NCERT Exemplar Class 10 Science Solutions Chapter 1 Chemical Reactions and Equations

Question 2. The following reaction is an example of a:
$4NH_3 (g) + 5O_2(g)\rightarrow 4NO(g) + 6H_2O(g)$
(i) displacement reaction
(ii) combination reaction
(iii) redox reaction
(iv) neutralisation reaction
(a) (i) and (iv) (b) (ii) and (iii)
(c) (i) and (iii) (d) (iii) and (iv)

Answer:

The correct answer is (c)

$4NH_3 (g) + 5O_2(g)\rightarrow 4NO(g) + 6H_2O(g)$
Oxygen displaces hydrogen of ammonia. Hence, this is a displacement reaction. Nitrogen of ammonia is getting oxidized with the addition of oxygen. This is an example of redox reaction.
Hence it is an example of displacement as well as redox reaction.

Question 3. Which of the following statements about the given reaction are correct?
$3Fe(s) + 4H_2O(g)\rightarrow Fe_3O_4(s) + 4H_2(g)$
(i) Iron metal is getting oxidised
(ii) Water is getting reduced
(iii) Water is acting as reducing agent
(iv) Water is acting as oxidising agent
(a) (i), (ii) and (iii) (b) (iii) and (iv)
(c) (i), (ii) and (iv) (d) (ii) and (iv)

Answer:

The correct answer is (c)

$3Fe(s) + 4H_2O(g)\rightarrow Fe_3O_4(s) + 4H_2(g)$
Oxygen combines with water to get oxidized.
Oxygen is removed from water hence it is getting reduced.
Water is providing oxygen and it acts as an oxidizing agent.

Question 4. Which of the following are exothermic processes?
(i) Reaction of water with quick lime
(ii) Dilution of an acid
(iii) Evaporation of water
(iv) Sublimation of camphor (crystals)
(a) (i) and (ii) (b) (ii) and (iii)
(c) (i) and (iv) (d) (iii) and (iv)

Answer:

The correct answer is (a)

When water reacts with water and acid it releases enormous amount of heat. Similarly, when water reacts with quick lime it releases a lot of heat.
When water reacts with quick lime, calcium hydroxide is formed along with release of heat. Hence, it is an exothermic reaction.
Dilution of an acid – Highly exothermic
Evaporation of water and sublimation of camphor are examples of endothermic reactions.

Question 5. Three beakers labelled as A, B and C each containing 25 mL of water were taken. A small amount of NaOH, anhydrous $CuSO_{4}$ and NaCl were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, whereas in case of beaker C, the temperature of the solution falls. Which one of the following statement(s) is(are) correct?
(i) In beakers A and B, exothermic process has occurred.
(ii) In beakers A and B, endothermic process has occurred.
(iii) In beaker C exothermic process has occurred.
(iv) In beaker C endothermic process has occurred.
(a) (i) only (b) (ii) only
(c) (i) and (iv) (d) (ii) and (iii)

Answer:

The correct answer is (c)

In beaker A:
$NaOH + H_2O \rightarrow Na^+ + OH^{-} + H_2O + heat$
In beaker B:
$CuSO_4 (s) + 5H_2O (l) \rightarrow CuSO_4 . 5H_2O(s)$
Both are exothermic processes. These will increase the temperature
In beaker C:
$NaCl + H_2O + heat \rightarrow Na^+ + Cl^{-} + H_2O$
This is an endothermic process and this will decrease the temperature.

Question 6. A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?
(a) $KMnO_{4}$ is an oxidising agent, it oxidises $FeSO_{4}$
(b)$FeSO_{4}$ acts as an oxidising agent and oxidises $KMnO_{4}$
(c) The colour disappears due to dilution; no reaction is involved
(d) $KMnO_{4}$ is an unstable compound and decomposes in presence of $FeSO_{4}$ to a colourless compound.

