NCERT Solutions Class 10 Science Chapter 4

Q.1 Carbon is the sixth most abundant element in the universe. It has four electrons in its valence shell and forms covalent bonds. Due to its unique ability to form bonds with the other atoms of carbon it gives rise to large molecules that are exceptionally stable.
(i) Which property of carbon enables it to form a large number of compounds?
(ii) What is the valency of carbon?
(iii) What are saturated and unsaturated hydrocarbons? Explain with example.
(iv) What learnings for life can be associated with this property (ability to form bonds with other atoms) of carbon?

Ans- (i) Catenation enables carbon to form a large number of compounds.
(ii) The valency of carbon is 4.
(iii) The hydrocarbons in which the carbon atoms are linked by only single bonds are called saturated hydrocarbons, for example ethane.
The hydrocarbon in which two carbon atoms are linked by a ‘double bond’ or a ’triple bond’ is called is called an unsaturated hydrocarbon, for example ethene.
(iv) The learnings for life that can be associated with this property (ability to form bonds with other atoms) of carbon are as follows:
• In union, there is strength.
• Even weak can become strong when they are united.

Q.2 Water freezes at 0 ºC and is used in automobiles to keep the engine cool. However, when the temperature falls much below 0 ºC water cannot be used as it freezes and forms ice. Under these conditions, a mixture of water and alcohol is used in radiators to keep the engine cool.
After reading the above passage, answer the following questions:
(i) Give two reasons for the use of the mixture of water and alcohol in car radiators in sub-zero conditions.
(ii) The alcohol-water mixture on fractional distillation gives an azeotropic mixture containing 95% alcohol and 5% water by mass. What is this mixture commonly called?
(iii) Name the liver disease caused by the excessive alcohol consumption.
(iv) Which value of alcohol is reflected in using it as antifreeze in radiators of vehicles in sub-zero conditions?

Ans- (i) The mixture of water and alcohol is used in car radiators in sub-zero conditions because of the following:
· They form a homogeneous mixture, i.e., they are completely miscible in all proportions.
· The mixture has much lower freezing point than water, therefore it does not freeze under low temperature conditions.
(ii) This mixture is commonly called rectified spirit.
(iii) The liver disease that is caused by the excessive alcohol consumption is liver cirrhosis.
(iv) We may not be able to complete a task or achieve a goal independently but together we can.

Q.3 In December 2011, at least 150 people died after consuming toxic alcohol in West Bengal. The residents of twelve villages in the South 24 Parganas district fell ill after drinking the alcohol. Many more people were being treated in the hospital, with fear of the death toll rising. Several died at home; fearing police would take action against them.
Toxic alcohol deaths are a regular occurrence in India.
After reading the above passage, answer the following questions:
(i) As a science student, explain the major reason behind such tragedies.
(ii) Do you think we can use alcohol as an additive in gasoline in India?
(iii) Suggest two activities to prevent such type of tragedies in future.

Ans- (i) The main component of drinking alcohol is ethanol. It tends to slow down the metabolic processes and depresses the central nervous system. Hence, the individual may feel relaxed after the consumption of ethanol. To prevent the misuse of the ethanol that is produced for the industrial or medicinal purposes, it is made unfit for drinking by adding poisonous substances like methanol to it. This is called denatured alcohol. Methanol is a poisonous alcohol which causes permanent blindness and death.

(ii)Yes, we can use alcohol as an additive in gasoline in India. The gasoline need of our country is mainly met by importing it. The import of gasoline increases our fiscal deficit. To meet the energy requirements and to reduce the oil pool deficit of our country one should support the use of ethanol as an additive in gasoline. However, it must be regulated by the Government.

