NCERT Solutions for Class 10 Science
Chapter 4: Carbon and its Compounds
NCERT Questions
Question 1
What would be the electron dot structure of carbon dioxide which has the formula \( CO_2 \)?
Answer: In order to complete its octet, the carbon atom shares its four valence electrons with the four electrons of two oxygen atoms, forming double bonds between the carbon atom and each oxygen atom.
Explanation: The atomic number (Z) for carbon is 6 and its electronic configuration is 2, 4. Carbon has four valence electrons. Each oxygen atom (Z = 8) has six valence electrons (2, 6). By sharing electrons, all atoms achieve a stable noble gas electron configuration.
Question 2
What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur?
Answer: Each sulphur atom is linked to similar atoms on either side by single covalent bonds to complete its octet. The \( S_8 \) molecule is in the form of a ring, often represented by a crown shape.
Explanation: The atomic number (Z) of sulphur is 16 and its electronic configuration is 2, 8, 6. The sulphur atom has six valence electrons. It needs two more electrons to complete its octet, which it gets by forming single bonds with two adjacent sulphur atoms.
Question 3
How many structural isomers can you draw for pentane?
Answer: There are exactly three structural isomers for pentane.
Explanation: Pentane (\( C_5H_{12} \)) has a skeleton of five carbon atoms. It can exist as a straight chain (n-pentane) as well as two branched chains (isopentane and neopentane).
Question 4
What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer: The two properties are Catenation and Tetravalency (Linking of carbon with other atoms).
Explanation:
(a) Catenation: Carbon has the unique property of self-linking. Any number of carbon atoms can be linked to one another by covalent bonds due to the stability of the C—C bonds (since the carbon atom is small).
(b) Tetravalency: Carbon is tetravalent and can readily unite with atoms such as hydrogen, oxygen, nitrogen, sulphur, etc. by electron sharing.
Question 5 & 6
What will be the formula of cyclopentane? Are structural isomers possible for bromopentane?
Answer: Cyclopentane is a cyclic compound with the formula \( C_5H_{12} \) (as per the source reference). Yes, structural isomers are possible for bromopentane.
Explanation: Bromopentane has a chain of five carbon atoms. It can exist in a number of forms known as structural isomers. They can be position isomers (differing in the position of the Br atom) or chain isomers (differing in the arrangement of the carbon skeleton).
Question 7
How would you name the following compounds based on IUPAC guidelines? (a) A 2-carbon chain with a bromine atom, (b) A 6-carbon chain with a triple bond at the first position, (c) A 1-carbon aldehyde.
Answer:
(a) Bromoethane
(b) Hex-1-yne
(c) Methanal
Explanation: IUPAC nomenclature uses standard prefixes for functional groups. Bromo is a prefix used for halogens. For alkynes, the suffix '-yne' is used along with the position number. For aldehydes, the suffix '-al' is used.
Question 8
Why is the conversion of ethanol into ethanoic acid an oxidation reaction?
Answer: It is considered an oxidation reaction because oxygen is gained and hydrogen is lost during the conversion.
Explanation: Ethanoic acid (\( CH_3COOH \)) has one oxygen atom more and two hydrogen atoms less than ethanol (\( C_2H_5OH \)). In chemistry, the loss of hydrogen or the gain of oxygen is defined as oxidation.
Question 9
A mixture of ethyne and oxygen is used for welding. Can you tell why a mixture of ethyne and air is not used?
Answer: A mixture of ethyne and air is not used because air contains a large amount of nitrogen, which does not support combustion. Pure oxygen ensures complete combustion of ethyne.
Explanation: When ethyne is burnt in oxygen, a large quantity of heat along with light is produced. This high temperature is needed to weld pieces of iron. Air is mostly nitrogen (4 parts) and only 1 part oxygen, which lowers the flame temperature drastically. Therefore, oxygen is used instead.
Question 10
How would you distinguish experimentally between an alcohol and a carboxylic acid?
Answer: They can be distinguished using the Litmus test and the Sodium hydrogen carbonate (\( NaHCO_3 \)) test.
Explanation:
(a) Litmus test: Carboxylic acid will change blue litmus paper to red. Alcohol is neutral and causes no color change.
(b) Sodium hydrogen carbonate test: Adding solid \( NaHCO_3 \) to carboxylic acid produces a brisk effervescence due to the release of carbon dioxide bubbles. Alcohol will show no such reaction.
Question 11
What are oxidising agents?
Answer: Oxidising agents are substances which either on their own or on reacting with another substance release oxygen in order to carry out oxidation reactions.
Explanation: Commonly used oxidising agents include ozone, bromine water, a mixture of potassium dichromate and sulphuric acid, or a mixture of potassium permanganate and sulphuric acid.
Question 12
Will you be able to check if water is hard by using a detergent?
Answer: No, it is not possible to check for hard water using a detergent.
Explanation: Detergents produce foam in any type of water, whether hard or soft. Therefore, a distinction between the two cannot be made with detergent. Soaps, however, form scum in hard water and can be used for this purpose.
Question 13
Why is agitation necessary to get clean clothes while washing?
Answer: Agitation helps in loosening the bonds between the dust or oil particles and the fabric of the clothes.
Explanation: The purpose of soap or detergent is to reduce friction between oil drops carrying dirt particles and water so they mix. Beating the clothes, scrubbing, or machine agitation physically forces the loosened dirt away from the fabric so it can be washed away.
Chapter End Questions
Question 1
Ethane, with the molecular formula \( C_2H_6 \) has:
(a) 6 covalent bonds
(b) 7 covalent bonds
(c) 8 covalent bonds
(d) 9 covalent bonds
Answer: (b) 7 covalent bonds
Explanation: Ethane has one carbon-carbon (C-C) covalent bond and six carbon-hydrogen (C-H) covalent bonds, totaling 7 covalent bonds.
