CBSE Class 10 Science Carbon and its Compounds VBQs Set 03

Question. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer: Catenation and tetravalency.

Question. How would you name the following compounds?
(i) \( CH_3—CH_2—Br \)
(ii) \( H—C=O(H) \)
(iii) \( H—C \equiv C—CH_2—CH_2—CH_2—CH_3 \)
Answer: (i) Bromoethane (ii) Methanal (iii) 1-Hexyne

Question. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?
Answer: \( CH_3CH_2OH \xrightarrow{[O]} CH_3COOH \)
Since the conversion of ethanol to ethanoic acid involves the addition of oxygen to ethanol, it is an oxidation reaction.

Question. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?
Answer: \( 2HC \equiv CH + 5O_2 \to 4CO_2 + 2H_2O + \text{Heat} \)
When ethyne is burnt in air, it gives a sooty flame due to incomplete combustion caused by limited supply of oxygen (in air). However, if ethyne is burnt with oxygen (at 3000 °C), it gives a clean flame because of complete combustion. This oxy-acetylene flame is used for welding. It is not possible to attain such a high temperature without mixing oxygen.

Question. How would you distinguish experimentally between an alcohol and a carboxylic acid?
Answer: We can distinguish between an alcohol and a carboxylic acid on the basis of their reaction with carbonates and hydrogen carbonates. Carboxylic acid reacts with carbonate and hydrogen carbonate to evolve carbon dioxide gas which turns lime water milky. On the other hand, alcohols do not react with carbonates and hydrogen carbonates.
Metal carbonate/Metal hydrogen carbonate + Carboxylic acid \( \to \) Salt + Water + Carbon dioxide

Question. What are oxidising agents? Give an example.
Answer: Those substances which give oxygen or replace hydrogen on reaction with other compounds are known as oxidising agents. For example, potassium permanganate (\( KMnO_4 \)).

Question. Would you be able to check if water is hard by using a detergent?
Answer: No. Detergents are sodium or potassium salts of sulphonic acids of hydrocarbons of alkene type. Unlike soap, they do not react with calcium and magnesium ions present in hard water to form scum. They give a good amount of lather with both soft water and hard water. Hence, it cannot be used to check whether the water is hard or not.

Question. 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?
Answer: A soap molecule has two parts namely hydrophobic and hydrophilic. With the help of these, it attaches to the grease or dirt particles and forms a cluster called micelle. These micelles remain suspended in solution as a colloid.
When water is agitated, the oily dirt tends to lift off from the dirty surface and dissociates into fragments. This gives an opportunity to other tails to stick to oil. This results in the formation of an emulsion in water. This emulsion now contains small globules of oil surrounded by soap or detergent molecules. The negatively charged heads present in water prevent the small globules from coming together and form clusters. Thus, the oily dirt is removed from the object.

EXERCISES

Question. 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.

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

Question. 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.
Answer: (b) This means that the fuel is not burning completely and unburnt carbon particles get deposited on the bottom of the vessel, making it black.

Question. Draw the electron dot structures for (a) ethanoic acid, (b) \( H_2S \), (c) propanone, (d) \( F_2 \).
Answer: (a) Ethanoic acid (\( CH_3COOH \)): Carbon is bonded to three hydrogens, another carbon, and two oxygens (one with a double bond, one with a single bond to hydrogen).
(b) \( H_2S \): Sulfur shares one electron each with two hydrogen atoms.
(c) Propanone (\( CH_3COCH_3 \)): Central carbon double-bonded to oxygen and single-bonded to two methyl groups.
(d) \( F_2 \): Two fluorine atoms sharing one pair of electrons.

Question. What is homologous series? Explain with an example.
Answer: It is a series of organic compounds having same general formula, same functional group, same general methods of preparation, similar chemical properties and gradation in physical properties, e.g. The general formula for the homologous series of alkanes is \( C_nH_{2n + 2} \). \( CH_4 \) (Methane), \( C_2H_6 \) (Ethane), \( C_3H_8 \) (Propane) and \( C_4H_{10} \) (Butane). Every homologous series have general formula. In alkane single bond is functional group, in alkene double bond and in alkyne triple bond.

Question. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?
Answer: Difference on the physical basis: Ethanol has pleasant smell, whereas ethanoic acid has vinegar like smell.
Difference on the chemical basis: Ethanol does not react with sodium hydrogen carbonate, whereas ethanoic acid liberates \( CO_2 \) on treatment with sodium hydrogen carbonate (\( NaHCO_3 \)).
\( CH_3COOH + NaHCO_3 \to CH_3COONa + H_2O + CO_2 \)
\( C_2H_5OH + NaHCO_3 \to \text{No Reaction} \)
When a few drops of alkaline \( KMnO_4 \) are added to alcohol and the resultant mixture is heated, the pink colour of alkaline \( KMnO_4 \) disappears. Ethanoic acid does not give this test.