(iii)We can prevent such tragedies in future by:
· Spreading awareness by organising campaigns and debates
· Making warning labels
· Using mass media

Q.4 In our country, river Ganga is worshipped as a goddess and is considered the holiest of all the rivers. Unfortunately, today it finds its name in the list of the five most polluted rivers of the world. The project manager of the Ganga Pollution Control Unit at Haridwar found that one of the major reasons for this was the continuous use of soaps and detergents in an indefinite proportion for bathing and washing purposes.
I believe, since Ganga is the life-line of crores of people, so it is our social responsibility to make it pollution free.
(i) What is soap? Give an example.
(ii) Government of India has introduced several plans to save Ganga from pollution. Will you support the government’s plans or consider it a violation of the human rights?
(iii) Which values are promoted by keeping Ganga fresh and clean?
(iv) Suggest two activities that can help you promote these values.

Ans- (i) Soap is the sodium or potassium salt of a long chain fatty acid, which has cleansing properties in water. For example, sodium stearate.
(ii) I will support the government’s plans to save Ganga from pollution as it is for the benefit of all.
(iii) If we keep Ganga fresh and clean, we promote the following values:
• Concern for environmental protection
• Reducing water pollution
(iv) Some of the activities that can help us promote the above mentioned values are as follows:
• Public education through programs including: Plays, Concerts, Painting competitions, Conventions
• Mobilising, involving and working with local government to clean Ganga

Q.5 On the occasion of Diwali, Rahul’s mother wanted to clean their house. She asked Rahul to buy a bottle of bleach from the market for all the household cleaning. However, Rahul bought a bottle of vinegar instead of bleach as his father suffered from severe asthma and his mother had pneumonia last year due to which she had trouble breathing.
(i) Would you support the use of vinegar instead of bleach for the cleansing purpose? Give reasons.
(ii) What is vinegar chemically?
(iii) What values are associated with Rahul’s decision of buying vinegar instead of bleach?

Ans- (i) Yes, I would support the use of vinegar instead of bleach.
Bleach consists of chlorine, which irritates the sensitive areas in the eyes, nose, and throat. It does not cause asthma but can aggravate it and cause breathing problems.
Vinegar, on the other hand, is an all-natural, non-toxic, alternative to the alarming number of harmful chemical cleaners that are available in the market today. It is a popular household cleanser, which is used to clean, disinfect and shine most of the commercial products and is effective for killing mold, bacteria, and germs.

(ii) Vinegar is an aqueous solution of ethanoic acid. It contains about 5 to 8 % percent of ethanoic acid.

(iii) Vinegar is natural, economical, and eco-friendly. It shows that he has taken the right decision as he has considered the health of his father and mother by replacing bleach with vinegar.

Q.6 (a) Which of the following hydrocarbons undergo addition reactions – C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄?
(b) Write characteristics of homologous series.

Ans- (a) Unsaturated hydrocarbons undergo addition reaction. Therefore, alkenes and alkynes will show addition reactions. General formula of alkenes is C_nH_2n and that of alkynes is C_nH_2n-2. Therefore, propene (C₃H₆) and ethyne (C₂H₂) will undergo addition reaction.
(b) The characteristics of homologous series are :
(i) The members of a homologous series are represented by the same general formula.
(ii) The members of homologous series differ by (-CH₂) group.
(iii)The members of same homologous series show similar chemical properties.
(iv)The members of same homologous series show different physical properties.

Q.7 Represent the following by balanced chemical reaction:
(a) Combustion of methane
(b) Reaction of methane with chlorine in presence of sunlight

$\begin{array}{l}{\text{<strong>Ans- </strong>(a)\hspace{0.33em}CH}}_4+2{\text{O}}_2\to {\text{\hspace{0.33em}CO}}_2+2{\text{H}}_2\text{O}\\ \\ {\text{(b) CH}}_4+{\text{Cl}}_2\stackrel{\text{Sunlight}}{\to }{\text{CH}}_3\text{Cl}+\text{HCl}\end{array}$

Q.8 An organic compound (A) is widely used as a preservative in pickles and has molecular formula C₂H₄O₂. It reacts with ethanol to form a sweet smelling compound ‘B’.
(i) Identify the compound (A).
(ii) Give the chemical reaction equation with ethanol to form compound (B).
(iii) How can we obtain compound (A) back from compound (B)?

