111 Most Probable Questions
(Explanations)
Decomposition Reaction:
1. Chemical Reactions and Equations
(i) ZnCO3(s) → ZnO(s) + CO2(g) (½ M)
1. (c) This is an oxidation-reduction reaction. A complete Heat
(vii) CaCO3(s) → CaO(s) + CO2(g) (½ M)
balanced equation is:
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2 Displacement Reaction
x = 4; y = 2; z = 1 (1 M) (ii) Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s) (½ M)
2. (c) In the given reaction, MnO2 loses oxygen and forms (v) Fe2O3 + 2Al → Al2O3 + 2Fe (½ M)
MnCl2. Therefore, it is said that MnO2 is reduced to Double Displacement Reaction
MnCl2. Whereas, HCl loses hydrogen and forms Cl2. (iii) NaBr(aq) + AgNO3(aq) → AgBr(s) + NaNO3(aq)
Therefore, it is said that HCl is oxidized to Cl2. (1 M)
3. (a) This is a thermal decomposition reaction. When lead 2. Acids, Bases and Salts
nitrate undergoes thermal decomposition, brown
fumes of nitrogen dioxide are produced. (1 M) 9. (b) Milk of magnesia is basic in nature having pH around
10. (1 M)
4. (a) Heat is released during an exothermic reaction along
10. (b) Calcium hydroxide turns milky due to the formation
with the formation of products. (1 M)
of calcium carbonate which on passing excess of CO2
5. (i) On mixing the two solutions, white precipitate of forms calcium hydrogen carbonate which makes the
barium sulfate will form. solution clear again. (1 M)
BaCl2(aq) + Na2SO4(aq) → BaSO4(s)+ 2NaCl(aq)
11. (a) (i) Since, compound X gets hardened when mixed
(1 M)
with water, therefore, X is Plaster of Paris and its
(ii) After 10 minutes, white precipitate will settle down at
chemical formula is CaSO4.1/2H2O. (1 M)
the bottom of the tube and solution above it becomes
(ii)
Baking soda is sodium hydrogen carbonate
colorless. (1 M)
(NaHCO3) and it is added to make cooking faster.
6. (i) To make water more conductive before electrolysis, (½ M)
acid is added to the solution. This makes it simple Baking powder is a mixture of baking soda and a
for the current to move through the solution. At the mild edible acid such as tartaric acid. (½ M)
cathode, hydrogen gas is released, whereas at the
12. (i) The ‘G’ and ‘C’ formulas are:
anode, oxygen gas is released. (1 M)
G = Cl2 (½ M)
The cathode gas collection volume is twice as large as
the anode gas collection volume. C = CaOCl2 (½ M)
2H2O(l) → 2H2(g) + O2(g) (1 M) (ii) The chemical equation involved is
(ii) Silver metal and chlorine gas are produced during the Ca(OH)2 + Cl2 → CaOCl2 + H2O (1 M)
decomposition of silver chloride when it is exposed to (iii) Bleaching Powder is the common name for chemical C.
light. The reaction is known as photolytic reaction. Calcium Oxychloride is its chemical name. (1 M)
(1 M) 13. (i) Dilution must take place by adding conc. acid into
7. The chemical equations for the various decomposition water by constant stirring. As dilution is an exothermic
reactions are: process, so it may be possible that acid may spill out
(i) 2H2O + electricity → 2H2(g) + O2 (1 M) if water is added to a bottle containing concentrated
(ii) CaCO3 + heat → CaO + CO2 (1 M) acid. (1 M)
(iii) 2AgBr + sunlight → 2Ag + Br2 (1 M) (ii) When H2SO4 is added to NaOH, formation of salt
(Na2SO4) takes place. As it is a neutral salt. Hence pH
8. Combination Reaction: of the salt is 7.
(iv) H2(g) + Cl2(g) → 2HCl(g) (½ M) 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)
(vi) 3H2(g) + N2(g) → 2NH3(g) (½ M) (1 M)
, (iii) HCl can produces H+ in aqueous solution. It can act Or
as acid in aq. solution. Hence, it changes the colour of A pH scale is a scale used for measuring hydrogen ion
litmus paper only in aqueous solution. (1 M) concentration in a solution. (½ M)
14. Recrystallisation of sodium carbonate gives washing soda. The salt obtained by the reaction of potassium hydroxide
(1 M) and sulphuric acid is potassium sulphate.
