# NCERT Solutions for Class 10 Science Chapter 4 Carbon and its Compounds

## NCERT Intext Questions

Page Number 61

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

Ans.

2. What would be electron dot structure of sulphur which is made up of eight atoms of sulphur.
[Hint: The eight atoms of sulphur are joined together in the form of a ring.]

Ans.

Page Number 68-69

1. How many structural isomers can you draw for pentane?

Ans. Three, structural isomers can be drawn for pentane, these are n-pentane, iso-pentane and neo-pentane.

2. What are the two properties of carbon which lead to the huge number of carbon compounds we see
around us ?

Ans. (i) Tetravalency: Carbon has valency 4 so it is capable of bonding with 4 other atoms of carbon or atoms of other element.

(ii) Catenation: Tendency of C-atom to link with one another through covalent bond to form chains and rings.

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

Ans. The molecular formula of cyclopentane is C5 H10 .

The electron dot structure of cyclopentane is given as below:

4. Draw the structures for the following compounds :

• (i) Ethanoic acid
• (ii) Bromopentane*
• (iii) Butanone
• (iv) Hexanal

* Are structural isomers possible for bromopentane?

Ans. (i) Ethanoic acid (CH3COOH)
(ii) Bromopentane (C5H11Br)
(iii) Butanone (CH3 — CH2 — COCH3)
(iv) Hexanal (C5H11CHO)

Structural isomers for bromopentane: There are three structural isomers for bromopentane depending on the position of Br at carbon 1, 2, 3.

5. How would you name the following compounds?

(i) CH3—CH2—Br
• Ans. (i) Bromoethane
• (ii) Methanal
• (iii) 1 – Hexyne
Page Number 71

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

Ans. Ethanoic acid is formed when 1 hydrogen molecule is removed from ethanol and an oxygen atom is added to it.

$$\underset{\text{Ethanol}}{\text{CH}_3\text{CH}_2\text{OH}+2[\text{O}]}\xrightarrow[\text{or Acidified} \text{K}_2\text{Cr}_2\text{O}_7]{\text{Alkaline KMnO}_4}\underset{\text{Ethanoic acid}}{\text{CH}_3\text{COOH} + \text{H}_2\text{O}}$$

Since the hydrogen atom is removed and oxygen is added, ethanol is transformed to ethanoic acid, which is an oxidation process.

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

Ans. A mixture of ethyne and air is not used for welding because burning of ethyne in air produces a sooty flame due to incomplete combustion, which is not enough to melt metals for welding.

Page Number 74

1. How would you distinguish experimentally between an alcohol and a carboxylic acid?

Ans. Difference between an alcohol and carboxylic acid are:

 Test Alcohol Carboxylic acid (i) Litmus test Alcohols do not respond to this test except phenol. Blue litmus solution turns red. (ii) Sodium hydrogen carbonate test C2H5OH + NaHCO3 → No reaction No brisk effervescence. CH3COOH + NaHCO3 → CH3COONa + H2O + CO2 Brisk effervescence due to evolution of CO2. (iii) Alkaline potassium permanganate On heating, pink colour disappears. Does not happen so.

2. What are oxidising agents?

Ans. An oxidising agent is a substance, that gains or accepts electron (donates H+) from a substance. Example: Halogens (Cl, F), oxygen, hydrogen peroxide (H2O2)
Page Number 76

1. Would you be able to check if water is hard by using a detergent ?

Ans. Detergents cannot be used to check the hardness of water, as they form foam or froth (lather) easily in both soft as well as hard water.

2. 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 ?

Ans. It is necessary to agitate to get clean clothes because the soap forms micelles with dirt such as grease. These micelles remain suspended as a colloid. To remove dirt in the form of micelles from clothes, agitation is necessary in order to get clean clothes.

## NCERT Exercise Questions

1. Ethane, with the molecular formula C2H6 has:

• (a) 6 covalent bonds
• (b) 7 covalent bonds
• (c) 8 covalent bonds
• (d) 9 covalent bonds
• Ans. (b) 7 covalent bonds.
Explanation :

7 covalent bonds are present.

2. Butanone is a four-carbon compound with the functional group:

• (a) 6 covalent bonds
• (b) 7 covalent bonds
• (c) 8 covalent bonds
• (d) 9 covalent bonds
• Ans. (b) 7 covalent bonds.
Explanation :

The functional group in Butanone is ketone (C = O).

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 fuel is wet.
• (d) the fuel is burning completely.
• Ans. (b) The fuel is not burning completely.
Explanation :

While cooking, if the bottom of the vessel is getting blackened on the outside, it means that the fuel is not burning completely. Sometimes, due to lack of oxygen, the fuel does not burn completely.

4. Explain the nature of the covalent bond using the bond formation in CH3Cl.

Ans. Covalent bond is formed by mutual and equal sharing of electrons between the atoms of different elements.

