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All questions of Carbon and Its Compounds for Class 8 Exam

Which type of coal is known as "brown coal" and has a lower carbon content?
  • a)
    Bituminous
  • b)
    Peat
  • c)
    Anthracite
  • d)
    Lignite
Correct answer is option 'D'. Can you explain this answer?

Diya Menon answered
Understanding Brown Coal
Brown coal is commonly referred to as lignite, which is a type of coal that is characterized by its lower carbon content and distinct properties.
Characteristics of Lignite (Brown Coal)
- Carbon Content: Lignite has a carbon content of about 25-35%, which is significantly lower than that of higher-ranked coals like bituminous and anthracite.
- Moisture Content: Lignite contains a high moisture content, often between 30-60%. This moisture affects its energy density and makes it less efficient compared to harder coals.
- Color and Texture: Lignite is usually brownish in color and has a soft, crumbly texture, making it easily breakable.
Formation and Uses
- Formation: Lignite is formed from peat, the precursor to coal, through geological processes that involve heat and pressure over millions of years. It is the earliest stage in the coal formation process.
- Uses: Due to its lower energy output, lignite is primarily used for electricity generation in power plants and is often found close to surface deposits, making it easier and cheaper to mine.
Comparison with Other Coal Types
- Bituminous Coal: Contains more carbon (45-86%) and is used for electricity generation and steel production.
- Anthracite: Contains the highest carbon content (86-97%) and is known for its high energy output and burning efficiency.
In summary, lignite or brown coal is distinguished by its lower carbon content, higher moisture, and unique formation. It serves important functions, especially in energy production, despite its limitations compared to higher-ranked coals.

Which of the following statements about carbon dioxide is true?
  • a)
    It is a colorless, odorless gas and does not support combustion.
  • b)
    It is a solid at room temperature.
  • c)
    It is a toxic gas that can cause suffocation.
  • d)
    It is lighter than air and rises in the atmosphere.
Correct answer is option 'A'. Can you explain this answer?

Nishtha Iyer answered
Understanding Carbon Dioxide
Carbon dioxide (CO2) is an essential gas in our atmosphere, and several key facts clarify its characteristics.
Colorless and Odorless
- CO2 is indeed a colorless and odorless gas, which means it cannot be seen or smelled in its natural state.
- This property makes it less noticeable in the environment, unlike other gases that might have distinct odors.
Non-flammable Nature
- One of the most important properties of carbon dioxide is that it does not support combustion.
- This means that fire cannot burn in the presence of CO2, making it a useful gas in fire extinguishers.
State at Room Temperature
- Contrary to option b, carbon dioxide is not a solid at room temperature.
- It exists as a gas under standard atmospheric conditions. However, it can be found in solid form as dry ice, but only at very low temperatures.
Toxicity and Suffocation
- While CO2 is not classified as a toxic gas in the same way that carbon monoxide is, high concentrations can lead to suffocation.
- This occurs because CO2 displaces oxygen in the air, leading to insufficient oxygen for breathing.
Density Compared to Air
- Option d is also incorrect as carbon dioxide is denser than air, causing it to settle rather than rise in the atmosphere.
In conclusion, the correct answer is option 'A': Carbon dioxide is a colorless, odorless gas that does not support combustion, making it a unique and important component of our atmosphere.

What type of bond is formed by the sharing of electrons between carbon atoms?
  • a)
    Covalent bond
  • b)
    Ionic bond
  • c)
    Hydrogen bond
  • d)
    Metallic bond
Correct answer is option 'A'. Can you explain this answer?

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A covalent bond is formed when atoms share electrons. Carbon's ability to form four covalent bonds enables it to create complex organic molecules, which are essential for life.

What type of carbon is primarily used as a reducing agent in metal extraction?
  • a)
    Graphite
  • b)
    Coke
  • c)
    Diamond
  • d)
    Charcoal
Correct answer is option 'B'. Can you explain this answer?

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Coke is primarily used as a reducing agent in the extraction of metals from their ores. Its high carbon content allows it to react with metal oxides, removing oxygen and yielding pure metals.

Which form of carbon is considered the hardest natural substance known?
  • a)
    Fullerene
  • b)
    Diamond
  • c)
    Charcoal
  • d)
    Graphite
Correct answer is option 'B'. Can you explain this answer?

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Diamond is recognized as the hardest natural substance due to its crystalline structure, where each carbon atom is tetrahedrally bonded to four other carbon atoms. This rigid arrangement makes diamond incredibly durable and resistant to scratching.

What is the chemical formula for carbon monoxide?
  • a)
    CO2
  • b)
    C3O2
  • c)
    C2O
  • d)
    CO
Correct answer is option 'D'. Can you explain this answer?

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The chemical formula for carbon monoxide is CO, indicating that it consists of one carbon atom and one oxygen atom. It is a colorless, odorless gas that is highly toxic and produced by incomplete combustion of carbon-containing fuels.

Which of the following carbon forms is primarily used in pencils?
  • a)
    Diamond
  • b)
    Charcoal
  • c)
    Graphite
  • d)
    Coal
Correct answer is option 'C'. Can you explain this answer?

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Graphite is primarily used in pencils because it is soft and can leave a mark on paper. Its layered structure allows the sheets to slide over each other easily, which is ideal for writing.

Which of the following is a property of diamond?
  • a)
    Soft and malleable
  • b)
    High melting point
  • c)
    Good conductor of electricity
  • d)
    Soluble in water
Correct answer is option 'B'. Can you explain this answer?