Its chemical equation is: KOH + H2SO4 → K2SO4 + H2O (1 M)
Na2CO3 + 10H2O → Na2CO3.10H2O (1 M) Potassium
It is a basic salt. (½ M) Sulphate
It is used for removing permanent hardness of water. Since, this salt is formed by the reaction of strong base and
(½ M) strong acid, therefore, its pH would be 7(neutral). (½ M)
15. Tooth enamel is made up of calcium hydroxyapatite
(a crystalline form of calcium phosphate). (½ M)
3. Metals and Non-Metals
Tooth enamel is the hardest substance in the body. It does 18. (b) Al2O3 is an amphoteric oxide which can react with
not dissolve in water and starts to corrode when pH of the HCl (an acid) as well as KOH (a base) to give
mouth becomes below 5.5. (1 M) corresponding salt and water. (1 M)
Bacteria present in the mouth produce acid by causing 19. (b) This is based on the concept of reactivity series in which
degradation of sugar and food particles remain in the more reactive metal will displace the less one from their
mouth after eating. (1 M) respective salt solutions. (1 M)
The best method to prevent this is to clean your mouth 20. (c) Metal- Mercury
after eating food. Toothpastes which are basic in nature Non-Metal- Bromine
can neutralize the excess acid and help in prevention of
The above mentioned elements exists in the liquid
tooth decay. (½ M)
state at the room temperature. (1 M)
16. The experimental setup is shown below:
6 volt battery Bulb
21. When silver is exposed to air, it combines with the sulphur
Switch
in the air to generate a coating of silver sulphide.
2Ag + H2S → Ag2S + H2
Silver sulphide (1 M)
Beaker Copper vessels lose their shiny brown surface as copper
reacts with moist CO2 in the air, forming a green layer of
Nail copper carbonate and copper hydroxide.
Alcohol or Glucose 2Cu + H2O + CO2 + O2 → Cu(OH)2 + CuCO3
solution Copper Copper
Rubber cork
(1) hydroxide carbonate (1 M)
In this experiment, the bulb not glowing in the presence of 22. Carbon cannot reduce the oxides of sodium, magnesium,
glucose and alcohol solutions suggests that these substances and aluminum because their reactivity exceeds that of
do not conduct electricity. This is because they do not carbon and also they have more affinity for oxygen.(1 M)
dissociate into ions in solution. (1 M) In the reactivity series, these metals are placed at the top
above carbon. (1 M)
Even though both glucose and alcohol contain hydrogen
Electrolytic reduction (Electrolysis) is a process used to
atoms, they do not release free ions (such as H⁺ or OH⁻) in
extract metals with high reactivity from their ores, such as
water, which are required for electrical conductivity. Thus,
sodium, magnesium, and aluminum. (1 M)
these solutions cannot be classified as acids. (1 M)
For example, sodium is obtained by the electrolysis of
17. (a) Sodium chloride is NaCl and is formed by the reaction its molten chloride. The metal is deposited at the cathode
of acid, HCl and the base, NaOH. (½ × 2 = 1 M) (the negatively charged electrode), whereas, chlorine is
(b) The cation and the anion present in calcium sulphate liberated at the anode (the positively charged electrode).
are Ca2+ and SO42– respectively. (½ × 2 = 1 M) (1 M)
(c) Salts containing the same positive or negative radicals The reactions are –
are known to belong to a family. (1 M) At cathode: Na+ + e– → Na (½ M)
Sodium chloride (NaCl) and washing soda At anode: 2Cl– → Cl2 + 2e– (½ M)
(Na2CO3⋅10H2O) both belong to the family of sodium 23. (i) In the electrolytic refining process, the impure metal
salts, since both contain the same positive radical i.e., is made the anode and a thin strip of pure metal is
sodium ions. (1 M) made the cathode. (1 M)
2 SCIENCE
, (ii) A solution of the metal salt is used as an electrolyte. (ii) • Ionic compounds conduct electricity in solution
In this case, the metal salt is acidified copper sulphate or in molten state. (½ M)
solution. (1 M) • Ionic compounds are generally soluble in water
(iii) (A) When electric current is passed through the cell, and insoluble in solvents. such as kerosene,
copper from the impure anode dissolves into the petrol, etc. (½ M)
electrolyte as copper ions. (1 M) (iii) (A) By losing one electron, Na attains noble gas
These ions then migrate towards the cathode, configuration and it forms ionic compound
where they gain electrons and deposit as pure NaCl. Cl gains one electron by Na to achieve
copper. (1 M) noble gas configuration.
