ISC Class 12 Chemistry Exam 2024 : Important Multiple Choice Questions Answers with Explanation

ISC Class 12 exams have started and you have very little time left for the Chemistry exam. Therefore, we are providing important multiple choice questions in this article. You can study them well and score well in your exams.

To help you prepare effectively, we have prepared a list of important multiple choice questions (MCQs) with detailed explanations. This guide aims to make your exam preparation easier by providing information on these questions.

Multiple choice questions are a common format in competitive exams including ISC Class 12 Chemistry exam. By practicing MCQs, you not only reinforce your knowledge but also learn to apply it effectively.

These questions cover various topics from the Chemistry syllabus. Remember, it is important to understand the logic behind each answer to score well.

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ISC Class 12 Chemistry Multiple Choice Questions 2024 

1. Sprinkling of salt helps in clearing the snow covered roads in hills. The phenomenon
involved in the process is :

(a) Lowering in vapour pressure of snow
(b) Depression in freezing point of snow
(c) Melting of ice due to increase in temperature by putting salt
(d) Increase in freezing point of snow

Ans. (b) Depression in freezing point of snow

Explanation : The phenomena of depression in freezing point of water helps in clearing the snow covered roads in hilly areas. When salt is sprinkled over snow covered roads, snow melts from the surface because depression of freezing point of water takes place due to addition of salt.

2. Which of the following condition is not satisfied by an ideal solution ?

(a) AH_mixing = O
(b) AV_mixing = O
(c) Raoult's Law is obeyed
(d) Formation of an azeotropic mixture

Ans. (d) Formation of an azeotropic mixture

Explanation : An ideal solution satisfies the following conditions :

AHmixing = O

AVmixing = O

It obeys Raoult's Law Whereas, azeotrophic mixtures are formed only in case of non-ideal solutions

3. The osmotic pressure of equimolar solution of NaCl, BaCl2 and glucose will be in the order of :

(a) NaCl > BaCl2 > Glucose
(b) BaCl2 > NaCl > Glucose
(c) Glucose > NaCl > BaCl2
(d) NaCl > Glucose > BaCl2

Ans. (b) BaCl2 > NaCl > Glucose

Explanation : Osmotic pressure is a colligative property, and more the number of particles more the osmotic pressure. πV = nRT

Where, π = Osmotic pressure

V = Volume
R = Gas Constant
T = Temperature

Hence, the order is BaCl2 > NaCl > Glucose since the concentration is same and i for BaCl2 is 3, NaCl is 2 and glucose is 1

4. Colligative properties depend on :

(a) The nature of solute particles in solution
(b) The number of solute particles in solution
(c) The nature of solute and solvent particles
(d) The physical properties of solute particles in solution

Ans. (b) The number of solute particles in solution

Explanation : A colligative property is a property of a solution that is dependent on the ratio between the total number of solute particles to the total number of solvent particles in a solution. It does not depend on the nature or identity of solute particles. The four colligative properties that can be exhibited by a solution are :

(i) Boiling point elevation
(ii) Freezing point depression
(iii) Relative lowering of vapour pressure
(iv) Osmotic pressure

5. Relative lowering of vapour pressure, osmotic pressure of a solution and elevation in boiling points are _________ properties. Osmosis is the passage of __________ through a semipermeable membrane from a solution of ___________ towards a solution of ____________ Osmotic pressure is equivalent to mechanical pressure which must be applied on ____________ to prevent osmosis.

(a) Colligative, solvent, lower concentration, higher concentration, solution.
(b) Colligative, solution, lower concentration, higher concentration, solvent.
(c) Colligative, solvent, higher concentration, lower concentration, solution.
(d) Colligative, solution, higher concentration, lower concentration, solution.

Ans. (a) Colligative, solvent, lower concentration, higher concentration, solution.

Explanation : Relative lowering of vapour pressure, osmotic pressure of a solution and elevation in boiling points are colligative properties. Osmosis is the passage of, solvent through a semipermeable membrane from a solution of lower concentration towards a solution of higher concentration. Osmotic pressure is equivalent to mechanical pressure which must be applied on solution to prevent osmosis.

6. Solutions which strictly obey ____________ law are called ____________ solutions. If the vapour pressure of non-ideal solution is _____________ than predicted by Raoult's law then it shows positive deviation and if it is ____________ than predicted by Raoult's law then it shows negative deviation

(a) Ideal, Raoult's, lower, higher
(b) Higher, Raoult's, ideal, lower
(c) Raoult's, ideal, higher, lower
(d) Raoult's, lower, higher, ideal

Ans. (c) Raoult's, ideal, higher, lower

Explanation : Solutions which strictly obey Raoult's law are called ideal solutions. If the vapour pressure of non-ideal solution is higher than predicted by Raoult's law then it shows positive deviation and if it is lower than predicted by Raoult's law then it shows negative deviation

7. A solution which does not obey Raoult's law at all range of concentration is called _____________ . A solution which distills without change in composition is called ____________ Ideal solutions obey __________ law and they __________ form azeotropic mixtures.