Answer:

The correct answer is (a)

$FeSO_4 +KMnO_{4}+H_{2}SO_{4}\rightarrow Fe_{2}(SO_{4})_{3}+MnSO_4+K_{2}SO_{4}+H_{2}O$
$KMnO_{4}$ acts as an oxidizing agent, it oxidizes $FeSO_4 (Fe^{+2}) to Fe_{2}(SO_{4})_{3} (Fe^{+3})$

Question 7. Which among the following is(are) double displacement reaction(s)?
(i) $Pb + CuCl_2 \rightarrow PbCl_2 + Cu$
(ii) $Na_2SO_4 + BaCl_2\rightarrow BaSO_4 + 2NaCl$
(iii) $C + O_2\rightarrow CO_2$
(iv) $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$
(a) (i) and (iv) (b) (ii) only
(c) (i) and (ii) (d) (iii) and (iv)

Answer:

The correct answer is (b)

Double displacement reactions are those in which ions of the reactants are exchanged to form new compounds as products.
Na⁺ ion combines with $Cl^ -$ ion and $SO_{4}^{2-}$ ion combines with $Ba^{2+}$ ion to form two new compounds as sodium chloride (NaCl) and barium sulphate $(BaSO_4)$respectively.
Sodium and Barium are displaced from each other’s salts hence it is a double displacement reaction.

Question 8. Which among the following statement(s) is(are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to:
(i) the formation of silver by decomposition of silver chloride
(ii) sublimation of silver chloride
(iii) decomposition of chlorine gas from silver chloride
(iv) oxidation of silver chloride
(a) (i) only (b) (i) and (iii)
(c) (ii) and (iii) (d) (iv) only

Answer:

The correct answer is (a)

$2AgCl\overset{light}{\rightarrow}2Ag +Cl_{2}$
Exposure of silver chloride to sunlight for a long duration yields silver metal and chlorine gas.
The white color of silver chloride changes to grey due to the formation of silver metal.

Question 9. Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. This process is called slaking of lime. Calcium hydroxide dissolves in water to form its solution called lime water. Which among the following is (are) true about slaking of lime and the solution formed?
(i) It is an endothermic reaction
(ii) It is an exothermic reaction
(iii) The pH of the resulting solution will be more than seven
(iv) The pH of the resulting solution will be less than seven
(a) (i) and (ii) (b) (ii) and (iii)
(c) (i) and (iv) (d) (iii) and (iv)

Answer:

The correct answer is (b)

$CaO + H_2O \rightarrow Ca(OH)_2 + heat$
When solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. It proves the reaction is exothermic.
pH of the solution will be more than 7 because oxides and hydroxides of metals are alkaline.

Question 10. Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved?
(i) Displacement reaction
(ii) Precipitation reaction
(iii) Combination reaction
(iv) Double displacement reaction
(a) (i) only (b) (ii) only
(c) (iv) only (d) (ii) and (iv)

Answer:

The correct answer is (d)

$BaCl_{2}+(NH_{4})_{2}SO_{4}\rightarrow BaSO_{4}+2NH_{4}Cl$
Ammonium and barium are getting displaced from their respective salts.
This reaction is an example of double displacement reaction as ions (both cations and anions) of the reacting molecules are exchanged.
Barium ion $(Ba^{2+})$ combines with sulphate ion $(SO_{4}^{2-})$ to form Barium sulphate $(BaSO_{4})$ and ammonium ion $(NH_{4}^{+})$ combines with chloride ion $(Cl^{-})$ ion to form ammonium chloride $(NH_4Cl)$.
The above reaction is also a precipitation reaction because barium and sulphate ions combine to form precipitates of barium sulphate.

Question 11. Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is:
(a) 1:1
(b) 2:1
(c) 4:1
(d) 1:2

Answer:

The correct answer is (b)

The reaction is shown as:
$2H_{2}O(l)\rightarrow 2H_{2}(g)+O_{2}(g)$
1 Mole of water has 2 moles of Hydrogen and 1 mole of water.
Therefore, the mole ratio of hydrogen and oxygen gas liberated is 2:1.