Question. 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: A soap molecule has both a hydrophilic and a hydrophobic end. The hydrophilic end is soluble in water, whereas the hydrophobic end is insoluble in water. When soap is added in water, the hydrophilic part gets dissolved in water but hydrocarbon tail being hydrophobic part forms clusters called micelles. As soap is soluble in ethanol, micelle formation will not take place in it.

Question. Why are carbon and its compounds used as fuels for most applications?
Answer: During the process of combustion of carbon and its compounds, a large amount of heat and light is released. Because of this carbon and its compounds are used as fuels.

Question. Explain the formation of scum when hard water is treated with soap.
Answer: Soaps are sodium or potassium salts of fatty acids. Hard water contains \( Ca^{2+} \) and \( Mg^{2+} \) ions, which react with soap to form calcium and magnesium salts of fatty acids which are insoluble and are called scum.
\( 2C_{17}H_{35}COONa + Ca^{2+} \to 2Na^+ + (C_{17}H_{35}COO)_2Ca \) (scum)
\( 2C_{17}H_{35}COONa + Mg^{2+} \to 2Na^+ + (C_{17}H_{35}COO)_2Mg \) (scum)

Question. What change will you observe if you test soap with litmus paper (red and blue)?
Answer: As soap solution is basic in nature, it will turn red litmus paper into blue but it will not affect blue litmus paper.

Question. What is hydrogenation? What is its industrial application?
Answer: Hydrogenation is a process in which a substance reacts with hydrogen in the presence of nickel or palladium as catalyst.
Industrial application: The process of hydrogenation is used to prepare vegetable ghee from vegetable oil.
Vegetable oil + \( H_2 \xrightarrow[Heat]{Ni} \) Vegetable ghee

Question. Which of the following hydrocarbons undergo addition reactions: \( C_2H_6, C_3H_8, C_3H_6, C_2H_2, CH_4 \)?
Answer: \( C_3H_6 \) and \( C_2H_2 \) are unsaturated hydrocarbons, therefore, they will undergo addition reactions.

Question. Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.
Answer: Add bromine water to each of them. Cooking oil will decolourise bromine water showing that it is unsaturated, whereas butter will not decolourise bromine water showing that it is saturated.

Question. Explain the mechanism of the cleaning action of soaps.
Answer: A soap molecule has hydrophobic and hydrophilic ends. In water, hydrophobic ends of soap, which consists of hydrocarbon chains, cluster together to form micelles. The oily dirt collects in the centre of micelle. These micelles stay in solution as a colloid and will not come together to form precipitate because of ion-ion repulsion. Thus the dirt suspended in the micelles can be easily rinsed away and hence, soaps are effective in cleaning.

SELECT NCERT EXEMPLAR PROBLEMS

Question. Carbon exists in the atmosphere in the form of
(a) carbon monoxide only
(b) carbon monoxide in traces and carbon dioxide
(c) carbon dioxide only
(d) coal
Answer: (c) carbon dioxide only

Question. Which of the following statements are usually correct for carbon compounds? These
(i) are good conductors of electricity
(ii) are poor conductors of electricity
(iii) have strong forces of attraction between their molecules
(iv) do not have strong forces of attraction between their molecules
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (ii) and (iv)
Answer: (d) (ii) and (iv)

Question. A molecule of ammonia (\( NH_3 \)) has
(a) only single bonds
(b) only double bonds
(c) only triple bonds
(d) two double bonds and one single bond
Answer: (a) only single bonds

Question. Buckminsterfullerene is an allotropic form of
(a) phosphorus
(b) sulphur
(c) carbon
(d) tin
Answer: (c) carbon

Question. Which of the following are correct structural isomers of butane?
(i) \( H—C—C—C—C—H \) (linear)
(ii) \( CH_3—CH(CH_3)—CH_3 \) (branched)
(a) (i) and (iii)
(b) (ii) and (iv)
(c) (i) and (ii)
(d) (iii) and (iv)
Answer: (c) (i) and (ii)

Question. Oils on treating with hydrogen in the presence of palladium or nickel catalyst form fats. This is an example of
(a) Addition reaction
(b) Substitution reaction
(c) Displacement reaction
(d) Oxidation reaction
Answer: (a) Addition reaction

Question. Which of the following is the correct representation of electron dot structure of nitrogen?
(a) \( :N : N: \)
(b) \( :N :: N: \)
(c) \( :N N: \)
(d) \( :N \equiv N: \)
Answer: (d) \( :N \equiv N: \)