(a) Non-Ideal, Azeotrope, Raoult's, do not
(b) Azeotrope, Raoult's, Non-Ideal, do not
(c) Raoult's, Non-Ideal, do not, Azeotrope
(d) Non-Ideal, Raoult's, Azeotrope, do not

Ans. (a) Non-Ideal, Azeotrope, Raoult's, do not

Explanation : A solution which does not obey Raoult's law at all range of concentration is called non-ideal. A solution which distills without change in composition is called azeotrope. Ideal solutions obey Raoult's law and they do not form azeotropic mixtures.

8. For sodium chloride solution, Van't Hoff factor is _______ When solvent starts flowing from ________ into ________ through semi-permeable membrane, the phenomenon is termed as reverse osmosis. Relative lowering of vapour pressure is equal to the mole fraction of the ________ .

(a) solvent, solution, Greater than 1, Solute
(b) Solute, solution, solvent, Greater than 1
(c) Greater than 1, solute, solvent, solution
(d) Greater than 1, solution, solvent, Solute

Ans. (d) Greater than 1, solution, solvent, Solute

Explanation : For sodium chloride solution, Van't Hoff factor is greater than 1. When solvent starts flowing from solution into solvent through semi- permeable membrane, the phenomenon is termed as reverse osmosis. Relative lowering of vapour pressure is equal to the mole fraction of the solute.

9. The minimum excess pressure that has to be applied on the solution to prevent the entry of the solvent into the solution through semi- permeable membrane is called as _______ The unit of molal depression constant is ________ . The relative lowering in vapour pressure is proportional to the ratio of number of _______ Solutions having same osmotic pressure are called as ________ .

(a) Atmospheric pressure, ppm, hypertonic solutiom, Solute molecules to the total number of solvent
(b) Atmospheric pressure, K kg mol-1, Solute molecules to the total number of molecules in solution, hypertonic solution
(c) Osmotic Pressure, K kg mol-1, Solute molecules to the total number of molecules in solution, isotonic solution
(d) Osmotic Pressure, ppm, Solute molecules to the total number of solvent, hypertonic solution

Ans. (c) Osmotic Pressure, K kg mol−1, Solute molecules to the total number of molecules in solution, isotonic solution

Explanation : The minimum excess pressure that has to be applied on the solution to prevent the entry of the solvent into the solution through semi- permeable membrane is called as osmotic pressure. The unit of molal depression constant is K kg mol-1. The relative lowering in vapour pressure is proportional to the ratio of number of Solute molecules to the total number of molecules in solution. Solutions having same osmotic pressure are called as isotonic solutions.

10. Which one of the following statements is TRUE for the Galvanic cell ?

(a) Electrons flow from copper electrode to zinc electrode
(b) Current flows from zinc electrode to copper electrode
(c) Cations move towards copper electrode
(d) Cations move towards zinc electrode

Ans. (c) Cations move towards copper electrode

Explanation : In Galvanic cell electrons flow from zinc electrode to copper electrode. So, current flows from copper electrode to zinc electrode. Hence, Cations move towards copper electrode

11. The number of Faradays required to reduce one mol of Cu²+ to metallic copper is :

(a) One
(b) Two
(c) Three
(d) Four

Ans. (b) Two

Explanation : Faraday's First Law of Electrolysis states that the chemical deposition due to the flow of current through an electrolyte is directly proportional to the quantity of electricity (coulombs) passed through it.

The reduction of one mole of Cu²+ to Cu can be represented as : Cu²⁺ + 2e- → Cu

Since in the reduction of one mole of Cu²⁺, two moles of electrons are required thus the amount of charge required is 2F.

12. On dilution, the specific conductance of a solution :

(a) Remains unchanged
(b) Increases
(c) Decreases
(d) First increases then decreases

Ans. (c) Decreases

Explanation : On dilution, the specific conductance of a solution decreases. Conductivity changes with the concentration of the electrolyte. On dilution as volume of solution increases. Thus, the number of ions per ml decreases and hence conductivity decreases. Thus, the specific conductivity decreases on dilution.

13. The standard reduction potential values of three metallic cations X, Y and Z are 0.52 V,-3.03 V and -1.18 V respectively. The order of reducing power of the corresponding metals is :

(a) Y > Z > X
(b) x > Y > Z
(c) Z > Y > X
(d) Z > X > Y

Ans. (a) Y > Z>X

Explanation : As the reduction potential drops, the reducing power of the electrode increases Y (-3.03 V) > Z (-1.18 V) > X (0.52 V)

14. When zinc granule is dipped into copper sulphate solution, copper is precipitated because :

(a) Both copper and zinc have a positive reduction potential.
(b) Both copper and zinc have a negative reduction potential.
(c) Reduction potential of zinc is higher than that of copper.
(d) Reduction potential of copper is higher than that of zinc.