Na → Na+ + e–
Or Cl + e– → Cl–
(iii) (B) In beaker ‘A’, the strip of zinc is dipped, and since On adding both equation:
zinc being more reactive than copper, therefore, Na+ + Cl– → NaCl (2 M)
will displace Cu from its salt solution. The Or
reaction that occurs is
(iii) (B) (i) Ionic compounds have rigid structure
Zn + CuSO4 → ZnSO4 + Cu (1 M) and movement of ions is not possible in
In beaker ‘B’, since, the strip of silver is dipped, and silver rigid structure. Hence, ionic solids do not
being less reactive than copper, therefore, will not displace conduct electricity. (1 M)
Ag from its salt solution and no reaction will occur. (1 M) (ii) H2 is produced at the cathode and O2 is
24. (a) When copper is heated in air, it combines with oxygen produced at the anode during the electrolysis
to form copper(II) oxide, a black oxide. (½ M) of a aqueous solution of NaOH. (1 M)
2Cu + O2→ 2CuO 26. (i)
(Copper) Copper(II) oxide (½ M) I: Silver (Ag) does not react with oxygen even at
(b) Some metal oxides, such as aluminum oxide and zinc high temperature. (½ M)
oxide, show both acidic as well as basic behaviour. II: Al reacts with oxygen at ordinary temperature
Such metal oxides that react with both acids, as well and forms a protective oxide layer which prevents
as bases to produce salts and water are known as the metal from further oxidation. (½ M)
amphoteric oxides. (½ M) III: Potassium catches fire when kept in open.(½ M)
Aluminum oxide reacts in the following manner with IV: Copper does not burn, but the hot metal is coated
acids and bases – with a black coloured layer of copper(II) oxide.
Al2O3 + 6HCl → 2AlCl3 + 3H2O (½ M)
Al2O3 + 2NaOH → 2NaAlO2 + H2O (½ M) (ii) Metal oxides which react with both acids as well
(Sodium as bases to produce salts and water are known as
alumnate) amphoteric oxides. (1 M)
(c) (i) Na2O(s) + H2O(l) → 2NaOH (aq) (1 M) Aluminium oxide reacts with acids and bases in the
(ii) Al2O3 + 2NaOH → 2NaAlO2 + H2O (1 M) following manner:
(Sodium Al2O3 + 6HCl → 2AlCl3 + 3H2O
alumnate) Al2O3 + 2NaOH → 2NaAlO2 + H2O
Or (Sodium
(i) SO2 gas is formed on burning sulphur in oxygen. aluminate) (1 M)
S + O2 → SO2 (½ M) (iii) An alkali is a base that dissolves in water. Most metal
(ii) SO2 (Sulphur dioxide) is formed. (½ M) oxides are insoluble in water but some of these dis-
(iii) SO2 formed is acidic in nature. (½ M) solve in water to form alkalis. (½ M)
(iv) SO2 turns moist blue litmus paper red but it does
Example: Sodium oxide dissolve in water to produce
not have any action on dry litmus paper. (½ M)
sodium hydroxide which is an alkali.
