Short & Long Answer Questions: Carbon & its compounds - Science Class 10

Short Answer Type Questions

Q1: A piece of black electrode used in dry cells on strong heating in the air gives a colorless gas that turns lime-water milky. What was the material of the electrode?
Answer: The piece of black electrode used in the dry cell is made of graphite (which is an allotrope of carbon element). This is confirmed by the fact that the piece of the electrode, on strong heating in air, gave a colorless gas carbon dioxide, which turned lime-water milky. Thus, the material of the electrode is graphite.

Q2: Why does graphite conduct electricity, but not a diamond?
Answer: In the case of a diamond, each carbon atom of a single crystal is surrounded by four other carbon atoms by covalent bonds such that they form four corners of a regular tetrahedron. Because of four covalent bonds with each carbon atom, there are no free electrons available. Due to the non-availability of free electrons within the crystalline structure, diamond acts as a bad conductor of electricity.

In the case of graphite, every carbon atom in a single crystal is covalently bonded to three carbon atoms. As each carbon atom has four valence electrons, one valence electron is left free for each carbon atom. These free electrons can be easily made to flow within the crystalline structure of graphite by applying electric potential. Thus, graphite is a good conductor of electricity.

Q3: Write three important uses of ethanol.
Answer: 

• Ethanol is used as a solvent to dissolve varnishes, medicines, and other organic compounds. 
• It is used as a beverage (for drinking as an intoxicant) in different forms, viz; Brandy, Whisky, etc.
• It is used for industrial uses in the name of denatured spirit.

Uses of Ethanol
Q4: State what you will observe when sugar crystals are heated strongly.
Answer: The sugar crystal will initially melt. Gradually, they turn brown and start swelling up. They will give off a large amount of steam. Finally, the black porous residue of carbon is left behind.

Q5: How are the molecules of aldehyde and Ketone structurally different?
Answer: In aldehyde, the carbon atom of the carbonyl group is attached to one alkyl group (R) and one hydrogen atom, but in ketone, the carbonyl group is attached to two alkyl groups. Aldehyde is a terminal functional group, whereas ketone is non-terminal and exists in the middle of the carbon chain.

Q6: What change has been made in the composition of detergents to make them biodegradable?
Answer: Detergents made from long-chain hydrocarbons having the minimum branching in their molecules are degraded more easily.

Q7: A hydrocarbon molecule contains 4 hydrogen atoms. Give its molecular formula, if it is an: 
(i) Alkane 
(ii) alkene
(iii) alkyne.
Answer: 
(i) An alkane containing 4 hydrogen atoms in its molecule is methane, CH₄.
(ii) An alkene containing 4 hydrogen atoms in its molecule is ethene, C₂H₄
(iii) An alkyne containing 4 hydrogen atoms in its molecule is propyne, C₃H_4.

Q8: Why is common salt added in soap making?
Answer: Common salt is added to the mixture to make the soap come out of the solution. Though most of the soap separates on its own, some of it remains in solution. Common salt is added to precipitate out all the soap from the aqueous solution.

Q9: What is meant by denatured alcohol? What is the need to denature alcohol?
Answer: The alcohol which is rendered unfit for drinking by mixing it with some poisonous substances, such as methanol, pyridine, copper sulphate, etc, is known as denatured alcohol. Ethanol is an important industrial chemical. Therefore, it is subjected to a very small excise duty. To prevent its misuse for drinking purposes, there is a need to denature alcohol.

Q10: What is meant by the term “functional group”?
Answer: A functional group in an organic compound is an atom or a group of atoms bonded together in a unique fashion, which is usually the site of chemical reactivity in an organic molecule.

Q11: Draw the structural formula of ethyne.

Answer:

Structural Formula: H – C ≡ C – H

Q12: Why are detergents better cleansing agents than soaps? Explain.
Answer: Detergents are better cleansing agents than soaps because we can use them even with hard water. Detergents have a more decisive cleansing action than soaps and are more soluble in water than soaps. The charged ends of detergents do not form insoluble precipitates with calcium and magnesium ions in hard water. When soap is used for washing clothes with hard water, it reacts with the calcium and magnesium ions of hard water to form an insoluble precipitate called scum. Scum sticks to the cloth to be washed, making cleaning clothes difficult.

Q13: Name the functional groups present in the following compounds
(a ) CH₃COCH₂CH₂CH₂CH₃
(b ) CH₃CH₂CH₂COOH
(c ) CH₃CH₂CH₂CH2CHO
(d ) CH₃CH₂OH
Answer:

(a ) A ketone functional group is present in the compound CH₃COCH₂CH₂CH₂CH₃.
(b ) A carboxylic acid functional group is present in the compound CH₃CH₂CH₂COOH.
(c ) An aldehyde functional group is present in the compound CH₃CH₂CH₂CH2CHO.
(d ) An alcohol functional group is present in the compound CH₃CH₂OH.

Q14: How is ethene prepared from ethanol? Give the reaction involved in it.
Answer: Ethanol is heated at 443 k more than concentrated sulphuric acid to obtain ethene.
CH₃CH₂OH + Conc. H₂SO₄ → CH₂ = CH₂ + H₂O

Q15: Intake of a small quantity of methanol can be lethal. Comment.
Answer: Intake of a small quantity of Methanol can be lethal as it reacts rapidly with the components of cells. It causes the protoplasm to coagulate. It also affects the optic nerve and causes blindness.

