NCERT Solutions for class 10th Science Chapter 4 Carbon and its Compounds

Question 1.What would be the electron dot structure of carbon dioxide which has the formula CO2?

Sol.The electron dot structure of CO2 is

Note: Electron dot structures also known as Lewis structures or Lewis electron dot structures, are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule.

Question 2.What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur? (Hint- The eight atoms are joined together in the form of a ring.)

Sol. Sulphur (atomic number= 16) has electronic configuration (2, 8, 6). It has six electrons in its valence shell. It requires two more electrons to attain the stable electronic configuration of its nearest noble gas i.e., argon (Ar). Each sulphur atom completes its octet by sharing two of its valence electrons with two separate sulphur atoms forming a eight – membered ring.

Question 3. How many structural isomers can you draw for pentane ?

Sol. Pentane has three structural isomers. Out of these one is a straight chain compound and two are branched chain compounds.

Note: Structural isomers are molecules which have the same molecular formula but have different arrangement of the atoms in space.

Question 4. What are the two properties of carbon which lead to the large number of carbon compounds we see around us ?

Sol. Carbon form large number of compounds due to the properties like Catenation and Tetravalency.
(i) Catenation: It is the property of element due to which it forms covalent bonds with other atoms of same element to form chain of atoms.For example, carbon shows the property of catenation. Carbon atoms can combine to each other to form long chains, branched chains and closed rings.
(ii) Tetravalency: Carbon has valency 4 so it is capable of bonding with 4 other carbon atoms or atoms of other compounds giving rise to compounds with specific properties depending on the elements present in the compound.

Question 5. What will be the formula and electron dot structure of cyclopentane ?

Sol. Cyclopentane is a cyclic compound with the formula C5H10. The structure of the compound may be represented as:

Question 6. Draw the structures of the following compounds.
(i) Ethanoic acid (ii) Bromopentane
(iii) Butanone (iv) Hexanal
Are structural isomers possible for bromopentane?

Sol. Structures of

Structural isomers for bromopentane: Yes, structural isomers are possible for bromopentane. There are three structural isomers for bromopentane depending on the position of Br at carbon I, 2, 3. Position 4 and 5 are same as 1, 2.

Question 7. How would you name the following compounds?

Sol.(i) Bromoethane (ii) Methanal (iii) Hex-1-yne.

Question 8. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

Sol. The conversion of ethanol to ethanoic acid is an oxidation reaction because it involves addition of oxygen to alcohol (ethanol) to convert it to ethanoic acid

Question 9. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

Sol. Ethyne is an unsaturated hydrocarbon. On combustion in air, it produces a yellow flame with lots of black smoke. Due to this incomplete combustion, heat produced is low and temperature obtained is also low. For welding purposes, high temperature (3000° C) is required. When a mixture of ethyne and oxygen is used, complete combustion is achieved and very high temperature required for welding is obtained. Hence, a mixture of ethyne and oxygen is used for welding instead of a mixture of ethyne and air.

Question 10.How would you distinguish experimentally between an alcohol and a carboxylic acids ?

Sol. The distinction can be made by following tests :
(i) Sodium bicarbonate test: Take both the compounds in two separate test
tubes. Now, add a small amount of solid sodium bicarbonate to both the test tubes. Brisk effervescence due to evolution of CO2 is noticed in test tube containing acetic acid (carboxylic acids) while no such
effervescence is noticed in other test tube containing alcohol

(ii) By Litmus test: Dip a strip of blue litmus separately in both the test-tubes containing carboxylic acid and alcohol. The colour changes from blue to red only in test-tube containing acid. Alcohol will not effect on litmus paper.

Question 11. What are oxidising agents?

Sol.The substances which can oxidise other substances by providing oxygen are called oxidising agents. E.g., alkaline potassium permanganate solution & acidified potassium dichromate solution are strong oxidising agent. Ozone, bromine water, mixture ofKMnO4 and H2S0 4 also act as oxidising agents.

Question 12. Would you be able to check if water is hard using a detergent?

Sol. Detergents are ammonium or sulphonate salts oflong chain carboxylic acids. They give a good amount oflather irrespective of whether the water is hard or soft. This means that detergents can be used in both soft and hard water. Therefore, it cannot be used to check whether the water is hard or not.

Question 13. People use a variety of methods to wash clothes. Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?

Sol. A soap molecule has two ends-one-end are hydrophobic (soap molecules note water) and the others end is hydrophilic (molecules lone water). With the help of these, it attaches to the grease or dirt particle and forms a cluster called micelle. These micelles remain suspended as a colloid. To remove these micelles (entrapping the dirt), it is necessary to agitate clothes.

Excercise:

Question 1. Ethane with the molecular formula C2H6 has
(a) 6 covalent bonds (b) 7 covalent bonds
(c) 8 covalent bonds (d) 9 covalent bonds

Sol. (b) Ethane has the following structure:

All the bonds are covalent. Thus, it has 7 covalent bonds.
Note: A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms.