Ans- (i)The compound (A) with molecular formula C₂H₄O₂ is ethanoic acid. It is commonly known as acetic acid.
Its structural formula is CH₃COOH. A dilute solution of acetic acid called vinegar is used for the preservation of pickles.

(ii)When acetic acid reacts with ethanol, it forms an ester. This reaction is called esterification.

${\text{CH}}_3\text{COOH}+{\text{C}}_2{\text{H}}_5\text{OH\hspace{0.33em}}\stackrel{\text{Acid}}{\to }{\text{\hspace{0.33em}CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$

(iii) Ethanoic acid can be obtained from ethyl ethanoate by reacting with water in the presence of dilute hydrochloric acid acting as catalyst.

Q.9 Name the functional group present in the following compounds.

Ans-

(a) Hydroxyl group
(b) Carboxyl group
(c) Carbonyl group

Q.10 Carbon has ability to form long chains of atoms. What is the name given to this property of carbon?

Ans- This property of carbon is called catenation.

Q.11 Name the following compounds.
(a)

(b)

Ans-

(a) Ethanal
(b) Pentyne

Q.12 Write the names of the functional groups present in the following compounds:
(i) CH₃-CHO
(ii) CH₃-CO-CH₃
(iii) CH₃-OH

Ans-

(i) Aldehyde group
(ii) Carbonyl group
(iii) Alcohol group

Q.13 Name the reaction.

$\underset{\text{Ethene}}{{\text{<strong>Ans- </strong>CH}}_2={\text{CH}}_2}+{\text{H}}_2\stackrel{\text{Ni Catalyst,}\Delta }{\to }\underset{\text{Ethane}}{{\text{CH}}_3-{\text{CH}}_3}$

The given reaction is a hydrogenation reaction because hydrogen is adding during the reaction.

Q.14 What happens when ethanoic acid reacts with
(i) Sodium hydrogen carbonate
(ii) Sodium carbonate
(iii) Sodium hydroxide

Ans- (i) CH3COOH+NaHCO3 → CH3COONa+H2O+CO2(ii) 2CH3COOH+Na2CO3 → 2CH3COONa+H2O+CO2(iii) CH3COOH+NaOH → CH3COONa+H2O

Q.15 (a) Write two characteristics of homologous series.
(b) Following reaction is called substitution reaction. Why?

${\text{<strong>Ans- </strong>CH}}_4+{\text{Cl}}_2\stackrel{\text{UV}}{\to }{\text{\hspace{0.33em}CH}}_3\text{Cl}+\text{HCl}$

(a) 1. All the members of a homologous series can be represented by a general formula.
2. The molecular formulae of any two successive members of a homologous series differ by –CH₂.
(b) In this reaction, hydrogen atom of methane is replaced by chlorine atom. Therefore, this reaction is called substitution reaction.

Q.16 Molecular formula of hydrocarbon (A) and (B) are C₃H₆ and C₄H₁₀ respectively.
Which of these compounds will undergo addition reaction and which will undergo substitution reaction easily?

Ans- Unsaturated hydrocarbon compounds undergo addition reaction. Substitution reactions are characteristic of saturated hydrocarbons. As molecular formula suggests, compound (A) is an unsaturated compound, therefore it will undergo addition reaction. Molecular formula of compound (B) suggests that it is a saturated hydrocarbon. Therefore, it will undergo substitution reaction.

Q.17 Write IUPAC names of the following compounds:
(a) CH₃COOH (b) CH₃COCH₃

Ans- (a) Ethanoic acid
(b) Propanone

Q.18 An organic compound (X) is an active ingredient of all alcoholic drinks. It reacts with ethanoic acid to form sweet smelling compound and water.
(a) Identify the compound (X) and write its formula.
(b) Write the name of reaction of compound (X) with acetic acid.
(c) Represent the following by balanced chemical reactions.
(i) Compound (X) reacts with sodium metal.
(ii) Compound (X) reacts with sulphuric acid at 170°C.
(iii) Compound (X) is heated with alk.KMnO₄.

