Chapter 13: Carbon: An Important Element
Solved textbook questions with step-by-step explanations.
Grade 9 Q&A: Chapter 13: Carbon: An Important Element
Welcome to the Questions and Answers section for Grade 9 Science, Chapter 13: "Carbon: An Important Element." This chapter explores the unique properties of carbon, its various forms (allotropes), the vast array of compounds it forms, and its indispensable role in life and the environment.
Important Questions and Answers
Q1: What is the atomic number and valency of carbon?
Answer: The atomic number of carbon is 6, and its valency is 4 (tetravalent).
Q2: Define allotropy. Name the crystalline allotropes of carbon.
Answer: Allotropy is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes. These allotropes have different physical properties but identical chemical properties. The crystalline allotropes of carbon are **Diamond, Graphite, and Fullerene**.
Q3: Describe the structure and properties of Diamond.
Answer:
- Structure: Each carbon atom in diamond is bonded to four other carbon atoms in a rigid, three-dimensional tetrahedral network structure.
- Properties: It is the hardest known natural substance, transparent, brilliant, has an extremely high melting point, and does not conduct electricity (insulator).
Q4: Describe the structure and properties of Graphite.
Answer:
- Structure: Carbon atoms in graphite are arranged in hexagonal rings, forming flat, two-dimensional layers. These layers are held together by weak Van der Waals forces, allowing them to slide over each other.
- Properties: It is soft, slippery, greyish-black, opaque, has a high melting point, and is a good conductor of electricity (due to free electrons between layers).
Q5: Give two uses of Diamond.
Answer: Two uses of diamond are:
- Used in jewelry due to its brilliance and hardness.
- Used in cutting tools for glass, rocks, and in drilling equipment due to its extreme hardness.
- Used as an abrasive for polishing hard surfaces.
Q6: Give two uses of Graphite.
Answer: Two uses of graphite are:
- Used as pencil leads (mixed with clay).
- Used as a lubricant in machinery due to its slippery nature.
- Used as electrodes in batteries and electric furnaces due to its electrical conductivity.
Q7: What are Fullerenes? Give an example.
Answer: Fullerenes are crystalline allotropes of carbon where carbon atoms are arranged in spherical, cylindrical, or ellipsoidal cage-like structures. They are relatively new allotropes. An example is **Buckminsterfullerene (C₆₀)**, which resembles a soccer ball.
Q8: Name some amorphous allotropes of carbon.
Answer: Some amorphous allotropes of carbon include Coal, Charcoal, Coke, and Lampblack (Soot).
Q9: Explain the property of 'Tetravalency' of carbon.
Answer: Tetravalency is the property of carbon atoms to form four covalent bonds with other atoms (carbon or other elements). This is because carbon has four valence electrons, and it needs to share four more electrons to achieve a stable octet configuration. This ability to form four bonds allows carbon to bond with a large number of other atoms, leading to a vast diversity of carbon compounds.
Q10: Explain the property of 'Catenation' of carbon.
Answer: Catenation is the unique property of carbon atoms to form strong covalent bonds with other carbon atoms. This allows carbon atoms to link together to form long straight chains, branched chains, or closed ring structures. This self-linking ability is the primary reason for the existence of an enormous number of organic compounds.
Q11: What are hydrocarbons? Name their main types.
Answer: Hydrocarbons are organic compounds that are composed solely of hydrogen and carbon atoms. Their main types are:
- Saturated Hydrocarbons (Alkanes): Contain only single bonds between carbon atoms.
- Unsaturated Hydrocarbons:
- Alkenes: Contain at least one carbon-carbon double bond.
- Alkynes: Contain at least one carbon-carbon triple bond.
Q12: Give the general formula and two examples of Alkanes.
Answer:
- General Formula: $C_nH_{2n+2}$
- Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈).
Q13: Give the general formula and two examples of Alkenes.
Answer:
- General Formula: $C_nH_{2n}$
- Examples: Ethene (C₂H₄), Propene (C₃H₆).
Q14: Give the general formula and two examples of Alkynes.
Answer:
- General Formula: $C_nH_{2n-2}$
- Examples: Ethyne (C₂H₂), Propyne (C₃H₄).
Q15: List five types of substances in daily life that are carbon compounds.
Answer: Five types of substances in daily life that are carbon compounds are:
- Fuels (e.g., petrol, diesel, LPG, CNG)
- Food (e.g., carbohydrates, proteins, fats, vitamins)
- Clothes (e.g., cotton, silk, nylon, polyester)
- Plastics (e.g., polyethylene, PVC)
- Medicines, Soaps, Detergents, Cosmetics.