Valence electrons in carbon = 4

Valence electrons in chlorine = 7

In order to complete the octet of carbon, it shares its electrons, one with each of the three hydrogen atoms and one with one chlorine atom. Since, bonds are formed because of sharing of electrons. Carbon forms 3 covalent bonds with hydrogen and 1 covalent bond with chlorine.

Three hydrogen atoms complete their shells by sharing three electrons (one electron each) of carbon atom.

Chlorine completes its outer shell by sharing its one out of seven electrons with one electron of carbon atom.

Thus carbon atom shares all its four electrons with three hydrogen atoms and one of chlorine atom and completes its outermost shell and single covalent bonds are formed in CH3Cl.

5. Draw the electron dot structures for:

• (a) ethanoic acid
• (b) propanone
• (c) H2S
• (d) F2
• Ans. (a) Ethanoic acid :

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

Ans. A homologous series is a series of C-compounds that have different number of C-atoms but having same functional group and same general formula.

Example: CH4 (methane), CH3CH3 (ethane), CH3CH2CH3 (propane), CH3CH2CH2CH3 (butane)
They all contain different C-atoms i.e., methane (1–C), ethane (2–C), propane (3–C), butane (4–C) and having functional group –CH2 in all compounds with general formula CnH2n+2 The series is alkane homologes series.

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

Ans. Difference between Ethanol and Ethanoic Acid on the basis of physical properties are:

 Property Ethanol Ethanoic acid (i) State, odour and appearance Colourless liquid with pleasant smell Colourless liquid with a pungent smell of vinegar (ii) Taste It has a burning taste. It has a sour taste. (iii) Melting point 156 K 290 K (iv) Boiling point 351 K 391 K

Difference on the basis of chemical properties are:

 Test Ethanol Ethanoic acid (i) Litmus test No change in the colour of litmus solution. Blue litmus solution turns red. (ii) Sodium hydrogen carbonate test C2H5OH + NaHCO3 → No reaction No brisk effervescence. CH3COOH + NaHCO3 → CH3COONa + H2O + CO2 Brisk effervescence due to evolution of CO2. (iii) Alkaline potassium perman-ganate On heating, pink colour disappears. No change takes place.

8. Why does micelle formation take place when soap is added to water ? Will a micell be formed in other solvents such as ethanol also ?

Ans. Micelle formation takes place when soap is added to water because the hydrocarbon chains of soap molecules are hydrophobic (water repelling) which are insoluble in water, but the ionic ends of soap molecules are hydrophilic (water attracting) and hence soluble in water. Such micelle formation will not be possible in other solvents like ethanol in which sodium salts of fatty acids do not dissolve.

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

Ans. Carbon and its compounds are used as a fuel because of the following reasons:

(i) They burn with a clean flame and no smoke is produced.

(ii) They give large amount of heat on combustion.

(iii) Their combustion can be controlled.

10. Explain the formation of scum when hard water is treated with soap.

Ans. When hard water is treated with soap, the molecules of soap react with the salts of calcium and magnesium of hard water and form an insoluble precipitate called scum. Soaps lose their cleansing property in hard water because of formation of scum.

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

Ans. The soap is basic or alkaline in nature so it will trun red litmus into blue and no effect on blue litmus paper.

12. What is hydrogenation ? What is its industrial application ?

Ans. Hydrogenation is a chemical reaction and in this reaction hydrogen is added to a substance in the presence of catalyst such as Ni or Pd.

Industrial applications:

(i) It is used to prepare vegetable ghee or vanaspati ghee from vegatable oils.

(ii) It is used in petrochemical industry to convert alkenes into alkanes (paraffins) and cycloalkanes.

13. Which of the following hydrocarbons undergo addition reactions : C2H6, C3H8, C3H6, C2H2 and CH4?
Ans. Addition reactions take place only in unsaturated hydrocarbons. So addition reaction takes place only in C3H6 and C2H2.

14. Give a test that can be used to differentiate between saturated and unstaurated hydrocarbons.

Ans. The test used to differentiate between saturated and unsaturated hydrocarbons is Bromine water test.

When Bromine water reacts with unsaturated hydrocarbon, bromine gets attached to hydrocarbon and thus red brown colour of Bromine gets decolourised, when bromine water reacts with saturated hydrocarbons the red-brown colour of Bromine water remains the same.

15. Explain the mechanism of the cleaning action of soaps.

Ans. When a dirty cloth is put in water containing dissolved soap, then the hydrocarbon ends of the soap molecules in micelle attach to the oil or grease particles present on the surface of dirty cloth. In this way the soap micelle entraps the oily or greasy particles by using its hydrocarbon ends. The ionic ends of the soap molecules in the micelles, however, remain attached to water. When the dirty cloth is agitated in soap solution, the oily and greasy particles present on its surface and entrapped by soap micelles get dispersed in water due to which the soap water becomes dirty but the cloth gets cleaned. The cloth is cleaned thoroughly by rinsing in clean water a number of times.