Ans-

(a) Compound (X) is ethanol C₂H₅OH.
(b) The reaction is esterification.
(c)

Q.19 (a) Name the products formed when ethanol burns in air.
(b) Will ethanol burn with sooty or non sooty flame? Give reasons also.

Ans-

$\begin{array}{l}{\text{(a)\hspace{0.33em}C}}_2{\text{H}}_5\text{OH}+3{\text{O}}_2\to 2{\text{CO}}_2+3{\text{H}}_2\text{O}\\ \end{array}$

(b) Ethanol is a saturated organic compound. The saturated hydrocarbons generally burn in air with a blue, non-sooty flame because the percentage of carbon in saturated hydrocarbons is comparatively low which gets oxidised completely by the oxygen present in air.

Q.20 Name the functional group present in ethyl alcohol (ethanol).

Ans- The functional group present in ethanol is alcohol group (-OH).

Q.21 What is molasses?

Ans- Molasses is the dark coloured, syrup like liquid left after the crystallization of sugar from cane juice.

Q.22 An organic compound (X) with molecular formula C₂H₆O is generally used as a solvent in cough syrups. It reacts with ethanoic acid to form sweet smelling compound (Y) and water.
(a) Identify the compound (X).
(b) Write the next member of homologous series to which compound (X) belongs.
(c) Write the balanced chemical reaction when compound (X) reacts with
(i) acetic acid
(ii) conc. H₂SO₄ at 443 K
(iii) sodium metal

Ans- (a) Compound (X) is ethanol (C₂H₅OH).
(b) C₃H₇OH (Propanol)
(c)

$\begin{array}{l}{\text{(i)\hspace{0.33em}CH}}_3\text{COOH}+{\text{C}}_2{\text{H}}_5\text{OH\hspace{0.33em}}\stackrel{\text{Acid}}{\to }{\text{\hspace{0.33em}CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}\\ {\text{(ii)\hspace{0.33em}CH}}_3{\text{CH}}_2\text{OH\hspace{0.33em}}\underset{{\text{H}}_2{\text{SO}}_{4(443K)}}{\overset{\text{Hot\hspace{0.33em}Conc.}}{\to }}{\text{\hspace{0.33em}CH}}_2={\text{CH}}_2+{\text{H}}_2\text{O}\\ \text{(iii)\hspace{0.33em}}2{\text{C}}_2{\text{H}}_5\text{OH}+2\text{Na\hspace{0.33em}}\to 2{\text{C}}_2{\text{H}}_5\text{ONa}+H_{\mathit{2}}\left(g\right)\end{array}$

Q.23 Write IUPAC name of the following compounds:

Ans-

(a) Ethanoic acid
(b) Propanone
(c) Propanol
(d) Propyne
(e) Methanal

Q.24 What are enzymes?

Ans- Enzymes are catalysts, which bring about bio-chemical changes.Example :- Lactase enzyme turns milk sugar, lactose into lactic acid.

Q.25 Give two uses of ethanoic acid.

Ans- (i) It is used for making synthetic vinegar.

(ii) It is used in the preparation of rayon fibre.

Q.26 What reagent will cause conversion of ethanol into ethanoic acid?

Ans- Alkaline KMnO₄ (Potassium per magnate) or acidified K₂Cr₂O₇ (Potassium dichromate) convert ethanol to ethanoic acid.

Q.27 Name the active bacteria for the fermentation of ethanol to produce vinegar.

Ans- The active bacteria is acetobacter (bacterium mycoderma acets).

Q.28 What are saturated hydrocarbons?

Ans- Hydrocarbons in which the carbon atoms are connected by only single bond are called saturated hydrocarbons.

Q.29 What is fermentation?