Ans. (d) Reduction potential of copper is higher than that of zinc.

Explanation : When zinc granule is dipped into copper sulphate solution, copper is precipitated because reduction potential of copper is higher than that of zinc. Therefore, copper get reduced more easily as compared to zinc.

15. The molar conductance of a solution _______ with dilution, while its specific conductance ________ with dilution. The electrode at which there is acceptance of electrons is called ________ .

(a) Decreases, increases, anode
(b) Remains same, decreases, Pt electrode
(c) Increases, decreases, cathode
(d) Decreases, remains same, anode

Ans. (c) Increases, decreases, cathode

Explanation : The molar conductance of a solution increases with dilution, while its specific conductance decreases with dilution. The electrode at which there is acceptance of electrons is called cathode.

16. The solution of sugar in water is __________ conductor of electricity. In a galvanised iron, the iron is coated with a layer of ____________ metal.

(a) Bad, zinc
(b) Good, copper
(c) Bad, less, chromium
(d) Good, Silver

Ans. (a) Bad, zinc

Explanation : The solution of sugar in water is bad conductor of electricity. In a galvanised iron, the iron is coated with a layer of zinc metal.

17. The rate of a chemical reaction is double for every 10°C rise in temperature because of :

(a) increase in the activation energy.
(b) decrease in the activation energy.
(c) increase in the number of molecular collisions.
(d) increase in the number of activated molecules.

Ans. (d) increase in the number of activated molecules.

Explanation : The rate of a chemical reaction is double for every 10°C rise in temperature because of increase in the number of activated molecules (i.e., the number of particles that have the minimum energy required). As the number of activated molecules is increased therefore, the rate of chemical reaction gets double.

18. In the reaction 2A + B → A₂B, if the concentration of A is doubled and that of B is halved, then the rate of the reaction will :

(a) increase 2 times
(b) increase 4 times
(c) decrease 2 times
(d) remain the same

Ans. (a) increase 2 times

Explanation : 

Reaction : 2A + B → A₂B

Rate = k[A]² [B]; k → rate constant

According to question, concentration of A is doubled and B is halved.

→ Rate = k[2A]²[B/2]

= k × 2[A]^² [B]

Thus, the rate of reaction increases 2 times when the concentration of A is doubled and that of B is halved.

19. A catalyst is a substance which :

(a) Changes the equilibrium constant of the reaction.
(b) Increases the equilibrium constant of the reaction.
(c) Supplies energy to the reaction.
(d) Shortens the time to reach equilibrium.

Ans. (d) Shortens the time to reach equilibrium.

Explanation : A catalyst is a substance which increases the rate of the reaction without itself being consumed during the course of the reaction. They shortens the time to reach equilibrium for a chemical reaction as they decreases the activation energy of the reactants.

20. Which transition metal has the highest density ?

(a) Os
(b) Zn
(c) Sc
(d) La

Ans. (a) Os

Explanation : The decrease in the metallic radius coupled with increase in atomic mass results in a general increase in the density of the elements. Thus, Osmium is the transition element which has the highest density of 22.

21. Which transition metal shows highest oxidation state ?

(a) Sc
(b) Ti
(c) Os
(d) Zn

Ans. (c) Os

Explanation : Osmium is the 5d series transition element that shows the highest oxidation state. The electronic configuration of Osmium is Os(76) = [Xe] 4f¹⁴ 5d⁶ 6s²

(i) Osmium has the highest oxidation state because the number of unpaired electrons in the outermost shell is more, i.e., 5d⁵ 6s².

(ii) If we consider all the transition metals the highest oxidation state is eight and the element which shows +8 oxidation state are Ruthenium (Ru) and Os (Osmium).

22. Which ion gives coloured solution ?

(a) Cu⁺
(b) Zn²⁺
(c) Ag⁺
(d) Fe²⁺

Ans. (d) Fe²⁺

Explanation : Fe²⁺ ions give coloured solution due to the presence of 4 unpaired electrons. Only the ions that have electrons in orbital and in which d-d transition is possible will be coloured. The ions in which d-orbitals are empty or completely filled will be colourless as no d-d transition is possible in those configurations.

23. Which of the following oxides of chromium is amphoteric in nature ?

(a) CrO
(b) Cr₂O₃
(c) CrO₃
(d) CrO₅

Ans. (b) Cr₂O₃

Explanation : Chromic oxide (or chromium (III) oxide) is an amphoteric compound. It has the ability to dissolve in acids, liberating hydrated chromiumions in the process. These hydrated Cr ions can react with bases to form complex salts.

24. Number of moles of K₂Cr₂O₇ reduced by one mole of Sn²⁺ ions is :

(a) 1/3
(b) 3
(c) 1/6
(d) 6

Ans. (a) 1/3

Explanation : The balanced chemical reactions (ionic reactions) for reduction of K2Cr2O7 by Sn²+ are : Cr2O72+ 14H++ 6e¯ → 2Cr3+ + 7H2O

( Sn2+ → Sn4+ + 2e- ) × 3

Balanced Net reaction : 3Sn2+ + Cr2O72 + 14H+ → 3Sn4+ + 2Cr3+ + 7H2O

Thus, according to the balanced reaction : 1 mole of Cr₂O7^2- will be reduced by 3 moles of Sn²⁺.