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Diamond possesses a very high melting point, around 3500°C, due to the strong covalent bonds between carbon atoms in its structure. This makes it suitable for high-temperature applications.

Which type of coal has the highest carbon content and burns with very little smoke?
  • a)
    Peat
  • b)
    Bituminous
  • c)
    Lignite
  • d)
    Anthracite
Correct answer is option 'D'. Can you explain this answer?

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Anthracite is the highest grade of coal, containing over 90% carbon. It burns efficiently and produces minimal smoke, making it a preferred choice for heating and industrial processes.

What is the process called when carbon compounds are broken down by heat in the absence of air?
  • a)
    Carbonization
  • b)
    Sublimation
  • c)
    Destructive distillation
  • d)
    Combustion
Correct answer is option 'C'. Can you explain this answer?

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The process of breaking down carbon compounds through heating in the absence of air is known as destructive distillation. This method is used to produce various forms of carbon, including charcoal and coke.

What is one major environmental concern associated with increased carbon dioxide levels in the atmosphere?
  • a)
    Decreased nitrogen levels
  • b)
    Decreased plant growth
  • c)
    Global warming
  • d)
    Increased oxygen levels
Correct answer is option 'C'. Can you explain this answer?

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The increase in carbon dioxide levels in the atmosphere is a significant contributor to global warming. CO2 is a greenhouse gas that traps heat, leading to changes in climate patterns and environmental conditions.

Which of the following gases is produced when carbon burns in sufficient oxygen?
  • a)
    Carbon dioxide
  • b)
    Nitrogen dioxide
  • c)
    Methane
  • d)
    Carbon monoxide
Correct answer is option 'A'. Can you explain this answer?

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When carbon burns in sufficient oxygen, it produces carbon dioxide (CO2). This gas is a significant contributor to the greenhouse effect and is vital for photosynthesis in plants.

Which carbon allotrope is known for its spherical structure and consists of 60 carbon atoms?
  • a)
    Charcoal
  • b)
    Diamond
  • c)
    Graphite
  • d)
    Buckminsterfullerene
Correct answer is option 'D'. Can you explain this answer?

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Buckminsterfullerene, commonly referred to as C60, is a unique allotrope of carbon with a spherical structure. Discovered in 1985, it has potential applications in materials science and nanotechnology.

What is the primary reason for carbon's ability to form a vast number of compounds?
  • a)
    Its valency of 4
  • b)
    Its position in the periodic table
  • c)
    Its ability to form ionic bonds
  • d)
    Its high atomic weight
Correct answer is option 'A'. Can you explain this answer?

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Carbon's ability to form a large variety of compounds is largely attributed to its valency of 4, which allows it to form stable covalent bonds with many other elements, including itself. This unique property enables carbon to create long chains and complex structures, leading to millions of known compounds.

What characteristic of charcoal makes it useful as an absorbent?
  • a)
    Its chemical composition
  • b)
    Its color
  • c)
    Its porous structure
  • d)
    Its density
Correct answer is option 'C'. Can you explain this answer?

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Charcoal's porous structure allows it to trap gases, liquids, and solids on its surface, making it an effective absorbent. This property is utilized in various applications, including air and water purification.

What is a common use of activated charcoal?
  • a)
    As a food preservative
  • b)
    In jewelry making
  • c)
    As a fuel
  • d)
    In water purification
Correct answer is option 'D'. Can you explain this answer?

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Activated charcoal is widely used in water purification due to its high surface area and porous structure, which allows it to adsorb impurities and contaminants effectively, making water safe to drink.

What happens to carbon dioxide when it is passed through lime water?
  • a)
    It evaporates
  • b)
    It turns the lime water milky
  • c)
    It turns the lime water clear
  • d)
    It forms a solid precipitate
Correct answer is option 'B'. Can you explain this answer?

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When carbon dioxide is passed through lime water, it reacts with calcium hydroxide to form calcium carbonate, which is insoluble in water and causes the lime water to turn milky. This reaction is often used as a test for the presence of CO2.

What is the main use of carbon dioxide in the baking industry?
  • a)
    To make the dough rise
  • b)
    As a preservative
  • c)
    As a flavor enhancer
  • d)
    To create a sour taste
Correct answer is option 'A'. Can you explain this answer?

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In the baking industry, carbon dioxide is used to make dough rise. When baking soda reacts with acidic components like vinegar, it produces CO2, which forms bubbles and causes the dough to expand, creating light and airy baked goods.

Which compound is formed when carbon reacts with a limited amount of oxygen?
  • a)
    Hydrocarbons
  • b)
    Carbon monoxide
  • c)
    Methane
  • d)
    Carbon dioxide
Correct answer is option 'B'. Can you explain this answer?

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When carbon reacts with a limited amount of oxygen, it forms carbon monoxide (CO). This gas is particularly dangerous because it can prevent oxygen from binding with hemoglobin in the blood, leading to asphyxiation.

Which process describes the formation of coal from ancient plant matter?
  • a)
    Fermentation
  • b)
    Photosynthesis
  • c)
    Carbonization
  • d)
    Destructive distillation
Correct answer is option 'C'. Can you explain this answer?

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Carbonization is the slow process through which ancient plant matter transforms into coal over millions of years. This process involves heat and pressure that gradually convert organic material into carbon-rich coal.

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