Question 2. Butanone is four-carbon compound with the functional group
(a) carboxylic acid (b) aldehyde
(c) ketone (d) alcohol

Sol.(c) The structure of butanone is:

Question 3. While cooking, if the bottom of the vessel is getting blackened on the outside, it means that :
(a) the food is not cooked completely.
(b) the fuel is not burning completely.
(c) the fuelis wet.
(d) the fuel is burning completely.

Sol. (b) As the fuel is not burning completely, so carbon particles gets deposited on the bottom of the vessel. That is the reason why bottom of the vessel got blackened from outside while cooking.

Question 4. Explain the nature of the covalent bond using the bond formation in CH3CI.

Sol. In chloromethane (CH3Cl), carbon atom has four valence electrons (Z = 6; i.e., 2, 4) whereas each hydrogen atom has 1 electron (Z = 1), chlorine atom have 7 valence electron (Z = 17, i.e., 2, 8, 7). In order to complete their octet, carbon shares its three valence electrons with three hydrogen atoms and one of its valence electron with chlorine atom. Since, bonds are formed because of sharing of electrons, hence these are covalent bonds.

Question 5. Draw the electron dot structure for
(a) Ethanoic acid (b) H2S
(c) Propanone (d) F2

Sol. (a) Ethanoic acid


Question 6. What is an homologous series? Explain with an example.

Sol. Homologous series is a series of compounds which have same functional group but have different number of carbon atoms.
Example: Methane, ethane, propane, butane, etc. are all part of the alkane homologous series. The general formula of this series is

Question 7. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?

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?

Sol. Micelles are lipid molecules that arrange themselves in a spherical form in aqueous solution. It formation is a response to the amphipathic nature of fatty acids. The molecules of soap are sodium or potassium salts of long-chain carboxylic acids. The acid end of soap dissolves in water while the carbon chain dissolves in oil. When soap is added to water, the hydrophilic (acid end) end will align along the surface of water and the hydrophobic tail (carbon chain) remains out of water. This cluster of molecules in which hydrophobic tails are in the interior of cluster and the ionic ends are on the surface of cluster is called micelle.

No, micelle formation will not be possible in ethanol because the alkyl chain of soap becomes soluble in alcohol.
Ethanol is not as polar as soap, so micelles is not formed in other solvents such as ethanol.

Question 9. Why are carbon and its compounds used as fuels for most applications?

Sol. Carbon and its compounds are used as fuels for most applications because they produce a large amount of heat and light, when burnt in air. Saturated hydrocarbons bum with clean flame and no smoke is produced.

Question 10. Explain the formation of scum when hard water is treated with soap.
Sol. On adding soap to hard water, the soluble calcium and magnesium salts present in hard water react with soap to give insoluble calcium salt of soap which produces scum.

Question 11. What change will you observe if you test soap with litmus paper (red and blue)?

Sol. Soap solution will turn red litmus paper blue because soap is alkaline in nature. It has no action on blue litmus paper.
Question 12. What is hydrogenation ? What is its industrial application ?

Sol. Unsaturated hydrocarbons undergo addition reaction with hydrogen in presence of catalysts like Pd, Pt or Ni, such a process is known as hydrogenation.

This reaction is used for the hydrogenation of vegetable oil into vanaspati ghee.

Question 13. Which of the following hydrocarbons undergo addition reactions: C2H6,C3H8, C2H2, and CH4

Sol. Unsaturated hydrocarbons undergo addition reaction. Out of ; C2H6,C3H8, C2H2, and CH4 : C3H6 and C2H2 are unsaturated hydrocarbon and hence undergo addition reactions. Other compounds i.e., C2H6, C3H8 and CH4 are saturated compounds.
Note: An addition reaction occurs when atoms are added to a compound containing a double or triple bond.

Question 14. Give a test that can be used to differentiate chemically between butter and cooking oil.

Sol. Cooking oil is unsaturated hydrocarbon whereas butter is saturated hydrocarbon. We know that brown colour of bromine disappears when a drop of bromine is added to an unsaturated compound whereas there is no reaction between saturated hydrocarbons and bromine. Thus, when a drop of bromine is added to cooking oil, its colour disappears whereas when a drop of bromine is added to butter, it becomes brown.

Question 15. Explain the mechanism of the cleaning action of soap.

Sol. Cleaning action of soap and detergent : Soap and detergent follow the same principle. They contain two parts, a large hydrocarbon part (tail) which is hydrophobic (water repelling) and a negative charged part (head), which is hydrophillic (water attracting). In solution, water molecules being polar in nature, surround the ions and not the organic part of the molecule. When a soap or detergent is dissolved in water the molecule gather together as clusters, called micelles. The tails stick inwards and the head outwards. The hydrocarbon tail attaches itself to oily dirt. When water is agitated, the oily dirt tends to lift off from the dirty surface and dissociates into fragments. The solution now contains small globules of oil surrounded by detergent molecules. The negatively charged heads present in water prevent the small globules from coming together and form aggregates. Thus, the oily dirt is removed from the object

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