Thus, 1 mole of Sn²⁺ will reduce = 1/3 moles of Cr₂07^2- .

25. Which of the following element belongs to actinoid series ?

(a) La
(b) Gd
(c) Lu
(d) Th

Ans. (d) Th

Explanation : Lu, Gd and La belongs to lanthanide series and Th belongs to actinide series.

26. Lanthanide contraction is caused due to:

(a) The same effective nuclear charge from Ce to Lu.
(b) The imperfect shielding on outer electrons by 4f electrons from the nuclear charge.
(c) The appreciable shielding on outer electrons by 4f electrons from the nuclear charge.
(d) the appreciable shielding on outer electrons by 5d electrons from the nuclear charge.

Ans. (b) The imperfect shielding on outer electrons by 4f electrons from the nuclear charge.

Explanation : The cause of the lanthanoid contraction is the poor shielding effect of 4f orbital due to which the electrons of s-orbitals attract more towards the nucleus due to which the size of the atoms decreases resulting in a smaller atomic radius.

27. In the complexes [Fe(CN)6]^³- and [Pt (en (H₂O)₂(NO₂) (Cl)]²⁺ the respective oxidation numbers of central metal atoms are :

(a) + 3 and + 4
(b) + 6 and + 4
(c) + 6 and + 3
(d) + 3 and + 3

Ans. (a) + 3 and + 4

Explanation : In the complexes [Fe(CN)₆]^3- and [Pt(en)(H₂O)₂(NO₂)(Cl)]²⁺, the respective oxidation numbers of central metal atoms are + 3 and + 4.

In the complex [Fe(CN)₆]^3-, the cyano ligand has - 1 oxidation number. Six cyano ligands contribute - 6 to the oxidation number. The complex ion has a net charge of - 3. Hence, the oxidation number of Fe is + 3.

In the complex [Pt(en)(H₂O)₂(NO₂)(Cl)]²⁺, en and water ligands have o-oxidation number and C1 ligand has −1 oxidation number and NO₂ ligand has -1 oxidation number. The net charge of the complex ion is + 2. Hence, the oxidation number of Pt is + 4.

28. What is the oxidation number of central atom in Na [Hg (CN)₂] ?

(a) +4
(b) +2
(c) 0
(d) +1

Ans. (d) +1

Explanation : The oxidation number of central atom is Na[Hg(CN)₂] is + 1. The oxidation state of Na is + 1 and of CN is - 1, so by calculating oxidation state of Hg is

x - 2 = -1

x = -1 + 2

x = + 1

Hence, the oxidation number of central metal Hg is +1.

29. What is the coordination number of central metal atom in [Pt(NH₃)₂C1₂] C1 ?

(a) 6
(b) 4
(c) 3
(d) 7

Ans. (b) 4

Explanation : NH₃ is neutral while Cr has - 1 charge. Hence cation part of complex has + 2 net charge on it. So, Pt has + 4 oxidation state.

30. The IUPAC name for the complex [Co(NO₂) (NH₃)₅]C1₂ is :

(a) Nitrito-N-pentaammine cobalt(III) chloride
(b) Nitrito-N-pentaammine cobalt(II) chloride
(c) Pentaammine nitrito-N-cobalt(II) chloride
(d) Pentaammine nitrito-N-cobalt(III) chloride

Ans. (d) Pentaammine nitrito-N-cobalt(III) chloride

Explanation : Oxidation number of Co is x - 1 = 2 → x = 3. Nitrogen is donating its lone pairs in the given compound. The IUPAC name is pentaammine nitrito-N-cobalt(III) chloride.

31. The coordination number of 'Co' in the complex [Co(en)₃]³⁺ is :

(a) 3
(b) 6
(c) 4
(d) 5

Ans. (b) 6

Explanation : Ethylenediamine is a bidentate chelating ligand for coordination compounds, with the two nitrogen atoms donating their lone pairs of electrons. In the complex [Co(en)₃]³⁺, there are three bidentate en ligands, and the coordination number of the cobalt(III) ion is six because 3 en ligands are present.

32. Which is not true about the coordination compound [Co(en)₂C1₂]C1 ?

(a) Exhibits geometrical isomerism
(b) Exhibits optical isomerism
(c) Exhibits ionisation isomerism
(d) Is an octahedral complex

Ans. (c) Exhibits ionisation isomerism

Explanation: The complex [Co(en)₂C1₂]Cl will possess two geometrical isomer (cis and trans) and its cis isomer is optically active that's why the total number of isomer is three. Also, the number of ligands attached are 6 therefore it is a octahedral complex. But it cannot show ionisation isomer because compounds which gives different ions in solution although they have same composition are called ionisation isomerism.