× × –
× ×× ×
× ×
× Cl × Na2O(s) + H2O(l) → 2NaOH(aq) (½ M)
Cl× 2+ ××
××
Mg 27. (i)
25. (a) (i) Mg + – (I) Cinnabar (HgS) is an ore of mercury. When it is
××
×Cl × × ×× ×
× × Cl ×
×× ×× heated in air, it is first converted into mercuric
(2,8,2) Two (2,8) Two oxide (HgO). Mercuric oxide is then reduced to
(1 M) mercury on further heating. (½ M)
111 Most Probable Questions 3
(Explanations)
Decomposition Reaction:
1. Chemical Reactions and Equations
(i) ZnCO3(s) → ZnO(s) + CO2(g) (½ M)
1. (c) This is an oxidation-reduction reaction. A complete Heat
(vii) CaCO3(s) → CaO(s) + CO2(g) (½ M)
balanced equation is:
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2 Displacement Reaction
x = 4; y = 2; z = 1 (1 M) (ii) Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s) (½ M)
2. (c) In the given reaction, MnO2 loses oxygen and forms (v) Fe2O3 + 2Al → Al2O3 + 2Fe (½ M)
MnCl2. Therefore, it is said that MnO2 is reduced to Double Displacement Reaction
MnCl2. Whereas, HCl loses hydrogen and forms Cl2. (iii) NaBr(aq) + AgNO3(aq) → AgBr(s) + NaNO3(aq)
Therefore, it is said that HCl is oxidized to Cl2. (1 M)
3. (a) This is a thermal decomposition reaction. When lead 2. Acids, Bases and Salts
nitrate undergoes thermal decomposition, brown
fumes of nitrogen dioxide are produced. (1 M) 9. (b) Milk of magnesia is basic in nature having pH around
10. (1 M)
4. (a) Heat is released during an exothermic reaction along
10. (b) Calcium hydroxide turns milky due to the formation
with the formation of products. (1 M)
of calcium carbonate which on passing excess of CO2
5. (i) On mixing the two solutions, white precipitate of forms calcium hydrogen carbonate which makes the
barium sulfate will form. solution clear again. (1 M)
BaCl2(aq) + Na2SO4(aq) → BaSO4(s)+ 2NaCl(aq)
11. (a) (i) Since, compound X gets hardened when mixed
(1 M)
with water, therefore, X is Plaster of Paris and its
(ii) After 10 minutes, white precipitate will settle down at
chemical formula is CaSO4.1/2H2O. (1 M)
the bottom of the tube and solution above it becomes
(ii)
Baking soda is sodium hydrogen carbonate
colorless. (1 M)
(NaHCO3) and it is added to make cooking faster.
6. (i) To make water more conductive before electrolysis, (½ M)
acid is added to the solution. This makes it simple Baking powder is a mixture of baking soda and a
for the current to move through the solution. At the mild edible acid such as tartaric acid. (½ M)
cathode, hydrogen gas is released, whereas at the
12. (i) The ‘G’ and ‘C’ formulas are:
anode, oxygen gas is released. (1 M)
G = Cl2 (½ M)
The cathode gas collection volume is twice as large as
the anode gas collection volume. C = CaOCl2 (½ M)
2H2O(l) → 2H2(g) + O2(g) (1 M) (ii) The chemical equation involved is
(ii) Silver metal and chlorine gas are produced during the Ca(OH)2 + Cl2 → CaOCl2 + H2O (1 M)
decomposition of silver chloride when it is exposed to (iii) Bleaching Powder is the common name for chemical C.
light. The reaction is known as photolytic reaction. Calcium Oxychloride is its chemical name. (1 M)
(1 M) 13. (i) Dilution must take place by adding conc. acid into
7. The chemical equations for the various decomposition water by constant stirring. As dilution is an exothermic
reactions are: process, so it may be possible that acid may spill out
(i) 2H2O + electricity → 2H2(g) + O2 (1 M) if water is added to a bottle containing concentrated
(ii) CaCO3 + heat → CaO + CO2 (1 M) acid. (1 M)
(iii) 2AgBr + sunlight → 2Ag + Br2 (1 M) (ii) When H2SO4 is added to NaOH, formation of salt
(Na2SO4) takes place. As it is a neutral salt. Hence pH
8. Combination Reaction: of the salt is 7.
(iv) H2(g) + Cl2(g) → 2HCl(g) (½ M) 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)
(vi) 3H2(g) + N2(g) → 2NH3(g) (½ M) (1 M)
, (iii) HCl can produces H+ in aqueous solution. It can act Or
as acid in aq. solution. Hence, it changes the colour of A pH scale is a scale used for measuring hydrogen ion
litmus paper only in aqueous solution. (1 M) concentration in a solution. (½ M)
14. Recrystallisation of sodium carbonate gives washing soda. The salt obtained by the reaction of potassium hydroxide
(1 M) and sulphuric acid is potassium sulphate.