Ans- The slow decomposition of large organic molecules by enzyme is known as fermentation.

Q.30 When ethanol reacts with ethanoic acid in the presence of conc. H₂SO₄, a substance with fruity smell is produced. answer the following:
a) State the class of compounds to which to which the fruity smeling compounds belong and how are they formed.
b) State the role of conc. H₂SO₄ in the reaction.

Ans-

a) Fruity smelling compounds belong to the class of esters.

$\underset{\text{Ethanoic\hspace{0.33em}acid}}{{\text{CH}}_3\text{COOH}}+\underset{\text{Ethanol}}{{\text{CH}}_3{\text{CH}}_2\text{OH}}\stackrel{{\text{Conc.\hspace{0.33em}H}}_{\text{2}}{\text{SO}}_{\text{4}}}{\to }\underset{\text{Ethyl\hspace{0.33em}ethanoate}}{{\text{CH}}_3{\text{COOCH}}_2{\text{CH}}_3}+{\text{H}}_2\text{O}$

b) Conc. H₂SO₄ acts as dehydrating agent and removes water.

Q.31 What change will you observe if you test soap with a litmus paper (red and blue)?

Ans- Red litmus will turn blue. There is no change on blue litmus paper.

Q.32 Name one alcohol which is poisonous

Ans- Methanol

Q.33 What substances are added to ethanol to make it unfit for drinking purposes?

Ans- Methanol and copper sulphate

Q.34 Which of the following hydrocarbon undergo addition reactions : C₂H₆, C₃H_8, C₃H₆, C₂H₂, and CH₄.

Ans- C₃H₆, and C₂H₂ undergo addition reactions.

Q.35 Explain the nature of the covalent bond using the bond formation in CH₄. Draw the structure also.

Ans- Covalent bond is formed by sharing of electrons between atoms. Carbon has atomic number 6 and has the electronic configuration 2,4. It has four electrons in its outermost shell and needs four more electrons to get the completely filled outermost shell. Each hydrogen atom has one electron and needs one more electron to get stable electronic configuration. Therefore atom of carbon shares its 4 electrons with four hydrogen atoms to form four covalent bonds. Thus, carbon and hydrogen both get stable electronic configuration of nearest noble gas.

Q.36 How are carbon fibers made?

Ans- Carbon fibers are made by heating synthetic fibers such as polyacrylonitrile at 3000ºC in the absence of oxygen. It is used for making sports goods, space crafts etc.

Q.37 What are synthetic detergents? Give one example of synthetic detergent. Write its two advantages over soap.

Ans- The sodium salt of the long chain benzene sulphonic acid or the sodium salt of a long chain alkyl hydrogen sulphate which has cleansing properties in water is known as synthetic detergent. For example, Sodium-n-dodecyl sulphate [CH₃-(CH₂)₁₀-CH₂-SO₄^–Na⁺].

Advantages:
a. Synthetic detergents have a strong cleansing action.
b. Synthetic detergents can be used for washing in hard water.

Q.38 What is meant by homologous series? State any four characteristics.

Ans- The members of the same class of organic compounds such as alkanes when arranged in ascending order of their molecular mass differ from each other by (-CH₂) group are called homologous and series is known as homologous series.

The characteristics of homologous series are :
a. The members of a homologous series are represented by the same general formula.
b. The members of homologous series differ by (-CH₂) group.
c. The members of same homologous series show similar chemical properties.
d. The members of same homologous series show different physical properties.

Q.39 Which of the following would yield an unsaturated hydrocarbon on cracking : C₆H₁₄, C₂H₆, C₂H₄?

Ans- Hexane will undergo cracking and yield an unsaturated hydrocarbon.
C₆H₁₄ (n-hexane) C₄H₁₀ (n-butane) + C₂H₄ (Ethene)
(unsaturated hydrocarbon)

Q.40 Name the products formed when methane burns in :

Ans- (a) Sufficient supply of air.
(b) Insufficient supply of air.
Write the chemical equations for above reactions.