33. In Fe(CO)₅, the Fe-C bond possesses :

(a) п-character only
(b) Both s and П- characters
(c) Ionic character
(d) s-character only

Ans. (b) Both s and п - characters

Explanation : It's called "synergic bonding". The ligand (CO) donates it's lone pair of electrons to the vacant orbitals of the iron atom and forms the sigma- bond. Since the iron atom also possesses some electrons in it's d-orbitals, it back donates those electrons to the molecular orbitals of the ligand forming a p-bond. In this way, the metal-carbon bond length is reduced and the complex donates it's electron pairs to the antibonding molecular orbital of CO, so the C-O bonds is weakened by this synergic bonding, leading to a larger C-O bond length in the complex (as opposed to a free CO molecule).

Metal-C bond length reduces, C-O bond length increases, complex gets stability.

It is important to note that the metal atom gets more stability. 

34. Carbylamine test involves heating a mixture of :

(a) Alcoholic KOH, methyl iodide, and sodium metal
(b) Alcoholic KOH, methyl iodide, and primary amine
(c) Alcoholic KOH, chloroform, and primary amine
(d) Alcoholic KOH, methyl alcohol, and primary amine

Ans. (c) Alcoholic KOH, chloroform, and primary amine 

Explanation : Aliphatic and aromatic primary amines on heating with CCl4 and alcoholic potassium hydroxide (alc. KOH) gives foul smelling alkyl isocyanides or carbylamines. So, Carbylamine test is performed in alcoholic KOH by heating a mixture of chloroform, and primary amine. 

35. A primary alkyl halide would prefer to undergo :

(a) S_N^¹ reaction 
(b) S_N² reaction
(c) a-Elimination
(d) Racemisation

Ans. (b) S_N² reaction

Explanation : As primary alkyl halide is least sterically hindered among primary, secondary and tertiary alkyl halides, therefore primary alkyl halides undergo S_N² reaction.

36. During the course of S_N¹ reaction, the intermediate species formed is :

(a) A free radical
(b) A carbanion
(c) A carbocation
(d) An intermediate complex

Ans. (c) A carbocation

Explanation : S_N¹ reactions are unimolecular and the rate of such reactions depends only on the concentration of one reactant. S_N¹ reactions happen in two steps :

(i) The leaving group leaves, and the substrate forms a carbocation which is an intermediate.

(ii) The nucleophile attacks the carbocation, forming the product.

37. The reaction : CH₃Br + OH− → CH₃OH + Br+ 

(i) The expected mechanism of the above reaction is :

(a) S_N¹ mechanism
(b) S_N² mechanism
(c) S_E¹ mechanism
(d) S_E² mechanism

Ans. (b) S_N² mechanism

Explanation : Usually, primary alkyl halides and methyl halides undergo substitution by S_N² mechanism. During the by S_N² mechanism, bond breaking and bond formation takes place.

(ii) The above reaction is :

(a) Elimination reaction
(b) Nucleophilic addition reaction
(c) Nucleophilic substitution reaction
(d) Electrophilic substitution reaction

Ans. (c) Nucleophilic substitution reaction

Explanation : CH₃Br + OH- → CH₃OH + Br-

In this reaction, nucleophile Br- is replaced by another nucleophile OH-. So, this is a nucleophilic substitution reaction.

38. In alkyl halides, due to greater polarity as well as higher molecular mass, as compared to the parent hydrocarbon, the intermolecular __________ and ___________ of attraction are stronger in the halogen derivatives.

(a) Dipole-dipole and Van der Waal's forces.
(b) Hydrogen bond and dipole-dipole forces.
(c) Van der Waal's and hydrogen bond forces.
(d) Dipole-dipole and London forces.

Ans. (a) Dipole-dipole and Van der Waal's forces.

Explanation : In alkyl halides, due to greater polarity as well as higher molecular mass, as compared to the parent hydrocarbon, the intermolecular dipole-dipole and Van der Waal's forces of attraction are stronger in the halogen derivatives.

39. Which of the following alcohol is least soluble in water ?

(a) N-Butyl alcohol
(b) Iso-Butyl alcohol
(c) Tert-Butyl alcohol
(d) Sec-Butyl alcohol

Ans. (a) N-Butyl alcohol 

Explanation : Amongst isomeric alcohols, as branching increases, the surface area of the non-polar hydrocarbon increases, consequently the solubility increases.

40. The ionisation constant of phenol is higher than that of ethanol because :

(a) Phenoxide ion is a stronger base than ethoxide ion
(b) Phenoxide ion is stabilised through delocalization
(c) Phenoxide ion is less stable than ethoxide ion
(d) Phenoxide ion is bulkier than ethoxide ion

Ans. (b) Phenoxide ion is stabilised through delocalisation

Explanation : As phenoxide ion is stabilized through delocalisation, thus the ionisation constant of phenol is higher than that of ethanol.