Question 12. Which of the following is(are) an endothermic process(es)?
(i) Dilution of sulphuric acid
(ii) Sublimation of dry ice
(iii) Condensation of water vapours
(iv) Evaporation of water
(a) (i) and (iii) (b) (ii) only
(c) (iii) only (d) (ii) and (iv)

Answer:

The correct answer is (d)

Dilution of sulphuric acid with water is a highly exothermic process as it occurs with the evolution of heat.
Sublimation of ice is a direct phase transition from solid to gas
Condensation is the change of water from its gaseous form (water vapor) into liquid water.
Evaporation is the process by which water changes from a liquid to a gas or vapor.
Change of solid to gas or liquid to gas absorbs heat hence it is an endothermic reaction.

Question 13.In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which of the following can be used in place of lead nitrate?
(a) Lead sulphate (insoluble)
(b) Lead acetate
(c) Ammonium nitrate
(d) Potassium sulphate

Answer:

The correct answer is (b)

$2KI (aq)+Pb(NO_{3})_{2}(aq)\rightarrow PbI_{2}(s)+2KNO_{3}$
Potassium Iodide(KI) and lead nitrate $Pb(NO_{3})_{2}$ dissociate in their aqueous states to form ions.
Precipitates of lead iodide $(PbI_2)$ are formed.
Lead sulphate being insoluble in water does not give $Pb^{2+}$ ions which can combine with $I^-$ to form $(PbI_2)$

Question 14. Which of the following gases can be used for storage of fresh sample of an oil for a long time?
(a) Carbon dioxide or oxygen
(b) Nitrogen or oxygen
(c) Carbon dioxide or helium
(d) Helium or nitrogen

Answer:

The correct answer is (d)

Helium can be used as it is an inert gas. Nitrogen is less reactive and it is cheaper than Helium.
Hence Helium, just as nitrogen, can be used for storage of a fresh sample of an oil for quite a while on the grounds that they avoid the oil from rancidity.
The contact of air and oil is forestalled by utilizing nitrogen or helium as covering gas.

Question 15. The following reaction is used for the preparation of oxygen gas in the laboratory
$2KClO_{3}(s) \xrightarrow[catalyst]{heat}2KCl(s)+3O_{2}(g)$
Which of the following statement(s) is(are) correct about the reaction?
(a) It is a decomposition reaction and endothermic in nature
(b) It is a combination reaction
(c) It is a decomposition reaction and accompanied by release of heat
(d) It is a photochemical decomposition reaction and exothermic in nature

Answer:

The correct answer is (a)

Potassium chlorate decomposes to give potassium chloride and oxygen. This is a decomposition reaction
Also as shown in the equation, heat is supplied for the reaction to take place. Hence it is endothermic in nature.

Question 16. Which one of the following processes involve chemical reactions?
(a) Storing of oxygen gas under pressure in a gas cylinder
(b) Liquefaction of air
(c) Keeping petrol in a china dish in the open
(d) Heating copper wire in presence of air at high temperature

Answer:

The correct answer is (d)

Storing of oxygen gas under pressure in a gas cylinder – does not involve chemical reactions
Liquefaction of air - Air is liquefied by the Linde process, in which air is alternately compressed, cooled, and expanded, each expansion results in a considerable reduction in temperature. With the lower temperature the molecules move more slowly and occupy less space, so the air changes phase to become liquid. Hence it does not involve chemical reactions
Keeping petrol in a china dish in the open – does not involve chemical reactions
When copper is heated in the presence of air in a very high temperature, a chemical reaction takes place.
$2Cu + O_2 \rightarrow 2CuO$

Question 17. In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products involved at reaction temperature?
(a) $2H_2 (l) + O_2 (l) \rightarrow 2H_2O (g)$
(b) $2H_2 (g) + O_2 (l)\rightarrow 2H_2O (l)$
(c) $2H_2 (g) + O_2 (g)\rightarrow 2H_2O (l)$
(d) $2H_2 (g) + O_2 (g)\rightarrow 2H_2O (g)$

Answer:

The correct answer is (c)

At room temperature hydrogen and oxygen are gases while water is liquid in nature hence option (c) represents correct states of reactants and products.

