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Class 10 Chemistry Solved Paper 2020

Section A

Q1. MCQs

Not Available

Class 10 Chemistry Solved Paper 2020

Section B


(i)    How do ultraviolet radiations break chlorofluorocarbons (CFCs)?

(ii)    Complete and balance the following chemical reactions:

(iii)   What is the function of DNA?

Ans.  The function of DNA is to store and transmit all the genetic information needed to build an organism. For instance, in human beings, the single fertilized egg cell carries the information for making legs, hands, head, liver, heart, kidneys, etc. DNA is found primarily in the cell nucleus.

(iv)  Write down the macroscopic characteristics of forward and reverse reaction.


Forward ReactionReverse Reaction
It is written from left to right.It is written from right to left.
Reactants produce products.Products produce reactants.
Rate is fastest in the beginning and gradually slows down.Its rate is zero in the beginning and gradually speeds up.

(v)   What do you know about dehydration reaction? Give one example.

Ans.    Dehydration means loss of water. Aldehydes dehydrate when their vapours are passed over heated alumina.

(vi)   Write chemical equation to show the preparation of ethyne from dehydrohalogenation of 1,2-dihalide.

Ans.  Vicinal dihalide on treatment with alcoholic potassium hydroxide eliminates two molecules of hydrogen halides from adjacent carbon atoms. The removal of two molecules forms a triple bond between two carbon atoms. The reaction occurs in two steps.

(vii)   Identify the functional group in following compounds and write names of these groups. Also encircle the functional group:

(viii)  Why does BF3 act as Lewis acid?

Ans:   Boron in BF3 has an incomplete octet. It has 6 electrons (3 electron pairs). So, it needs an electron pair to complete its octet. Hence BF3 is an electron pair acceptor or Lewis acid.

(ix)   What is the name of bond formed between two amino acids in building a protein? Also show its formation.

Excluded from the syllabus for Annual Exams 2021

(x)   Differentiate between soft and hard water.


Soft WaterHard Water
Water that easily gives lather with soap and does not form scum is called soft water.Water that gives little lather or forms scum with soap is called hard water.
No Calcium or Magnesium Salts are present in soft water.Calcium and Magnesium Salts are present in hard water.
Example of soft water is drinking water.Example of hard water is seawater.

(xi)  Describe how temperature changes as one moves from Earth’s surface into atmosphere up to 50 km?

Ans.    When we move from Earth’s surface to 12km, this region of the atmosphere is called Troposphere, and temperature changes from 17˚C to -55˚C.

           From 12 km above the Earth’s surface to the height of 50km, the region of the atmosphere is called Stratosphere. The temperature changes from -55 ˚C to -5 ˚C.

(xii)  What are lipids?

Excluded from the syllabus for Annual Exams 2021

(xiii)  Distinguish mono, di and trisaccharide. Give one example of each.

Ans. Mono, di, and trisaccharides are the types of oligosaccharides.

Monosaccharides are the simplest carbohydrates. They can not be hydrolyzed. They have general formula (CH2O)n. For example, Glucose C6H12O6.

Oligosaccharides that produce two monosaccharide units on hydrolysis are called disaccharides. For example, Sucrose C12H22O11.

Oligosaccharides that produce three monosaccharide units on hydrolysis are called trisaccharides. For example, Raffinose C18H32O16.

(xiv)   What do you mean by the term Destructive Distillation of coal?  

Ans. Destructive Distillation of Coal
When coal is heated in the absence of air at high temperatures, it is converted into coal gas, coal tar, and coke. This process is called destructive distillation.

(xv)  Write down any three natural processes that contribute toward the air pollution.

Ans.    The three natural processes that contribute towards air pollution are as under:
(a) Forest fires and dust storms release smoke and dust particles into the air.
(b) Volcanos emit clouds of dust and poisonous gases along with ash.
(c) Termites and cows also release a large amount of methane into the air.