41. The correct order of boiling points for primary (1°), secondary (2°) and tertiary alcohol (3°) is :

(a) 1° > 2° >3°
(b) 3° > 2° > 1°
(c) 2° > 1° >3°
(d) 2° >3° > 1°

Ans. (a) 1° 2° >3°

Explanation : For alcohols boiling points depends upon 3 factors, viz. molecular weight, number of available H-bonds and the surface area of the molecule. If the molecular weight of all the alcohols are more or less same, then the boiling point will also be nearly same. Now, both the number of available H-bonds and the surface area of the molecule are least In 3° alcohols and maximum in 1° alcohols. Hence, 3° alcohols have a least boiling point while 1° alcohols have a maximum boiling point. Hence the correct sequence will be, 1° > 2° > 3°. 

42. Lucas test is used for distinction of :

(a) Alcohols
(b) Phenols
(c) Alkyl halides
(d) Aldehydes

Ans. (a) Alcohols

Explanation : Lucas test in alcohols is conducted to distinguish between primary, secondary, and tertiary alcohols. It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides through a substitution reaction :

ROH + HC1 → RC1 + H₂O

43. The boiling point of methanol is greater than that of methyl thiol because :

(a) There is intermolecular hydrogen bonding in methanol and no hydrogen bonding in methyl thiol.
(b) There is intramolecular hydrogen bonding in methanol and no hydrogen bonding in methyl thiol.
(c) There is intramolecular hydrogen bonding in methanol and intermolecular hydrogen bonding in methyl thiol.
(d) There is no hydrogen bonding in methanol and intermolecular hydrogen bonding in methyl thiol

Ans. (a) There is intermolecular hydrogen bonding in methanol and no hydrogen bonding in methyl thiol.

Explanation : Methanol has high boiling point than methyl thiol because there is intermolecular hydrogen bonding in methanol and no hydrogen bonding in methyl thiol.

44. Alcohols can be obtained from all methods except :

(a) Hydroboration-oxidation
(b) Oxymercuration-demercuration
(c) Reduction of aldehyde/ketones with Zn-Hg/HCl
(d) By fermentation of starch

Ans. (c) Reduction of aldehyde/ketones with Zn-Hg/HCl

Explanation : As per the Clemmensen reduction, reduction of aldehydes and ketones with Zn(Hg)/HCl yields alkanes.

45. When phenol is treated with excess of bromine water, it gives :

(a) m-bromophenol
(b) o-and p-bromophenol
(c) 2,4-dibromophenol
(d) 2, 4, 6-tribromophenol

Ans. (d) 2, 4, 6-tribromophenol

Explanation : When phenol is treated with excess bromine water, it gives 2, 4, 6-tribromophenol

46. _______ is an example of trihydric alcohol and _______ is an example of dihydric alcohol. Ethyl bromide on reaction with moist silver oxide gives _______ as the main product.

(a) iso-propanol, methanol, ethanol
(b) Glycol, ethanol, methanol
(c) Ethanol, glycol, propanol
(d) Glycerol, glycol, ethanol

Ans. (d) Glycerol, glycol, ethanol

Explanation : Glycerol is an example of trihydric alcohol and glycol is an example of dihydric alcohol. Ethyl bromide on reaction with moist silver oxide gives ethanol. as the main product.

47. Alcohols act as __________ due to the presence of unshared electron pairs on oxygen atom. Ethers are ___________ in nature.

(a) Bronsted bases, basic
(b) Bronsted acids, acidic
(c) Bronsted acid, neutral
(d) Bronsted bases, amphoteric

Ans. (a) Bronsted bases, basic

Explanation : Alcohols act as bronsted bases due to the presence of unshared electron pairs on oxygen atom. Ethers are basic in nature.

48. The addition of HCN to carbonyl compounds is an example of :

(a) Nucleophilic addition
(b) Electrophilic addition
(c) Free radical addition
(d) Electromeric addition

Ans. (a) Nucleophilic addition

Explanation : The reaction of HCN with carbonyl compounds is an example of the nucleophilic addition reaction. The base is used as a catalyst and it deprotonates the HCN molecule to produce a stronger nucleophile i.e., cyanide ion. The cyanide ion thus formed acts as a nucleophile which attacks at the double bond of carbon and oxygen and forms an addition product.

49. Which of the following compounds will undergo Cannizzaro reaction ?

(a) CH₃CHO
(b) CH₃COCH₃
(c) C₆H₅CHO
(d) C₆H₅CH₂CHO

Ans. (c) C₆H₅CHO

Explanation : Self-oxidation and reduction of aldehydes which do not possess an alpha hydrogen atom in the presence of concentrated alkali (aqueousor alcoholic) is known as Cannizzaro reaction. The reaction products are alcohol and salt of carboxylic acid.