Q16: What is the carbon cycle? Why is it important?
Answer: The carbon cycle is the biogeochemical cycle by which carbon moves through the Earth's atmosphere, oceans, land, and living organisms. It is important because it maintains the balance of carbon dioxide in the atmosphere, which is crucial for the greenhouse effect and maintaining Earth's temperature. It also ensures the continuous availability of carbon, an essential element for all life forms.
Q17: Name two processes that add carbon dioxide to the atmosphere in the carbon cycle.
Answer: Two processes that add carbon dioxide to the atmosphere are:
- Respiration: Released by living organisms.
- Combustion: Burning of fossil fuels (coal, oil, gas) and biomass.
- Decomposition: Breakdown of dead organic matter by microbes.
Q18: Name the process that removes carbon dioxide from the atmosphere in the carbon cycle.
Answer: **Photosynthesis** is the process by which green plants and other producers absorb carbon dioxide from the atmosphere (or dissolved in water) to produce organic compounds (food) using sunlight energy.
Q19: How does the structure of diamond make it very hard?
Answer: Diamond's extreme hardness is due to its unique structure. Each carbon atom is covalently bonded to four other carbon atoms in a regular, strong, three-dimensional tetrahedral arrangement. This forms a giant covalent network structure where all atoms are held together by strong bonds throughout the crystal, making it incredibly difficult to break.
Q20: Why does graphite conduct electricity, while diamond does not?
Answer: Graphite conducts electricity because each carbon atom in graphite is bonded to only three other carbon atoms, forming hexagonal layers. The fourth valence electron of each carbon atom is free to move between these layers, forming a 'sea' of delocalized electrons, which allows graphite to conduct electricity. In contrast, in diamond, all four valence electrons of each carbon atom are involved in strong covalent bonds, so there are no free electrons to conduct electricity.
Q21: What is the difference between crystalline and amorphous allotropes of carbon?
Answer:
- Crystalline Allotropes: Have a regular, definite, and ordered arrangement of carbon atoms in a repeating pattern (e.g., diamond, graphite, fullerene). They have sharp melting points and specific geometric shapes.
- Amorphous Allotropes: Do not have a regular or definite arrangement of carbon atoms. Their structure is irregular and disordered (e.g., coal, charcoal, coke, lampblack). They do not have sharp melting points.
Q22: Give an example of a straight-chain hydrocarbon and a ring-structure hydrocarbon.
Answer:
- Straight-chain hydrocarbon: Butane (C₄H₁₀) or Propane (C₃H₈).
- Ring-structure hydrocarbon: Cyclohexane (C₆H₁₂) or Benzene (C₆H₆).
Q23: How is charcoal produced? What is one of its important uses?
Answer: Charcoal is produced by heating wood in the absence of air, a process called destructive distillation. One important use of charcoal is as a fuel, and it is also used as an adsorbent (e.g., in water filters or gas masks) due to its porous nature.
Q24: Why is carbon considered the 'backbone' of organic chemistry?
Answer: Carbon is considered the 'backbone' of organic chemistry because of its unique properties of tetravalency (forming four bonds) and catenation (forming strong bonds with other carbon atoms to create long chains, branched chains, and rings). These properties allow carbon to form an enormous number and variety of stable compounds, which form the basis of all living organisms and many synthetic materials.
Q25: What are saturated hydrocarbons? Give an example.
Answer: Saturated hydrocarbons are organic compounds composed of carbon and hydrogen atoms, where all carbon-carbon bonds are single bonds. This means they are 'saturated' with hydrogen atoms. They are generally less reactive. Example: Methane (CH₄) or Ethane (C₂H₆).
Exercise Solutions (Based on Maharashtra Board Textbook)
Q1: Choose the correct option and rewrite the statements.
- The atomic number of carbon is 6.
- Diamond is a crystalline allotrope of carbon.
- The property of carbon atoms to form long chains with other carbon atoms is called catenation.
- Graphite is a good conductor of electricity.
- The general formula for alkanes is CnH2n+2.
Q2: Match the pairs.
| Group A | Group B |
|---|---|
| Diamond | Hardest natural substance |
| Graphite | Used in pencil leads |
| Fullerene | Cage-like structure |
| Methane | Alkane |
| Ethene | Alkene |
Q3: Give scientific reasons.
- Diamond is used in cutting tools.
Answer: Diamond is used in cutting tools (like glass cutters, rock drills, and abrasives) because it is the hardest known natural substance. Its unique three-dimensional network structure, where each carbon atom is strongly bonded to four others, gives it exceptional rigidity and resistance to scratching and wear. This extreme hardness allows it to cut through or abrade very tough materials. - Graphite is used as a lubricant.
