WBBSE Class 8 School Science Chapter 4 Occurrence Of Carbon And Its Compounds In Nature SAQs

WBBSE Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Short Answer Type Questions

Question 1. What do you mean, by allotropy? Why does this occur?
Answer:

Allotropy:

The phenomenon in which some elements like carbon, sulfur, etc. exist in more than one form generally having the same chemical properties but some different physical properties in the same physical state is known as allotropy.

This occurs due to various reasons. For example, allotropes of an element differ in their molecular structure or in the relative arrangement of molecules within a crystal.

For example diamond and graphite. Sometimes it occurs due to the difference in the number of atoms present in a molecule. For example oxygen (02) and ozone (O2).

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Question 2. Schematically show the different allotropes of carbon.
Answer:

The Different Allotropes Of Carbon:

WBBSE Class 8 Carbon Compounds Short Answer Questions

Question 3. Suggest an experiment to prove that different allotropes of carbon are all made up of carbon.
Answer:

Carbon has several different allotropes such as graphite, diamond, charcoal, etc. They appear different, but one thing is common in all of them – they are all made of carbon.

If a fixed mass of all these allotropes is separately heated strongly in the presence of pure oxygen, the same mass of carbon dioxide is formed in all the cases.

Question 4. Diamond is one of the hardest substances known to us. What is the reason behind this?
Answer:

Diamond is one of the hardest substances known to us.

It is one of the hardest substances known to us. This hardness of a diamond can be related to its internal structure.

Here, every carbon atom is surrounded by four other carbon atoms. This arrangement is known as the tetrahedral arrangement. This imparts rigidity to diamonds and it is also responsible for their high density and very high melting point.

Question 5. Graphite is soft and slippery—Explain. Graphite has a layered structure where two-dimensional sheets made of carbon are arranged parallel one over the other.
Answer:

Graphite is soft and slippery:

In each two-dimensional sheet, carbon atoms are arranged in a hexagonal planar fashion. The distance between two successive layers is relatively large and hence the force of attraction between the successive layers is weak.

So when force is applied, one layer slides over the other. That is why graphite is soft and slippery.

Occurrence of Carbon Chapter 4 WBBSE Solutions

Question 6. Mention some uses of diamond and graphite. Diamond has the following uses :
Answer:

Diamond has the following uses:

1. It is used as a precious stone.
2. It is used to cut glasses.
3. It is used for making the tip of a boring drill that bores through rocks.

Graphite has the following uses:

1. It is used for making the tip of the pencil.
2. It is used as an electrode in batteries and in electrochemical cells.
3. Suspension of graphite in oil is used as a lubricant.

Question 7. Why low-grade coal is not used in thermal power plants?
Answer:

In thermal power plants, water is boiled to produce steam to rotate the turbine wheels to produce electricity. Since the calorific value of low-grade coal is quite low, hence burning low-grade coal will not produce enough heat to boil large amounts of water. So to produce a large amount of steam, a much larger amount of low-grade coal has to be burnt.

Question 8. Why dilute sulphuric acid is not used during the preparation of carbon dioxide from calcium carbonate?
Answer:

Dilute sulphuric acid is not used as a mineral acid. Initially sulphuric acid reacts with CaC03 to produce CO₂ and CaSOA.

⇒ \(\mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{CaSO}_4+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}\)

But after some time, the reaction stops. This is because dilute H.SO. reacts with CaC0o to produce calcium sulfate which has very low solubility in water and it forms an insoluble coating around the pieces of CaCOy thus preventing the reaction between dilute H2SOA and CoC03. So the formation of CO₂ is stopped.

WBBSE Class 8 Science Short Answer Format

Question 9. Briefly describe the physical properties of carbon dioxide.
Answer:

Physical properties of CO₂:

1. Carbon dioxide (CO₂) is a colorless gas having a faint pungent smell and an acidic taste.
2. It is fairly soluble in water and the solubility increases with pressure. Soda water is made by dissolving CO₂ in water under high pressure.
3. The gas is 1.5 times heavier than air. Its density is 1.958 grams per liter at 0°C and 1 atmospheric pressure.
4. It can be liquefied by application of pressure at any temperature below 31 °C.
5. The gas is not poisonous but causes suffocation in want of oxygen in an atmosphere of carbon dioxide.