Its chemical equation is: KOH + H2SO4 → K2SO4 + H2O (1 M)
Na2CO3 + 10H2O → Na2CO3.10H2O (1 M) Potassium
It is a basic salt. (½ M) Sulphate
It is used for removing permanent hardness of water. Since, this salt is formed by the reaction of strong base and
(½ M) strong acid, therefore, its pH would be 7(neutral). (½ M)
15. Tooth enamel is made up of calcium hydroxyapatite
(a crystalline form of calcium phosphate). (½ M)
3. Metals and Non-Metals
Tooth enamel is the hardest substance in the body. It does 18. (b) Al2O3 is an amphoteric oxide which can react with
not dissolve in water and starts to corrode when pH of the HCl (an acid) as well as KOH (a base) to give
mouth becomes below 5.5. (1 M) corresponding salt and water. (1 M)
Bacteria present in the mouth produce acid by causing 19. (b) This is based on the concept of reactivity series in which
degradation of sugar and food particles remain in the more reactive metal will displace the less one from their
mouth after eating. (1 M) respective salt solutions. (1 M)
The best method to prevent this is to clean your mouth 20. (c) Metal- Mercury
after eating food. Toothpastes which are basic in nature Non-Metal- Bromine
can neutralize the excess acid and help in prevention of
The above mentioned elements exists in the liquid
tooth decay. (½ M)
state at the room temperature. (1 M)
16. The experimental setup is shown below:
6 volt battery Bulb
21. When silver is exposed to air, it combines with the sulphur
Switch
in the air to generate a coating of silver sulphide.
2Ag + H2S → Ag2S + H2
Silver sulphide (1 M)
Beaker Copper vessels lose their shiny brown surface as copper
reacts with moist CO2 in the air, forming a green layer of
Nail copper carbonate and copper hydroxide.
Alcohol or Glucose 2Cu + H2O + CO2 + O2 → Cu(OH)2 + CuCO3
solution Copper Copper
Rubber cork
(1) hydroxide carbonate (1 M)
In this experiment, the bulb not glowing in the presence of 22. Carbon cannot reduce the oxides of sodium, magnesium,
glucose and alcohol solutions suggests that these substances and aluminum because their reactivity exceeds that of
do not conduct electricity. This is because they do not carbon and also they have more affinity for oxygen.(1 M)
dissociate into ions in solution. (1 M) In the reactivity series, these metals are placed at the top
above carbon. (1 M)
Even though both glucose and alcohol contain hydrogen
Electrolytic reduction (Electrolysis) is a process used to
atoms, they do not release free ions (such as H⁺ or OH⁻) in
extract metals with high reactivity from their ores, such as
water, which are required for electrical conductivity. Thus,
sodium, magnesium, and aluminum. (1 M)
these solutions cannot be classified as acids. (1 M)
For example, sodium is obtained by the electrolysis of
17. (a) Sodium chloride is NaCl and is formed by the reaction its molten chloride. The metal is deposited at the cathode
of acid, HCl and the base, NaOH. (½ × 2 = 1 M) (the negatively charged electrode), whereas, chlorine is
(b) The cation and the anion present in calcium sulphate liberated at the anode (the positively charged electrode).
are Ca2+ and SO42– respectively. (½ × 2 = 1 M) (1 M)
(c) Salts containing the same positive or negative radicals The reactions are –
are known to belong to a family. (1 M) At cathode: Na+ + e– → Na (½ M)
Sodium chloride (NaCl) and washing soda At anode: 2Cl– → Cl2 + 2e– (½ M)
(Na2CO3⋅10H2O) both belong to the family of sodium 23. (i) In the electrolytic refining process, the impure metal
salts, since both contain the same positive radical i.e., is made the anode and a thin strip of pure metal is
sodium ions. (1 M) made the cathode. (1 M)
2 SCIENCE
, (ii) A solution of the metal salt is used as an electrolyte. (ii) • Ionic compounds conduct electricity in solution
In this case, the metal salt is acidified copper sulphate or in molten state. (½ M)
solution. (1 M) • Ionic compounds are generally soluble in water
(iii) (A) When electric current is passed through the cell, and insoluble in solvents. such as kerosene,
copper from the impure anode dissolves into the petrol, etc. (½ M)
electrolyte as copper ions. (1 M) (iii) (A) By losing one electron, Na attains noble gas
These ions then migrate towards the cathode, configuration and it forms ionic compound
where they gain electrons and deposit as pure NaCl. Cl gains one electron by Na to achieve
copper. (1 M) noble gas configuration.