Question. Structural formula of ethyne is
(a) \( H—C \equiv C—H \)
(b) \( H_3C—C \equiv C—H \)
(c) \( H_2C=CH_2 \)
(d) \( H—CH_2—CH_2—H \)
Answer: (a) \( H—C \equiv C—H \)

Question. Chlorine reacts with saturated hydrocarbons at room temperature in the
(a) absence of sunlight
(b) presence of sunlight
(c) presence of water
(d) presence of hydrochloric acid
Answer: (b) \( CH_4(g) + Cl_2(g) \xrightarrow{Sun light} CH_3Cl + HCl \)

Question. Pentane has the molecular formula \( C_5H_{12} \). It has
(a) 5 covalent bonds
(b) 12 covalent bonds
(c) 16 covalent bonds
(d) 17 covalent bonds
Answer: (c) 16 covalent bonds

Question. Ethanol reacts with sodium and forms two products. These are
(a) sodium ethanoate and hydrogen
(b) sodium ethanoate and oxygen
(c) sodium ethoxide and hydrogen
(d) sodium ethoxide and oxygen
Answer: (c) \( 2CH_3CH_2OH + 2Na \to 2CH_3CH_2ONa + H_2 \)

Question. Vinegar is a solution of
(a) 50 % – 60 % acetic acid in alcohol
(b) 5 % – 8 % acetic acid in alcohol
(c) 5 % – 8 % acetic acid in water
(d) 50 % – 60 % acetic acid in water
Answer: (c) 5 % – 8 % acetic acid in water

Question. Carbon forms four covalent bonds by sharing its four valence electrons with four univalent atoms, e.g. hydrogen. After the formation of four bonds, carbon attains the electronic configuration of
(a) helium
(b) neon
(c) argon
(d) krypton
Answer: (b) neon

Question. The correct electron dot structure of a water molecule is
(a) \( H \cdot O \cdot O \)
(b) \( H \cdot O : O \)
(c) \( H : O : H \)
(d) \( H : O : O \)
Answer: (c) \( H : O : H \)

Question. Which among the following are unsaturated hydrocarbons?
(i) \( H_3C—CH_2—CH_2—CH_3 \)
(ii) \( H_3C—C \equiv C—CH_3 \)
(iii) \( H_3C—CH(CH_3)—CH_3 \)
(iv) \( H_3C—C(CH_3)=CH_2 \)
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (ii) and (iv)
(d) (iii) and (iv)
Answer: (c) (ii) and (iv)

Question. Which of the following does not belong to the same homologous series?
(a) \( CH_4 \)
(b) \( C_2H_6 \)
(c) \( C_3H_8 \)
(d) \( C_4H_8 \)
Answer: (d) \( C_4H_8 \)

Question. The heteroatoms present in \( CH_3—CH_2—O—CH_2—CH_2Cl \) are
(i) oxygen (ii) carbon (iii) hydrogen (iv) chlorine
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv)
Answer: (d) (i) and (iv)

Question. Which of the following represents saponification reaction?
(a) \( CH_3COONa + NaOH \xrightarrow{CaO} CH_4 + Na_2CO_3 \)
(b) \( CH_3COOH + C_2H_5OH \xrightarrow{H_2SO_4} CH_3COOC_2H_5 + H_2O \)
(c) \( 2CH_3COOH + 2Na \to 2CH_3COONa + H_2 \)
(d) \( CH_3COOC_2H_5 + NaOH \to CH_3COONa + C_2H_5OH \)
Answer: (d) \( CH_3COOC_2H_5 + NaOH \to CH_3COONa + C_2H_5OH \)

Question. In which of the following compounds, —OH is the functional group?
(a) Butanone
(b) Butanol
(c) Butanoic acid
(d) Butanal
Answer: (b) \( CH_3CH_2CH_2CH_2OH \) has —OH as functional group.

Question. The soap molecule has a
(a) hydrophilic head and a hydrophobic tail
(b) hydrophobic head and a hydrophilic tail
(c) hydrophobic head and a hydrophobic tail
(d) hydrophilic head and a hydrophilic tail
Answer: (a) —COONa head is hydrophilic and the hydrocarbon chain is the hydrophobic tail.

Question. Identify the unsaturated compounds from the following
(i) Propane (ii) Propene (iii) Propyne (iv) Chloropropane
(a) (i) and (ii)
(b) (ii) and (iv)
(c) (iii) and (iv)
(d) (ii) and (iii)
Answer: (d) Propene (\( CH_3—CH=CH_2 \)) and propyne (\( HC \equiv C—CH_3 \)) are unsaturated.