50. Which of the following does not answer iodoform test ?

(a) n-Butyl alcohol
(b) Ethylmethyl ketone
(c) Acetophenone
(d) Acetaldehyde

Ans. (a) n-Butyl alcohol

Explanation : Iodoform test is given by the compounds containing CH₃CO-group or CH₃CHO-group. From the given options, acetophenone, acetaldehyde and ethylmethyl ketone, all will give positive iodoform test because all are having CH₃CO present. But the n-butyl alcohol will not give positive iodoform test as it does not have (CH₃)RCH-OH group that can be oxidized to CH₃CO.

51. Which of the following compound will undergo self-aldol condensation in the presence of cold dilute alkali ?

(a) CH = C-CHO
(b) CH₂ = CHCHO
(c) C₆H₅CHO
(d) CH₃CH₂CHO

Ans. (d) CH₃CH₂CHO

Explanation : Since CH3CH2CHO has a-hydrogen atom, therefore it will undergo self-aldol condensation in the presence of cold dilute alkali.

52. Trimer of acetaldehyde is named as _________ .

(a) Paraldehyde
(b) Formaldehyde
(c) Metaldehyde
(d) Pentaldehyde

Ans. (a) Paraldehyde

Explanation : Trimer of acetaldehyde is named as Paraldehyde which is the cyclic trimer of acetaldehyde molecules. Formally, it is a derivative of 1,3,5- trioxane, with a methyl group substituted for a hydrogen atom at each carbon.

53. A ____________ precipitate is obtained on adding iodine and sodium hydroxide to ____________ .

(a) Red, acetic acid
(b) Yellow, ketone
(c) Orange, propanol
(d) Blue, acetaldehyde

Ans. (b) Yellow, ketone

Explanation : When acetone reacts with iodine in the presence of alkali, iodoform is formed which is a light yellow color precipitate and the reaction is known as iodoform reaction. The reaction is as follows :

C₆H₅COCH₃ + NaOH + I₂ → C₆H₅Ca₂Na + CHI₃

54. Benzaldehyde undergoes __________ reaction on treatment with concentrated sodium hydroxide because it has _________ atom.

(a) Cannizzaro, no alpha-hydrogen
(b) Aldol condensation, alpha hydrogen
(c) Wurtz, no alpha-hydrogen
(d) HVZ reaction, alpha hydrogen

Ans. (a) Cannizzaro, no alpha-hydrogen

Explanation : Benzaldehyde undergoes cannizzaro reaction on treatment with concentrated sodium hydroxide because it has no alpha-hydrogen atom. One molecule of benzoic acid is oxidised to benzoate ion and other molecule is reduced to benzyl alcohol.

55. The reaction of a primary amine with chloroform and ethanolic KOH is called :

(a) Carbylamine reaction
(b) Kolbe's reaction
(c) Reimer-Tiemann reaction
(d) Wurtz-Fitting reaction

Ans. (a) Carbylamine reaction

Explanation : The reaction of a primary amine with chloroform and ethanolic KOH is called as carbylamine reaction, which is given as :

RNH₂ + CHC1₃ + 3KOH → RN⁺ = C^- + 3KC1 + 3H₂O

56. Which of the following does not react with Hinsberg reagent ?

(a) Ethyl amine
(b) (CH₃)₂NH
(c) (CH₃CH₂)₃N
(d) Propane-2-amine

Ans. (c) (CH₃CH₂)₃N

Explanation : N,N-Diethylethanamine (CH₃CH₂)₃N is a Tertiary amine. Tertiary amines (R₃N) does not react with Hinsberg reagent (C₆H₅SO₂C1) as there are no freely available hydrogen on the amine group which can donate electrons to the sulphur of the Hinsberg reagent.

57. The linkage that holds monosaccharide units together in a polysaccharide is called :

(a) Peptide linkage
(b) Glycoside linkage
(c) Ester linkage
(d) Ionic linkage

Ans. (b) Glycoside linkage

Explanation : In polysaccharides, the linkage connecting monosaccharide is called a glycosidic linkage. Glycosidic linkage is the covalent bond between two or more monosaccharides to form a polysaccharide. It is formed between the hemiacetal or hemiketal group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some compound such as alcohol.

58. ____________ is a monosaccharide which is sweeter than sucrose.

(a) Fructose
(b) Glucose
(c) Galactose
(d) Maltose

Ans. (a) Fructose

Explanation : Fructose is a naturally occurring monosaccharide which is sweeter than sucrose. It is 1.2-1.8 times sweeter than sucrose. Fructose metabolism does not require insulin and has a low impact on blood glucose levels.

59. a-helix refers to :

(a) Primary structure of proteins
(b) Secondary structure of proteins
(c) Tertiary structure of proteins
(d) Quaternary structure of proteins

Ans. (b) Secondary structure of proteins

Explanation : Proteins are the polypeptides of amino acid and they have different structure which is because of a different arrangement of amino acid. a-helix refers to secondary structure of proteins. In other words, the folding of the linear polypeptide chain into a specific structure that has the alpha helix is called the secondary structure.