Q16: Gas is evolved when ethanol reacts with sodium. Name the gas that evolved and write the balanced chemical equation of the reaction involved.
Answer: Hydrogen gas is evolved when ethanol reacts with sodium.
2 Na + 2 CH₃CH₂OH → 2 CH₃CH₂ONa + H₂

Q17: Ethene is formed when ethanol at 443 K is heated with excess concentrated sulphuric acid. What is the role of sulphuric acid in this reaction? Write the balanced chemical equation of this reaction.
Answer: Concentrated sulphuric acid removes water from ethanol, thereby acting as a dehydrating agent.
CH₃CH₂OH + Conc. H₂SO₄ → CH₂ = CH₂ + H₂O

Q18: Catenation is the ability of an atom to form bonds with other atoms of the same element. Both carbon and silicon exhibit it. Compare the ability of catenation of the two elements. Give reasons.
Answer: Both carbon and silicon show catenation. However, compounds made with silicon are more reactive and less stable. In contrast, bonds formed by carbon are very strong, so organic compounds are more stable than silicon compounds. Thus, we can say that carbon shows better catenation than silicon.

Q19: Unsaturated hydrocarbons contain multiple bonds between the two C-atoms and show addition reactions. Give the test to distinguish ethane from ethene.
Answer: The bromine water test can be used to distinguish between saturated and unsaturated hydrocarbons. Saturated compounds don’t give an additional reaction. Hence, there won’t be any change in the reaction mixture. In contrast, if an unsaturated hydrocarbon is added to bromine water, its color will decolorise.
Saturated hydrocarbon + Br₂ → No Reaction (No Colour Change)
Unsaturated hydrocarbon + Br₂ → Reaction will occur (Decolourise)

Long Answer Type Questions

Q1: A salt X is formed, and gas is evolved when ethanoic acid reacts with sodium hydrogen carbonate. Name the salt X and the gas evolved. Describe an activity and draw a diagram of the apparatus to prove that the evolved gas is the one you have named. Also, write a chemical equation of the reaction involved.
Answer: The salt X is sodium ethanoate (CH₃COONa), and the evolved gas is carbon dioxide (CO₂).
Take a test tube and add ethanoic acid (CH₃COOH). Add sodium bicarbonate (NaHCO₃) to the acid, close the test tube’s mouth with a cork, and attach a delivery tube.
Take lime water in another test tube and attach it to the delivery tube. The lime water turns milky. This indicates that the evolved gas is carbon dioxide.
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
The milkiness is due to the formation of CaCO₃.
Reaction Involved: CH₃COOH + NaHCO₃ → CH₃COONa + H2O + CO₂ (g)

Q2: (a ) What are hydrocarbons? Give examples.
(b ) What is a functional group? Give examples of four different functional groups.
Answer:

(a ) A hydrocarbon is any of a class of organic compounds made up of carbon and hydrogen. Methane and Ethane are examples of hydrocarbons.
(b ) A functional group is the atoms group in a molecule that specifies the chemical behavior of the molecule. Atoms in a functional group are bonded by the covalent bond.

Q3: Name the reaction which is commonly used in the conversion of vegetable oils to fats. Explain the reaction involved in detail.
Answer: Vegetable oils generally have long chains of unsaturated carbons, while animal fats have long chains of saturated carbons. An addition reaction is used in the conversion of vegetable oils to fats. It is known as the hydrogenation of oil.
An addition reaction is carried out in the presence of Ni as a catalyst.

Q4: (a ) Write the formula and draw the electron dot structure of carbon tetrachloride.
(b ) What is saponification? Write the reaction involved in this process.
Answer:
(a ) The formula of carbon tetrachloride is CC₄.
Electron dot structure of Carbon tetrachloride:

(b ) Saponification is a methodology of manufacturing soap by the hydrolysis of fats or oils with a base like sodium hydroxide.
Reaction: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH

Q5: A compound C (molecular formula, C₂H₄O₂) reacts with Na – metal to form a compound R and evolves into a gas that burns with a pop sound. Compound C, on treatment with alcohol A in the presence of an acid, forms a sweet-smelling compound S (molecular formula, C₃H₆O₂). In addition to NaOH to C, it also gives R and water. S on treatment with NaOH solution gives back R and A. Identify C, R, A, and S and write down the reactions involved.
Answer: Here, compound C is ethanoic acid (CH₃COOH), compound R is sodium ethanoate (CH₃COONa), compound A is ethanol (C₂H₅OH), and compound S is ethyl ethanoate (CH₃COOC₂H₅).
Ethanoic acid (CH₃COOH) reacts with sodium metal to form sodium ethanoate (CH₃COONa).
2 CH₃COOH + 2 Na → 2 CH₃COONa + H₂
Ethanoic acid (CH₃COOH) on treatment with ethanol (C₂H₅OH) in the presence of an acid forms a sweet-smelling ethyl ethanoate (CH₃COOC₂H₅).
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
On adding NaOH to ethanoic acid (CH₃COOH), it also gives sodium ethanoate (CH₃COONa) and water.
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
Thus, compound C is Ethanoic acid.