Answer: Graphite is used as a lubricant because of its unique layered structure. Carbon atoms in graphite are arranged in flat, hexagonal layers that are held together by weak Van der Waals forces. These weak forces allow the layers to slide easily over one another. This slippery nature makes graphite an excellent solid lubricant, especially in high-temperature environments where liquid lubricants might decompose. - Carbon forms a large number of compounds.
Answer: Carbon forms an exceptionally large number of compounds due to two unique properties:- Tetravalency: Carbon has a valency of four, meaning it can form four strong covalent bonds with other carbon atoms or atoms of other elements.
- Catenation: Carbon atoms have the unique ability to form strong covalent bonds with other carbon atoms, leading to the formation of long straight chains, branched chains, and ring structures. This self-linking property allows for an immense variety and complexity of carbon compounds.
Q4: Write answers to the following questions in your own words.
- Explain the different allotropes of carbon.
Answer: Allotropes are different structural forms of the same element in the same physical state. Carbon exhibits allotropy, mainly in two forms:- Crystalline Allotropes: These have a regular and definite arrangement of carbon atoms.
- Diamond: Each carbon atom is bonded to four others in a tetrahedral network, making it extremely hard and an electrical insulator.
- Graphite: Carbon atoms form hexagonal layers held by weak forces, making it soft, slippery, and a good electrical conductor.
- Fullerene: Carbon atoms form spherical or cage-like structures (e.g., C₆₀, Buckminsterfullerene).
- Amorphous Allotropes: These forms have an irregular arrangement of carbon atoms and are generally impure. Examples include coal, charcoal, coke, and lampblack.
- Crystalline Allotropes: These have a regular and definite arrangement of carbon atoms.
- Describe the importance of carbon compounds in daily life.
Answer: Carbon compounds are indispensable in our daily lives:- Fuels: Petroleum, natural gas, coal, LPG, CNG are all carbon-based fuels that power our vehicles, homes, and industries.
- Food: All major food components like carbohydrates, proteins, fats, and vitamins are organic compounds containing carbon, essential for our survival.
- Clothes: Natural fibers (cotton, silk, wool) and synthetic fibers (nylon, polyester) are carbon compounds.
- Plastics: Most plastics (e.g., polyethylene, PVC) are synthetic polymers made from carbon compounds, used in countless products.
- Medicines: The vast majority of pharmaceutical drugs are complex organic compounds.
- Other products: Soaps, detergents, cosmetics, dyes, paints, and many industrial chemicals are also carbon-based.
- Explain the carbon cycle with a diagram.
Answer: The carbon cycle is the continuous movement of carbon atoms among the atmosphere, oceans, land, and living organisms.
**Key processes:**- Photosynthesis: Plants absorb carbon dioxide (CO₂) from the atmosphere to make food.
- Respiration: Plants and animals release CO₂ into the atmosphere during breathing.
- Decomposition: When plants and animals die, decomposers (bacteria and fungi) break down organic matter, releasing CO₂ back into the atmosphere and soil.
- Combustion: Burning of fossil fuels (formed from ancient organic matter) and biomass releases large amounts of CO₂.
- Ocean Exchange: Oceans absorb and release CO₂ from the atmosphere.
(Diagram: A simplified diagram showing arrows connecting carbon reservoirs: Atmosphere (CO₂), Plants (Photosynthesis, Respiration), Animals (Respiration, Consumption), Decomposers (Decomposition), Fossil Fuels (Formation, Combustion), Oceans (Absorption, Release).)
Q5: Numericals (from typical textbook exercises).
- If a hydrocarbon has 4 carbon atoms and is an alkane, write its molecular formula.
Solution: For alkanes, the general formula is $C_nH_{2n+2}$. Given $n=4$ (number of carbon atoms). Number of hydrogen atoms = $2n+2 = 2(4)+2 = 8+2 = 10$.
Molecular formula = C₄H₁₀ (Butane). - A compound has 3 carbon atoms and contains a double bond. Write its molecular formula.
Solution: A compound with a double bond is an alkene. For alkenes, the general formula is $C_nH_{2n}$. Given $n=3$ (number of carbon atoms). Number of hydrogen atoms = $2n = 2(3) = 6$.
Molecular formula = C₃H₆ (Propene).
References
- Maharashtra State Board Science and Technology Standard Nine Textbook (Latest Edition) - Chapter 13: Carbon: An Important Element.
- Maharashtra State Board 9th Standard Science Syllabus.
- Balbharati Science and Technology Textbook Part 1.