a mixture. (True)
• Reason: A mixture is formed when two or more substances are mixed without chemical change. (True)
• Link: Does the reason explain why air is a mixture? Yes, because air is formed by mixing gases without them reacting chemically. Answer: (i) Both Assertion and Reason are true and Reason is the correct explanation.

3. Properties of Water vs Elements

Statement: Water, a compound, has different properties compared to Oxygen and Hydrogen. Justify. Justification:

1. State: Hydrogen and Oxygen are gases; Water is a liquid.
2. Combustibility: Hydrogen burns with a pop sound (fuel). Oxygen supports burning. Water puts out fire.
3. Composition: Hydrogen and Oxygen are elements. Water is a compound ($H_2O$).

4. Matching Examples

• (i) Elements - water (Compound) -> Incorrect.
• (ii) Uniform mixtures - minerals, seawater, bronze, air. -> Correct. (Minerals can be compounds or mixtures, but often uniform. Seawater, bronze, air are uniform).
• (iii) Pure substances - Sugar (Compound), Iron (Element), Oxygen (Element). -> Correct.
• (iv) Non-uniform - Air (Uniform) -> Incorrect. Answer: (iii) Pure substances— carbon dioxide, iron, oxygen, sugar. (ii) is also largely correct depending on the mineral definition, but (iii) is definitively chemically correct.

5. Iron Oxide and Magnesium Oxide Reactions

• Iron (Element) + Oxygen/Moisture (Mixture) $\rightarrow$ Rust/Iron Oxide (Compound).
• Magnesium (Element) + Oxygen (Element) $\rightarrow$ Magnesium Oxide (Compound). Justification: Elements cannot be broken down. Compounds are formed by chemical combination. Air is a mixture of gases.

6. Classification Table

Pure Substances: All Elements + All Compounds listed above.

7. Heating Iron and Sulfur

• Substance Formed: Iron Sulfide ($FeS$).
• Difference: It is a black solid, non-magnetic, and properties are different from iron or sulfur.
• Equation: Iron + Sulfur $\xrightarrow{\text{Heat}}$ Iron Sulfide.

8. Element AND Compound?

Answer: No. A substance is either an Element (one type of atom) or a Compound (different atoms bonded). It cannot be both. However, both are Pure Substances.

9. Water as a Mixture?

If water were a mixture of H and O:

1. It would be a gas (mixture of two gases).
2. Lighting a match near “water” would cause an explosion (Hydrogen burns!).
3. Life as we know it would not exist because liquid water is essential for biological processes.

10. Fig 8.24 Analysis (Gas A)

• Reaction: Iron + Dilute HCl $\rightarrow$ Iron Chloride + Hydrogen Gas.
• Gas A: Hydrogen.
• Word Equation: Iron + Hydrochloric Acid $\rightarrow$ Iron Chloride + Hydrogen.

11. Compounds from Non-metals

1. Water ($H_2O$): Drinking, Washing.
2. Carbon Dioxide ($CO_2$): Photosynthesis, Fire Extinguishers, Carbonated Drinks.

12. Gold: Mineral and Metal?

• Metal: Chemically, Gold ($Au$) is a metal (shiny, conductor).
• Mineral: Geologically, it is found naturally in the earth’s crust in its native form (Native Gold). Since minerals are naturally occurring solid substances, native Gold is considered a mineral.