Question 2
Butanone is a four carbon compound with the functional group:
(a) carboxylic acid
(b) aldehyde
(c) ketone
(d) alcohol
Answer: (c) ketone
Explanation: The suffix "-one" is used for the ketone functional group in IUPAC nomenclature.
Question 3
While cooking, if the bottom of the vessel is blackened on the outside, it means that:
(a) the food is not cooked completely
(b) the fuel is not burning completely
(c) the fuel is wet
(d) the fuel is burning completely.
Answer: (b) the fuel is not burning completely
Explanation: Incomplete combustion produces unburnt carbon particles (soot) which stick to the bottom of vessels, turning them black.
Question 4
Explain the nature of the covalent bond using the bond formation in \( CH_3Cl \).
Answer: In \( CH_3Cl \), carbon forms single covalent bonds by sharing three valence electrons with three hydrogen atoms and one valence electron with a chlorine atom.
Explanation: The molecule of chloromethane consists of Carbon (Z = 6, config: 2,4), Hydrogen (Z = 1), and Chlorine (Z = 17, config: 2,8,7). To complete its octet, carbon shares its 4 valence electrons with the 3 hydrogen atoms and the 1 chlorine atom, resulting in 4 covalent bonds.
Question 5 & 6
What is a homologous series? Explain with an example.
Answer: A homologous series is a series of carbon compounds that have different numbers of carbon atoms but contain the same functional group and can be represented by the same general formula.
Explanation: Compounds of a homologous series differ by a \( CH_2 \) unit from their consecutive members and show similar chemical and physical properties. For example, alkanes (methane, ethane, propane, butane) belong to the same homologous series.
Question 7
How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?
Answer: They can be differentiated by their smell, boiling points, litmus test, and reaction with bases like sodium hydrogen carbonate.
Explanation:
Physical: Ethanol has a pleasant alcoholic smell and a boiling point of 351 K. It is neutral to litmus. Ethanoic acid has a vinegar-like smell, a boiling point of 391 K, and turns blue litmus red.
Chemical: Ethanoic acid reacts with \( NaHCO_3 \) to evolve carbon dioxide gas (brisk effervescence) and reacts with alkalis (NaOH/KOH). Ethanol does neither.
Question 8
Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?
Answer: Micelle formation occurs in water because soap molecules have a hydrophobic (water-repelling) tail and a hydrophilic (water-attracting) head. Micelles will not form in ethanol.
Explanation: In water, the nonpolar alkyl tails point inwards towards oil drops, while the hydrophilic carboxylate heads face outwards towards the water, creating a stable micelle bridge. In organic solvents like ethanol, soap is completely soluble, so micelles do not form.
Question 9
Why are carbon and its compounds used as fuels in most cases?
Answer: Carbon and its compounds burn in oxygen to form carbon dioxide and water, in reactions that are highly exothermic (releasing large amounts of heat and light).
Explanation: Most common fuels like coal, methane, propane, butane, petrol, and kerosene are rich in carbon and hydrogen. Their combustion provides excellent heat energy, making them efficient fuels.
Question 10
Explain the formation of scum when hard water is treated with soap.
Answer: Scum is formed when soap (sodium or potassium salts of fatty acids) reacts with calcium and magnesium ions present in hard water to form insoluble precipitates.
Explanation: Hard water contains dissolved \( Ca^{2+} \) and \( Mg^{2+} \) salts. When soap is added, it undergoes a displacement reaction to form calcium and magnesium salts of fatty acids, which do not dissolve and float as a white precipitate known as scum.
Question 11
What change will you observe by testing soap with litmus paper (blue or red)?
Answer: Soap solution will change the colour of red litmus paper to blue.
Explanation: When soap is dissolved in water, the resulting solution is alkaline in nature due to the formation of strong alkalis like NaOH or KOH during hydrolysis. It has no effect on blue litmus.
Question 12
What is hydrogenation? What is its industrial application?
Answer: Hydrogenation is an addition chemical reaction where molecular hydrogen (\( H_2 \)) is added to an unsaturated compound in the presence of a catalyst like nickel, palladium, or platinum.
Explanation: Industrially, catalytic hydrogenation is used to convert unsaturated alkenes or aromatics into saturated alkanes (paraffins) or cycloalkanes, reducing their toxicity and reactivity. It is heavily utilized in petrochemical processes.
Question 13
Which of the listed hydrocarbons undergo addition reactions: \( C_2H_6 \), \( C_3H_8 \), \( C_3H_6 \), \( C_2H_2 \), and \( CH_4 \)?
Answer: \( C_3H_6 \) (propene) and \( C_2H_2 \) (ethyne) will undergo addition reactions.
Explanation: Only unsaturated hydrocarbons (containing double or triple bonds) undergo addition reactions. Alkanes like \( C_2H_6 \), \( C_3H_8 \), and \( CH_4 \) are already saturated and will not undergo addition.
Question 14
Give a test that can be used to differentiate between butter and cooking oil.
Answer: The Bromine water test can be used.
Explanation: Cooking oil is unsaturated (contains double bonds) and will discharge the yellow/brown color of bromine water. Butter is composed of saturated fats and will not react or change the color of bromine water.
Question 15
Explain the mechanism of cleansing action of soap.
Answer: Soap cleanses by forming spherical clusters called micelles that trap dirt and oil.
Explanation: Soap molecules have two parts: a long nonpolar hydrocarbon tail (hydrophobic) and a negatively charged head (hydrophilic). In water, the hydrophobic tails attach themselves to oily or dirty parts, enveloping the dirt inside the micelle. The hydrophilic heads face outwards, making the entire micelle soluble in water so it can be washed away.