Question 10. Give equations of some reactions that involve the preparation of CO₂ from metallic carbonates and metallic bicarbonates by heating.
Answer:

Metallic carbonates (except those of sodium, potassium, and barium) such as calcium carbonate (CaCOJ or metallic bicarbonates (such as calcium bicarbonate or potassium bicarbonate) decompose on heating, liberating CO₂ gas.

⇒ \(\begin{gathered}
\mathrm{CaCO}_3 \rightarrow \mathrm{CaO}+\mathrm{CO}_2 \\
\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2 \rightarrow \mathrm{CaCO}_3+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \\
2 \mathrm{KHCO}_3 \rightarrow \mathrm{K}_2 \mathrm{CO}_3+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}
\end{gathered}\)

Question 11. What do you mean by the carbon cycle?
Answer:

Carbon Cycle:

The carbon cycle is the circulation and transformation of carbon back and forth between living things and the environment. The total amount of carbon present in the earth and the earth’s atmosphere is fixed. But the amount of carbon present in different compounds is always changing.

So, in other words, the carbon cycle is the bio-geo-chemical cycle by which carbon is exchanged or cycled among earth’s oceans, atmosphere, ecosystem, and geosphere.

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Question 12. What is fullerene? Mention its uses.
Answer:

Fullerene:

Fullerene, an allotrope of carbon, is a large spherical polyhedral cluster of 60-70 carbon atoms arranged in the form of a geodesic dome by fusing pentagons and hexagons.

1. It is a brown or black powder and is soluble in common solvents like benzene, toluene, etc.
2. Uses of fullerene:
3. As industrial catalyst
4. In photovoltaic cells
5. In drug delivery system in cancer therapy

Question 13. What does CO₂ form after reacting with rain water? What is its nature?
Answer:

CO₂ dissolves in rainwater to form weak carbonic acid.

⇒ \(\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}=\mathrm{H}_2 \mathrm{CO}_3 .\)

It is a weak unstable dibasic acid that turns blue litmus dull red, but it cannot change the color of methyl orange.

Chapter 4 Occurrence of Carbon Practice Questions

Question 14. Soda acid type of fire extinguisher is not useful for fire caused by inflammable liquids-why?
Answer:

Soda acid type of fire extinguisher is not useful for fire caused by inflammable liquids

The solution coming out of the cylinder, being denser than inflammable oils like petrol, sinks in the oil and disables it to cut off the supply of air to the fire.

Hence soda acid type of fire extinguisher is not useful for fires caused by inflammable liquids.

Question 15. Fire due to the burning of metals cannot be extinguished by CO₂– why?
Answer:

Fire due to the burning of metals cannot be extinguished by CO₂

A burning Mg ribbon or metallic sodium continues to burn in an atmosphere of CO₂ with the separation of black carbon.

⇒ \(2 \mathrm{Mg}+\mathrm{CO}_2 \rightarrow 2 \mathrm{MgO}+\mathrm{C} \quad 4 \mathrm{Na}+\mathrm{CO}_2 \rightarrow 2 \mathrm{Na}, \mathrm{O}+\mathrm{C}\)

During the burning of such metals, a large amount of heat is liberated and the temperature becomes so high that CO₂ decomposes into carbon and O₂. It is the oxygen that helps in the burning of the metals.

Short Answer Solutions for Class 8 Carbon

Question 16. X is a substance that is used as a building material. It is insoluble in water. When reacted with dilute HCl, it produces a greenhouse gas (Y) that turns lime water milky. Identify X and Y.
Answer:

X is a substance that is used as a building material. It is insoluble in water.

When reacted with dilute HCl, it produces a greenhouse gas (Y) that turns lime water milky.

X is calcium carbonate (CaCOJ that exists in marble.

⇒ \(\mathrm{CaCO}_3+2 \mathrm{HCl} \rightarrow \mathrm{CaCl}_2+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2 \uparrow\)

The greenhouse gas (Y) is CO₂ which turns clear lime water milky.