## NCERT solutions for class 10 science-Exercise Solutions

Reviewed By:
Krishna Kant Majee
M.Sc., B.Ed.

## Chapter 04-Carbon and Its Compounds-Exercise Solutions

### NCERT Book Exercise Page Number-77-78

Q.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.

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

Ans. (c) Ketone.

Q.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.

Q.4. Explain the nature of the covalent bond using the bond formation in $CH_{3}Cl$.
Ans. The structure of $CH_{3}Cl$ is given below :

Carbon has four valence electrons. It shares 1 electron each with 3 hydrogen atoms and 1 electron with chlorine. The bond between C and Cl atoms is covalent but due to higher value of electronegativity of Cl, the C—Cl bond is polar in nature.

Q.5. Draw the electron dot structures for :

(a) ethanoic acid (b) $H_{2}S$ (c) propanone (d) $F_{2}$.

Ans. (a) Ethanoic acid $(CH_{3}COOH)$:

(b) $H_{2}S$ (Hydrogen sulphide) :

(c) Propanone

(d) Fluorine

Q.6. What is homologous series? Explain with an example.
Ans. A homologous series is a series of carbon compounds that have different numbers of carbon atoms but contain the same functional group.
For example, methane, ethane, propane, butane, etc. are all part of the alkane homologous series. The general formula of this series is $C_{n}H_{2n+2}$.
Methane $CH_{4}$
Ethane $CH_{3}CH{3}$
Propane $CH_{3}CH_{2}CH_{3}$
Butane $CH_{3}CH_{2}CH_{2}CH_{3}$
It can be noticed that there is a difference of $— CH_{2}$ unit between each successive compound.

Q.7. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?
Ans. Ethanol and Ethanoic acid be differentiated on the basis of their following properties:
• Ethanol is a liquid at room temperature with a pleasant smell. Ethanoic acid has a melting point of 17 °C. Since it is below the room temperature so, it freezes during winter. Moreo¬ver, ethanoic acid has a smell like vinegar.
• Ethanol does not react with metal carbonates while, ethanoic acid reacts with metal car¬bonates to form salt, water and carbon dioxide. For example :
$2CH_{3}COOH + Na_{2}CO_{3}\rightarrow 2CH_{3}COONa + CO_{2} + H_{2}O$
• Ethanol does not react with NaOH while ethanoic acid reacts with NaOH to form sodium ethanoate and water. For example :
$CH_{3}COOH + NaOH \rightarrow CH_{3}COONa + H_{2}O$
• Ethanol is oxidized to give ethanoic acid in presence of acidified $KMnO_{4}$ while, no reaction takes place with ethanoic acid in presence of acidified $KMnO_{4}$.

Q.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?
Ans. Soap molecule has two ends. One end is hydrophilic and another end is hydrophobic. When soap is dissolved in water and clothes are put in the soapy solution, soap molecules converge in a typical fashion to make a structure, called micelle. The hydrophobic ends of different molecules surround a particle of grease and make the micelle, which is a spherical structure. In this, the hydrophilic end is outside the sphere and hydrophobic end is towards the centre of the sphere. This is why micelle formation takes place when soap is added to water.
Since ethanol is not as polar as water, so micelles will not be formed in other solvents such as ethanol.

Q.9. Why are carbon and its compounds used as fuels for most applications?
Ans. Carbon and its compounds give large amount of heat on combustion due to high percentage of carbon and hydrogen. Carbon compounds used as fuel have optimum ignition temperature with high calorific values and are easy to handle. Their combustion can be controlled. Therefore, carbon and its compounds are used as fuels.

Q.10. Explain the formation of scum when hard water is treated with soap.
Ans. Hard water often contains salts of calcium and magnesium. Soap molecules react with the salts of calcium and magnesium and form a precipitate. This precipitate begins floating as an off-white layer over water. This layer is called scum. Soaps lose their cleansing property in hard water because of formation of scum.
$2C_{17}H_{35}COONa + CaCl_{2}\rightarrow(C_{17}H_{35}COO)_{2}Ca + 2NaCl$
Sodium stearate(soap) Calcium chloride(salt) Calcium stearate(scum)

Q.11. What change will you observe if you test soap with litmus paper (red and blue)?
Ans. Since soap is basic in nature, it will turn red litmus blue. However, the colour of blue litmus will remain blue.

Q.12. What is hydrogenation? What is its industrial application?
Ans. Hydrogenation is the chemical reaction between hydrogen and other compounds in the presence of catalyst. Hydrogenation is used mainly to reduce unsaturated hydrocarbons. Hydrogenation is an addition reaction. For Example: When ethene is heated with the catalyst nickel it is reduced to ethane.
Ni catalyst
$CH_{2} = CH_{2} + H_{2}\rightarrow CH_{3}—CH_{3}$
Ethene Ethane
Industrial application :
• Hydrogenation is used in many industrial applications. For example, in Petrochemical Industry, hydrogenation is used to convert alkenes into alkanes (paraffins) and cycloalkanes.
• It is also used to prepare vegetable ghee from vegetable oils.

Q.13. Which of the following hydrocarbons undergo addition reactions : $C_{2}H_{6}, C_{3}H_{8}, C_{3}H_{6}, C_{2}H_{2} ~~and~~ CH_{4}$.
Ans. Unsaturated hydrocarbons undergo addition reactions. Being unsaturated hydrocarbons, $C_{3}H_{6}$ and $C_{2}H_{2}$ undergo addition reactions.

Q.14. Give a test that can be used to differentiate chemically between butter and cooking oil.
Ans. Butter contains saturated fats. Therefore, it cannot be hydrogenated. On the other hand, oil has unsaturated fats. That is why it can be hydrogenated to saturated fats (solids).

Q.15. Explain the mechanism of the cleaning action of soaps.
Ans. The dirt present on clothes is organic in nature and insoluble in water. Therefore, it cannot be removed by only washing with water. When soap is dissolved in water, its hydrophobic ends attach themselves to the dirt and remove it from the cloth. Then, the molecules of soap arrange themselves in micelle formation and trap the dirt at the centre of the cluster. These micelles remain suspended in the water. Hence, the dust particles are easily rinsed away by water.