Class 10 Science Chapter 4 Question Answers - Carbon and its compounds

Q1: Why does graphite conduct electricity, but not a diamond?
Answer: Graphite is a crystalline allotropic form of carbon. It is a soft black lustrous form of carbon that conducts electricity.

• In graphite, each carbon atom is bonded to three other carbon atoms through strong bonds. The fourth electron, which is not involved in these bonds, is free to move around. This free movement of the fourth electron allows graphite to conduct electricity.
• In contrast, diamonds, with a tetrahedral carbon bonding arrangement, lack free electrons for conductivity, making them poor conductors.
• The distinct atomic structures determine their electrical properties, making graphite a conductor and diamond a bad conductor of electricity.

Q2: 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, eg. Brandy, Whisky, etc.
• It is used for industrial uses in the name of denatured spirit.

Uses of EthanolQ3: 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. The substance is known as Charred sugar. This is a chemical change.

C₁₂H₂₂O₁₁ + heat → 12 C + 11 H₂O + other compounds 

Q4: 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.

Q5: 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.

Q6: 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.  This is because soap is less soluble in salt water than in pure water. When salt is added, the soap molecules aggregate and form solid clumps, which can be separated from the liquid. This process is called salting out.
The chemical reaction for soap making is:

Fat+NaOH→Soap+Glycerol
This is an example of a saponification reaction, where a fat or oil reacts with a base to produce soap and glycerol.

Q7: 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.

Q8: 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.

Q9: Draw the structural formula of ethyne.

Answer: Ethyne is the simplest alkyne with the chemical formula C2H2. It has a triple bond between two carbon atoms and a single bond with one hydrogen atom on each carbon. The structural formula of ethyne is:
H – C ≡ C – H

Q10: Why are detergents better cleansing agents than soaps? Explain.
Answer: Detergent and soap are both cleaning agents with distinct properties. Detergent, the sodium salt of a long-chain benzene sulfonic acid, is effective in hard water and acidic solutions. This makes it a preferred choice over soap, which, being the sodium and potassium salts of long-chain fatty acids, tends to form scums in hard water and decompose in acidic solutions, making it less versatile for washing. Detergents, therefore, offer better cleaning efficiency in various water conditions, making them a more practical option for everyday use.

Q11: 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.

Q12: Draw the electron dot structure of ethyne and also draw its structural formula.
Ans: Ethyne, C₂H₂

Q13: Draw the structures of diamond and graphite.
Ans: In diamond, each carbon atom is bonded to four other carbon atoms forming a rigid three dimensional structure.
In graphite, each carbon atom is bonded to three other carbon atoms in the same plane giving a hexagonal array. One of these bonds is a double bond.

Q14: 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.

Q15: 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)

Q16: 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)

Q17: (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.
Examples of functional groups

• Alcohol: An alcohol is an organic compound having an aliphatic carbon atom having the hydroxyl,
  - OH functional group.
• Ketone: A ketone is a functional group with a carbonyl carbon    .
• Carboxylic: An organic acid with a carboxyl group is known as a carboxylic acid - COOH.
• Aldehyde: An aldehyde is a functional group with a carbonyl carbon,  

 Q18: 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.

Q19: (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

Q20: 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.