Q.41 How is an ester prepared in the lab? Write its one use in daily life.

Ans- Esters are prepared by the reaction of a carboxylic acid with an alcohol in the presence of concentrated sulphuric acid.

$\underset{\text{Ethanoic\hspace{0.33em}acid}}{{\text{CH}}_3\text{COOH}}+\underset{\text{1-Propanol}}{{\text{CH}}_3{\text{CH}}_2{\text{CH}}_2\text{OH}}\stackrel{{\text{Conc.\hspace{0.33em}H}}_{\text{2}}{\text{SO}}_{\text{4}}}{\to }\underset{\text{Ester}}{{\text{CH}}_3{\text{COOCH}}_2{\text{CH}}_2{\text{CH}}_3}+{\text{H}}_2\text{O}$

Ester is used in making artificial perfumes and making artificial flavours and essences used in cold drinks and ice-creams.

Q.42 Compound ‘A’ is obtained from wood-tar distillation. It has specific smell and burning taste. It burns with blue flame to give CO₂ and H₂O. It is soluble in water. It is used as a fuel and solvent. It reacts with sodium metal and gives out hydrogen gas. It reacts with acetic acid to form pleasant fruity smelling compound ‘B’ in presence of concentrated H₂SO₄. Identify ‘A’ and ‘B’ and give equations for all the reactions involved.

Ans- The compound A is methanol obtained from wood-tar distillation has specific smell and burning taste.

$2{\text{CH}}_3\text{OH (methanol)}+3{\text{O}}_2\to 2{\text{CO}}_2+4{\text{H}}_2\text{O.}$

It reacts with sodium metal to give hydrogen gas.

$\underset{\text{(Methanol)}}{2{\text{CH}}_3\text{OH}}+\underset{\text{(Sodium)}}{2\text{Na}}\to \underset{\text{(Sodium\hspace{0.33em}methoxide)}}{2{\text{CH}}_3\text{ONa}}+\underset{\text{(Hydrogen\hspace{0.33em}gas)}}{{\text{H}}_2}$

It reacts with acetic acid (CH₃COOH) in presence of concentrated sulphuric acid. It forms methyl ethanoate (ester) which has pleasant fruity smell.

$\underset{\text{(Methanol)}}{{\text{CH}}_3\text{OH}}+\underset{\text{(Acetic\hspace{0.33em}acid)}}{{\text{CH}}_3\text{COOH}}\stackrel{{\text{Conc. H}}_2{\text{SO}}_4}{\to }\underset{\text{(Methyl\hspace{0.33em}ethanoate)}}{{\text{CH}}_3{\text{COOCH}}_3}+\underset{\text{(Water)}}{{\text{H}}_2\text{O}}$

So, compound A is methanol and compound B is methyl ethanoate.

Q.43 (a) How is ethanol prepared by fermentation?

(b) Give two uses of ethyl alcohol.

(c) What are the harmful effects of drinking alcohol?

Ans- (a) The fermentation of sugar is carried out at a controlled temperature of 20ºC to 30ºC. Fermentation produces a dilute solution of ethanol in water. Ethanol is separated from water and purified by the process of distillation.
(n) Uses of ethyl alcohol:
1. Ethanol is used for making anti-freeze mixtures.
2. Ethanol is used as fuel in internal combustion engines and in spirit lamps.

(c) Harmful effects of drinking alcohol :
a. Ethanol is harmful for the lungs and causes addiction.
b. Ethanol can lead to heart problems.
c. It affects the central nervous system, delaying response time and reflexes.

Q.44 What is the optimum temperature for fermentation?

Ans- The optimum temperature for fermentation is 20-30°C.

Q.45 Why are carbon and its compounds used as fuels for most of the applications?

Ans- Carbon and its compounds are used as fuel because they release very large amount of heat and light on burning.

Q.46 What is hydrogenation?

Ans- The addition of hydrogen to a double bond or a triple bond is known as hydrogenation.