Question 18. Which of the following are combination reactions?
(i)$2KClO_3\overset{heat}{\rightarrow} 2KCl + 3O_2$
(ii)$MgO + H_2O\rightarrow Mg(OH)_2$
(iii) $4Al + 3O_2\rightarrow 2Al_2O_3$
(iv) $Zn + FeSO_4\rightarrow ZnSO_4 + Fe$
(a) (i) and (iii) (b) (iii) and (iv)
(c) (ii) and (iv) (d) (ii) and (iii)

Answer:

The correct answer is (d)
A combination reaction is a reaction where two or more elements or compounds combine to form a single compound.
(i) )$2KClO_3\overset{heat}{\rightarrow} 2KCl + 3O_2$
Decomposition reaction.
(ii)$MgO + H_2O\rightarrow Mg(OH)_2$
Combination reaction as magnesium oxide and water combine to form magnesium hydroxide
(iii) $4Al + 3O_2\rightarrow 2Al_2O_3$
Combination reaction as aluminium combines with oxygen to form aluminium oxide
(iv)$Zn + FeSO_4\rightarrow ZnSO_4 + Fe$
Displacement reaction.

Question 19. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.
(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas.
(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.
(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated $H_2SO_4 .$
(d) Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.

Answer:

Chemical Reactions and Equations
(a$N_2 (g) + 3H_2 (g)\rightarrow 2NH_3 (g)$
Combination reaction.
(b)$NaOH (aq) + CH_3COOH(l) \rightarrow CH_3COONa (aq) + H_2O(l)$
Neutralisation reaction between a strong base and weak acid
(c)$C_2H_5OH(aq) + CH_3COOH(l) \rightarrow CH_3COOC_2H_5 (aq) + H_2O(l)$
Double displacement reaction.
(d) $C_2H_4(g) + 3O_2(g) \rightarrow 2CO_2(g) + 2H_2O(g) + Heat + Light$
Oxidation reaction.

Question 20. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.
(a) Thermit reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.
(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.
(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.
(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.

Answer:

(a) Thermit reaction, iron (III) oxide reacts with aluminum and gives molten iron and aluminum oxide.
$Fe_2O_3 + 2Al\rightarrow Al_2O_3(s) + 2Fe + heat$
Single displacement reaction
(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.
$3 Mg(s) + N_2\rightarrow Mg_3N_2$
Combination reaction
(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.
$Cl_2 (g) + 2KI (aq) \rightarrow 2KCl (aq) + I_2(s)$
Displacement reaction
(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.
$C_2H_5OH + 3O_2\rightarrow 2CO_2 + 3H_2O + heat$
Combustion reaction

Question 21. Complete the missing components/variables given as x and y in the following reactions
(a) $Pb(NO_3)_2 (aq) + 2KI(aq) \rightarrow PbI_2 (x) + 2KNO_3(y)$
(b) $Cu(s) + 2AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + x(s)$
(c) $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(x) + H_2(y)$
(d) $CaCO_3(s) \overset{x}{\rightarrow} CaO(s) + CO_2(g)$

Answer:

a) x (s), y (aq)
$Pb(NO_3)_2 (aq) + 2KI(aq) \rightarrow PbI_2 (s) + 2KNO_3(aq)$
b) x is 2Ag
$Cu(s) + 2AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2Ag(s)$
c) x (aq), y(g)
$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$
d) x is heat
$CaCO_3(s) \overset{heat}{\rightarrow} CaO(s) + CO_2(g)$

Question 22. Which among the following changes are exothermic or endothermic in nature?
(a) Decomposition of ferrous sulphate
(b) Dilution of sulphuric acid
(c) Dissolution of sodium hydroxide in water
(d) Dissolution of ammonium chloride in water

Answer:

(a) Decomposition of ferrous sulphate
$2FeSO_{4}(s)\overset{heat}{\rightarrow} Fe_{2}O_{3}(s)+SO_{3}(g)+SO_{2}(g)$
Endothermic reaction as it involves absorption of heat
(b) Dilution of sulphuric acid
Dilution of sulphuric acid with water is a highly exothermic process as it occurs with the evolution of heat.
$H_{2}SO_{4}+2H_{2}O\rightarrow 2H_{3}O^{+}+SO_{4}^{2-}+ Heat$
Exothermic reactions
(c) Dissolution of sodium hydroxide in water
It produces negative hydroxide ions and positive sodium ions (Na+)
$NaOH\overset{water}{\rightarrow}Na^{+}+OH^{-}+Heat$
Exothermic reactions
(d) Dissolution of ammonium chloride in water
$NH_{4}Cl(s)\rightarrow NH^{+}_{4}(aq)+Cl^{-}(aq)$
Endothermic reaction

Question 23. Identify the reducing agent in the following reactions
(a) $4NH_3 + 5O_2\rightarrow 4NO + 6H_2O$
(b) $H_2O + F_2 \rightarrow HF + HOF$
(c) $Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$
(d) $2H_2 + O_2 \rightarrow 2H_2O$

Answer:

Reducing agent is the one which reduces other substance (by removal of oxygen or addition of hydrogen) and itself gets oxidised.
In the given reactions, reducing agents are as follows:
a) $NH_{3}$ - Ammonia
b) $F_{2}$ - Fluorine
c) $CO$ - Carbon monoxide
d) $H_{2}$ - Hydrogen

Question 24. Identify the oxidising agent (oxidant) in the following reactions
(a) $Pb_3O_4 + 8HCl \rightarrow 3PbCl_2 + Cl_2 + 4H_2O$
(b) $2Mg + O_2 \rightarrow 2MgO$
(c) $CuSO_4 + Zn \rightarrow Cu + ZnSO_4$
(d) $V_2O_5 + 5Ca \rightarrow 2V + 5CaO$
(e) $3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$
(f) $CuO + H_2\rightarrow Cu + H_2O$

Answer:

An oxidizing agent is a reactant that removes electrons from other reactants during a redox reaction. The oxidizing agent typically takes these electrons for itself, thus gaining electrons and being reduced.
In the above reactions, oxidizing agents are:
a) $Pb_3O_4$
b) $O_2$
c) $CuSO_4$
d) $V_2O_5$
e) $H_2O$
f) $CuO$

Question25. Write the balanced chemical equations for the following reactions
(a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogencarbonate.
(b) Sodium hydrogencarbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide.
(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide $(Cu_2I_2)$, liberates iodine gas and also forms potassium sulphate.

Answer:

(a) $Na_2CO_3 + HCl \rightarrow NaCl + NaHCO_3$
(b) $NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$
(c) $2CuSO_4 + 4KI \rightarrow 2K_2SO_4 + Cu_2I_2 + I_2$

Question 26. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction?

Answer:

$KCl(aq) + AgNO_3(aq) \rightarrow AgCl(s) + KNO_3(aq)$
This is a double displacement (both reactants exchange ions to form new products) and precipitation reaction (white precipitate of silver chloride (AgCl) is formed).

Question 27. Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.

Answer:

$2FeSO_4(s) \overset{heat}{\rightarrow} Fe_2O_3 (s) +SO_2 (g) +SO_3 (g)$
This is a decomposition reaction.
Ferrous sulphate decomposes on heating to ferric oxide.

Question 28. Why do fire flies glow at night?

Answer:

Fire flies produces an enzyme called as luciferase which carries oxidation of Magnesium Because of oxidation of Magnesium in presence of atmospheric oxygen fire flies’ glows in the night. This phenomenon of glowing by living organisms is called bioluminescence.

Question 29. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

Answer:

Grapes on the plant do not ferment because of defense mechanism of plants. When grapes are plucked from plant, grapes reacts with yeast to carry out fermentation. Here sugar changes to alcohol and it is a chemical change.