Section C

Q3.a.   Describe different stages of Solvay process. (Flow chart is not required)

Ans. Stages of Solvay Process
The Solvay process consists of the following steps:

i) Preparation of ammonical brine:
Ammonical brine is prepared by dissolving ammonia gas in brine. Ammonical brine is fed into the carbonating tower.

ii) Carbonation:
In the carbonating tower, carbon dioxide is passed through ammonical brine. Following reaction takes place in it.
CO2(g) + NH3(g) + H2O(l) → NH4HCO3(aq)
NH4HCO3(aq) + NaCl(aq) → NaHCO3(s) + NH4Cl(aq)

In the lower compartments of carbonating tower, the temperature of the mixture is lowered to 15°C. At this temperature, NaHCO3 precipitates out.

iii) Filtration
Precipitates of NaHCO3 are separated from the milky solution by filtration. It is used as baking soda.

iv) Calcination
Sodium hydrogen carbonate is heated to get sodium carbonate.
2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g)

Carbon dioxide released is re-cycled in the process.

v) Preparation of carbon dioxide and slaked lime
Carbon dioxide is produced by heating limestone in a kiln.
CaCO3(l) → CaO(s) + CO2(g)
Slaked lime is pumped to the ammonia recovery tower.

vi) Recovery of ammonia
The solution containing ammonium chloride produced in the carbonation tower is heated with slaked lime.
2NH4Cl(aq) + Ca (OH)2(aq) → 2NH3(g) + CaCl2(aq) + 2H2O(l)
Almost all the ammonia is recovered in this process. It is reused in the process.

Q3.b.  Write the methods to remove permanent hardness of water.

Ans.  Methods to remove permanent hardness
i) By adding washing soda
On a large scale, permanent hardness in water can be removed by adding washing soda (Na2CO3.10H2O). Ca+2 and Mg+2 ions are removed as their insoluble carbonates.

ii) By ion Exchange Resins
The hard water is passed through a container filled with a suitable resin containing sodium ions. Zeolite is one of the natural ion exchangers. Chemically it is sodium aluminum silicate. It is usually written as Na2Z. The Ca+2 or Mg+2 ions causing the hardness are exchanged with Na+ ions in the resin.

Q4.a.   State Law of Mass Action. Derive the expression for equilibrium constant.

Ans.  Law of Mass Action
It states that “the rate at which a substance reacts is directly proportional to its active mass. The rate at which the reaction proceeds, is directly proportional to the product of the active masses of the reactants”.

The term “active mass” represents the concentration of reactants and products in mol.dm-3 for a dilute solution and is expressed in terms of square brackets [ ].

Consider a hypothetical reaction in which ‘a’ moles of reactant A and ‘b’ moles of reactant B react to give ‘c’ moles of product C and ‘d’ moles of product D at equilibrium.

aA(g) + bB(g)    cC(g) + dD(g)  
According to the law of mass action;

Rate of forward reaction α [A]a [B]b
Rate of forward reaction = kf [ A]a [B]b

Rate of reverse reaction α [C]c [D]d
Rate of reverse reaction= kr [C]c [D]d

Where kf and kr are the rate constants for forward and the reverse reactions respectively

At equilibrium state:
Rate of forward reaction = Rate of reverse reaction
Thus                       kf [ A]a [B]b = kr [C]c [D]d

Q4.b.  What is chemical nature of oligosaccharides? Describe their structure as well.

Ans.    Oligosaccharides
Carbohydrates which upon hydrolysis form 2 to 9 molecules of monosaccharides or simple sugars are called oligosaccharides.

Oligosaccharides are white crystalline solids. They are further clarified as disaccharides, trisaccharides, etc. depending upon the number of monosaccharides units, they produce on hydrolysis.

Q5. a.   Write down disadvantages of hard water.

Excluded from the syllabus for Annual Exams 2021

Q5.b.  Write down sources and uses of proteins.

Excluded from the syllabus for Annual Exams 2021

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