Science and Technology Class 9 Important Questions Chapter 13 Maharashtra Board

Carbon : An important element

Short Questions

1. What is the symbol and atomic number of carbon?

• The symbol of carbon is C, and its atomic number is 6.

2. What is the valency of carbon?

• The valency of carbon is 4, due to its electron configuration (2,4).

3. What remains after the complete combustion of an organic compound?

• Carbon dioxide (CO₂) and water (H₂O) remain after complete combustion.

4. What type of element is carbon?

• Carbon is a non-metallic element with an electron configuration of 2,4.

5. What are organic compounds?

• Organic compounds are compounds obtained from plants and animals, containing carbon.

6. What is allotropy in carbon?

• Allotropy is the property of carbon to exist in different forms like diamond and graphite.

7. What is the hardest natural substance known?

• Diamond, a crystalline allotrope of carbon, is the hardest natural substance.

8. What is the structure of graphite?

• Graphite has a layered hexagonal structure with carbon atoms bonded to three others.

9. What is fullerene?

• Fullerene is a carbon allotrope with molecules like buckyballs (C₆₀) or buckytubes.

10. What is a non-crystalline form of carbon?

• Coal is a non-crystalline form of carbon used as a fossil fuel.

11. What are hydrocarbons?

• Hydrocarbons are organic compounds made of only carbon and hydrogen, like CH₄.

12. What is a saturated hydrocarbon?

• A saturated hydrocarbon, like ethane (C₂H₆), has only single bonds between carbon atoms.

13. What is an unsaturated hydrocarbon?

• An unsaturated hydrocarbon, like ethene (C₂H₄), has at least one double or triple bond.

14. What gas is produced when coal is ignited in air?

• Carbon dioxide (CO₂) is produced when coal reacts with oxygen (C + O₂ → CO₂).

15. What is the molecular formula of methane?

• The molecular formula of methane is CH₄, with a molecular mass of 16.

16. Why is methane called marsh gas?

• Methane is called marsh gas because it is found on the surface of marshy places.

17. What is the chemical reaction for methane combustion?

• Methane burns with oxygen to form carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O.

18. What is the main component of biogas?

• Methane (CH₄) constitutes about 55-60% of biogas produced in a biogas plant.

19. How is carbon dioxide used in fire extinguishers?

• Carbon dioxide (CO₂) is used in fire extinguishers to displace oxygen and extinguish fires.

20. What is the reaction when CO₂ passes through limewater?

• Carbon dioxide reacts with limewater to form calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.

Long Questions

1. Why is carbon considered an essential element in organic compounds?

• Carbon is essential because it forms the backbone of all organic compounds due to its ability to form four covalent bonds. It is present in vital molecules like DNA, proteins, and carbohydrates.

2. How does the structure of diamond make it the hardest substance?

• In diamond, each carbon atom is bonded to four others in a three-dimensional covalent structure, making it extremely hard. This structure, with strong C-C bonds, gives diamond its high melting point (3500°C) and density (3.5 g/cm³).

3. What are the differences between diamond and graphite?

• Diamond has a three-dimensional structure, making it hard and a non-conductor, while graphite has a layered structure, making it soft and a conductor. Their properties differ due to distinct arrangements of carbon atoms.

4. How is graphite used as a conductor of electricity?

• Graphite conducts electricity because its layered structure allows free electrons to move within each layer. This property makes it useful in electrodes and pencils (C + clay mixture).

5. What are the uses of fullerene in modern technology?

• Fullerenes, like C₆₀, are used as insulators, catalysts in water purification, and exhibit superconductivity at certain temperatures. Their unique cage-like structure enables these applications.

6. Why are coal and its types considered fossil fuels?

• Coal, including peat, lignite, bituminous, and anthracite, is formed from ancient plant remains, making it a fossil fuel. It is used for electricity generation and as a fuel in industries.

7. How is methane produced in a biogas plant?

• In a biogas plant, anaerobic microbes decompose organic matter like dung to produce organic acids, which methanogenic bacteria convert into methane (CH₃COOH → CH₄ + CO₂). This methane is used as a fuel.

8. What are the physical properties of carbon dioxide?

• Carbon dioxide (CO₂) is a colorless, odorless gas with a density higher than air and is sparingly soluble in water. Its melting point is -56.6°C, and it turns limewater milky (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O).

9. How does carbon dioxide help in fire extinguishing?

• Carbon dioxide in fire extinguishers displaces oxygen, stopping combustion, and does not conduct electricity, making it safe for electrical fires. The reaction 2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂ produces CO₂ in some extinguishers.

10. Why are covalent compounds like methane poor conductors of electricity?

• Covalent compounds like methane (CH₄) lack free electrons or ions to carry an electric current, making them poor conductors. Their bonds, such as C-H in CH₄, involve electron sharing, not transfer.