Question 30. Which among the following are physical or chemical changes?
(a) Evaporation of petrol
(b) Burning of Liquefied Petroleum Gas (LPG)
(c) Heating of an iron rod to red hot.
(d) Curdling of milk
(e) Sublimation of solid ammonium chloride

Answer:

Physical changes only change the appearance of a substance, not its chemical composition.
Chemical changes cause a substance to change into an entirely substance with a new chemical formula.
Physical changes
(a) Evaporation of petrol
(b) Heating of an iron rod to red hot.
(e) Sublimation of solid ammonium chloride
Chemical changes
(c) Burning of Liquefied Petroleum Gas (LPG)
(d) Curdling of milk

Question 32. A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved

Answer:

Compound X is Calcium oxide.
CaO is extensively used in cement industry. On treatment with water, CaO produces Ca(OH)₂ which is alkaline in nature and turns red litmus to blue color.
$CaO(s) + H_2O(l)\rightarrow Ca(OH)_2(aq)$

Question 33. Write a balanced chemical equation for each of the following reactions and also classify them.
(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.
(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.
(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas.
(d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

Answer:

(a) $Pb(CH3COO)_2 + 2HCl \rightarrow PbCl_2 + CH_3COOH$
This is a Double Displacement reaction.
(b) $2Na + 2C_2H_5OH \rightarrow 2C_2H_5O^-Na^+ + H_2$
This is a Displacement reaction.
(c) $Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$
This is a redox reaction.
(d) $2H_2S + O_2 \rightarrow 2S + 2H_2O$
This is a redox reaction.

Question 34. Why do we store silver chloride in dark coloured bottles?

Answer:

Silver chloride decomposes into silver and chlorine gas when exposed to sunlight.
$2AgCl \rightarrow 2Ag + Cl_2?$
Hence Silver chloride is stored in dark colored bottles.

Question 35. Balance the following chemical equations and identify the type of chemical reaction.
(a) $Mg(s) + Cl_2(g) \rightarrow MgCl_2(s)$
(b) $HgO(s) \rightarrow Hg(l) + O_2(g)$
(c) $Na(s) + S(s) \rightarrow Na_2S(s)$
(d) $TiCl_4 (l) + Mg(s) \rightarrow Ti(s) + MgCl_2(s)$
(e) $CaO(s) + SiO_2(s) \rightarrow CaSiO_3(s)$
(f) $H_2O_2 (l) \rightarrow H_2O(l) + O_2(g)$

Answer:

(a)$Mg(s) + Cl_2(g) \rightarrow MgCl_2(s)$
This type of reaction is called a combination reaction or a synthesis reaction.
(b) $2HgO(s) \overset{heat}{\rightarrow} 2Hg(l) + O_2(g)$
This is an example of thermal decomposition reaction.
(c) $2Na(s) + S(s) \rightarrow Na_2S(s)$
This is an example of a Combination reaction.
(d) $TiCl_4 (l) + Mg(s) \rightarrow Ti(s) + 2MgCl_2 (s)$
This reaction falls under the category of Displacement reactions
(e) $CaO(s) + SiO_2(s) \overset{UV}{\rightarrow} CaSiO_3(s)$
This is a synthesis reaction.
(f) $2H_2O_2 (l) \rightarrow 2H_2O (l) + O_2 (g)$
This is a decomposition reaction.

Question 36. A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y.
(a) Write the chemical formulae of X and Y.
(b) Write a balanced chemical equation, when X is dissolved in water.

Answer:

(a) $2Mg + O_2 \rightarrow 2MgO$
Compound X: MgO [Magnesium Oxide]
Compound Y: $Mg_{3}N_{2}$ [Magnesium Nitride]
(b) If Magnesium Oxide is dissolved in water, the product is Magnesium Hydroxide
$MgO + H_{2}O \rightarrow Mg(OH)_{2}$

Question 37. Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?

Answer:

Zinc is more reactive than copper as Zinc is placed above Hydrogen and Copper is placed below hydrogen in the activity series of metals. Because of this Zinc reacts with HCl whereas Copper will not react.
$Zn > Fe > Pb > H > Cu > Hg$
$Zn + 2HCl (dil) \rightarrow ZnCl_2 + H_2?$

Question 38. A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.
(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.
(b) Name the black substance formed and give its chemical formula.