60. The non-proteinous substances which certain enzymes require for their activity are called :

(a) Catalysts
(b) Inhibitors
(c) Co-enzymes
(d) Epimers

Ans. (c) Co-enzymes

Explanation : The non-proteinous substances which certain enzymes require for their activity are called co-enzymes that are organic molecules required by certain enzymes to carry out catalysis. Co-enzymes function as an intermediate carriers of electrons, specific atoms or functional groups that are transferred in the overall reaction. They bond to the active site of the enzyme and participate in catalysis but are considered substrates of the reaction.

61. Amino acids are least soluble in water :

(a) At pH = 7
(b) At pH > 7
(c) At pH < 7
(d) At isoelectric point

Ans. (d) At isoelectric point

Explanation : Amino acids are least soluble in water when the pH of the solution is the same as the isoelectric point for the amino acid.

62. Which of the following amino acid is not optically active ?

(a) Alanine
(b) Glycine
(c) Phenylanine
(d) All are optically active

Ans. (b) Glycine

Explanation : All amino acids have an amine and a carboxyl group. They also have a functional group which is unique to each of them. Depending on the polarity of the functional group the amino acid can be rendered optically inactive or active. The simplest of all amino acids, Glycine which has H as a functional group lacks optical acitivity, as the saturated alpha carbon is unsubstituted. Others among the 20 amino acids have D or L stereoisomers.

63. At isoelectric point the amino acids have ___________ solubility in water.

(a) Maximum
(b) Zero
(c) Least
(d) None of these

Ans. (c) Least

Explanation : At isoelectric point the amino acids have least solubility in water. The solubility of amino acids depend on the pH of the solution. The intermediate pH at which a protein molecule has a net charge of zero is called the isoelectric point of that protein. In general, the net charge on the protein, either positive or negative, can interact with water molecules, meaning that it is more likely water soluble. As a result, amino acid is the least soluble when the pH of the solution is at its isoelectric point.

64. Neutral amino acids have __________ in the pH range of 5.6 - 6.3.

(a) Iso-electric point
(b) Electric point
(c) Piezoelectric point
(d) None of the above

Ans. (a) Iso-electric point

Explanation : The isoelectric point (IP) is the pH at which the amino acid has an overall zero charge. The isoelectric point (IP) of neutral amino acids
range from 5.6 - 6.3.

65. A nucleoside is made up of :

(a) A base and sugar
(b) A base and phosphoric acid
(c) A sugar and phosphoric acid
(d) A sugar, a base and phosphoric acid

Ans. (a) A base and sugar

Explanation : Nucleoside is sugar + base. Nucleosides are glycosylamines that can be thought of as nucleotides without a phosphate group. A nucleoside consists simply of a nucleobase (also termed as nitrogeneous base) and a 5-carbon sugar (either ribose or deoxyribose).

66. The sequence of bases on m-RNA molecule, synthesised on the GCATA strand of DNA is :

(a) CGUAU
(b) CGTAT
(c) TACGC
(d) ATCGC

Ans. (a) CGUAU

Explanation : The sequence of bases on m-RNA molecule synthesised on the GCATA strand of DNA is CGUAU. Within the RNA double helix, A forms two hydrogen bonds with U on the opposite strand, and G forms three hydrogen bonds with C on the opposite strand.

67. Which of the following is correct about H-bonding in nucleotide ?

(a) A-T, G-C
(b) A-G, T-C,
(c) G-T, A-C
(d) A-A, T-T

Ans. (a) A-T, G-C

Explanation : The double helix structure of DNA molecule is made up of polynucleotide chains which are held together by H-bonds. In these helixes, the adenine (A) base is linked with thymine (T) by two H-bonds and guanine (G) is linked with cytosine (C) by three H-bonds as A = T and G = C.

68. The chemical changes in DNA molecule that could lead to synthesis of proteins with an altered amino acid sequence is called __________ .

(a) Transcription
(b) Translation
(c) Mutation
(d) Replication

Ans. (c) Mutation

Explanation : The chemical changes in DNA molecule that could lead to synthesis of proteins with an altered amino acid sequence is called mutation.

69. The sugar in RNA is __________ .

(a) D-ribose
(b) L-ribose
(c) Deoxyribose
(d) D-ribonucleic

Ans. (a) D-ribose

Explanation : Ribose, also called D-ribose is a five-carbon sugar found in RNA, where it alternates with phosphate groups to form the "backbone" of the RNA polymer and binds to nitrogenous bases. Thus, the sugar in RNA is D-ribose.

70. The helical structure of protein is stabilised by :

(a) Hydrogen bonds
(b) Ether bonds
(c) Peptide bonds
(d) Dipeptide bonds

Ans. (a) Hydrogen bonds

Explanation : The helical structure of protein is stabilised by hydrogen bonds between amide group of the same peptide chain. These bonds are formed by -NH- group of one unit and oxygen of carbonyl group of the third unit. This H-bonding is responsible for holding helix in a position.

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ISC Class 12 Study Material