Na → Na+ + e–
Or Cl + e– → Cl–
(iii) (B) In beaker ‘A’, the strip of zinc is dipped, and since On adding both equation:
zinc being more reactive than copper, therefore, Na+ + Cl– → NaCl (2 M)
will displace Cu from its salt solution. The Or
reaction that occurs is
(iii) (B) (i) Ionic compounds have rigid structure
Zn + CuSO4 → ZnSO4 + Cu (1 M) and movement of ions is not possible in
In beaker ‘B’, since, the strip of silver is dipped, and silver rigid structure. Hence, ionic solids do not
being less reactive than copper, therefore, will not displace conduct electricity. (1 M)
Ag from its salt solution and no reaction will occur. (1 M) (ii) H2 is produced at the cathode and O2 is
24. (a) When copper is heated in air, it combines with oxygen produced at the anode during the electrolysis
to form copper(II) oxide, a black oxide. (½ M) of a aqueous solution of NaOH. (1 M)
2Cu + O2→ 2CuO 26. (i)
(Copper) Copper(II) oxide (½ M) I: Silver (Ag) does not react with oxygen even at
(b) Some metal oxides, such as aluminum oxide and zinc high temperature. (½ M)
oxide, show both acidic as well as basic behaviour. II: Al reacts with oxygen at ordinary temperature
Such metal oxides that react with both acids, as well and forms a protective oxide layer which prevents
as bases to produce salts and water are known as the metal from further oxidation. (½ M)
amphoteric oxides. (½ M) III: Potassium catches fire when kept in open.(½ M)
Aluminum oxide reacts in the following manner with IV: Copper does not burn, but the hot metal is coated
acids and bases – with a black coloured layer of copper(II) oxide.
Al2O3 + 6HCl → 2AlCl3 + 3H2O (½ M)
Al2O3 + 2NaOH → 2NaAlO2 + H2O (½ M) (ii) Metal oxides which react with both acids as well
(Sodium as bases to produce salts and water are known as
alumnate) amphoteric oxides. (1 M)
(c) (i) Na2O(s) + H2O(l) → 2NaOH (aq) (1 M) Aluminium oxide reacts with acids and bases in the
(ii) Al2O3 + 2NaOH → 2NaAlO2 + H2O (1 M) following manner:
(Sodium Al2O3 + 6HCl → 2AlCl3 + 3H2O
alumnate) Al2O3 + 2NaOH → 2NaAlO2 + H2O
Or (Sodium
(i) SO2 gas is formed on burning sulphur in oxygen. aluminate) (1 M)
S + O2 → SO2 (½ M) (iii) An alkali is a base that dissolves in water. Most metal
(ii) SO2 (Sulphur dioxide) is formed. (½ M) oxides are insoluble in water but some of these dis-
(iii) SO2 formed is acidic in nature. (½ M) solve in water to form alkalis. (½ M)
(iv) SO2 turns moist blue litmus paper red but it does
Example: Sodium oxide dissolve in water to produce
not have any action on dry litmus paper. (½ M)
sodium hydroxide which is an alkali.
× × –
× ×× ×
× ×
× Cl × Na2O(s) + H2O(l) → 2NaOH(aq) (½ M)
Cl× 2+ ××
××
Mg 27. (i)
25. (a) (i) Mg + – (I) Cinnabar (HgS) is an ore of mercury. When it is
××
×Cl × × ×× ×
× × Cl ×
×× ×× heated in air, it is first converted into mercuric
(2,8,2) Two (2,8) Two oxide (HgO). Mercuric oxide is then reduced to
(1 M) mercury on further heating. (½ M)
111 Most Probable Questions 3