Answer:

a) Silver reacts with $H_2S$ present in the atmosphere to form a black color compound Silver Sulphide. This phenomenon is called as corrosion.
b) Black color compound formed is Silver Sulphide
$2Ag+ H_2S \rightarrow Ag_2S + H_2$

Question 39. On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed
(a) Write a balanced chemical equation of the reaction.
(b) Identity the brown gas X evolved.
(c) Identity the type of reaction.
(d) What could be the pH range of aqueous solution of the gas X?

Answer:

Blue coloured powder of copper (II) nitrate = $Cu(NO_3)_2$
(a) $2Cu(NO_3)_2 \rightarrow 2CuO + 4NO_2 + O_2$
(b) The brown gas X is nitrogen dioxide, $NO_2$
(c) Reaction involved is Thermal decomposition.
(d) pH< 7 because $NO_2$ dissolves in water to form acidic solution (pH range below 7).

Question 40. Give the characteristic tests for the following gases
(a) $CO_2$
(b) $SO_2$
(c) $O_2$
(d) $H_2$

Answer:

(a) Pass $CO_2$ into limewater which will turn limewater milky. This is the confirmation test for the presence of carbon-dioxide.
$Ca(OH)_2 (aq) + CO_2 \rightarrow CaCO_3 (White color)+ H_2O$
(b) 1) Smell is the characteristic feature of $SO_2$ which smells like rotten egg.
2) Sulphur dioxide gas turns moist litmus paper from blur to red as sulphur dioxide is acidic in nature.
3) Sulphur dioxide gas turns acidified potassium dichromate solution from orange to green (Conformation Test)
(c) Test for oxygen involves burning of match stick near oxygen. It makes the match stick burn even more brightly.
(d) When burning matchstick is brought near $H_2$ gas, the flame burns with the pop sound. This is the test to confirm Hydrogen gas.

Question 41. What happens when a piece of
(a) zinc metal is added to copper sulphate solution?
(b) aluminium metal is added to dilute hydrochloric acid?
(c) silver metal is added to copper sulphate solution?
Also, write the balanced chemical equation if the reaction occurs

Answer:

(a) When Zinc is added to copper sulphate solution Zinc displaces copper to form Zinc sulphate.
$Zn(s)+ CuSO_4(aq) \rightarrow ZnSO_4 (aq) + Cu(s)$
(b) Aluminium metal reacts with dilute HCl to for Aluminum Chloride and Hydrogen gas is evolved in the reaction.
$2Al (s)+ 6HCl (aq) \rightarrow 2AlCl_3(aq) + 3H_2$
(c) When silver metal is added to Copper Sulphate solution there will not be any reaction as silver is non-reactive metal.

Question 42. What happens when zinc granules are treated with dilute solution of H₂SO₄, HCl, HNO₃, NaCl and NaOH, also write the chemical equations if reaction occurs.

Answer:

When zinc granules are treated with dilute solution of the following:

(i) $\mathrm{H}_2 \mathrm{SO}_4$

When zinc granules are treated with dilute solution of sulphuric acid, zinc sulphate solution is formed along with hydrogen gas.

$\mathrm{Zn}(\mathrm{s}) \quad+\mathrm{H}_2 \mathrm{SO}_4(\mathrm{aq}) \quad \rightarrow \quad \mathrm{ZnSO}_4(\mathrm{aq})+\mathrm{H}_2(\mathrm{~g})$

(ii) HCl

When zinc granules are treated with dilute solution of hydrochloric acid, zinc chloride solution is obtained with the evolution of hydrogen gas.

$\mathrm{Zn}(\mathrm{s}) \quad+2 \mathrm{HCl}(\mathrm{aq}) \quad \rightarrow \quad \mathrm{ZnCl}_2(\mathrm{aq}) \quad+\mathrm{H}_2(\mathrm{~g})$

(iii) $\mathrm{HNO}_3$

When zinc granules are treated with dilute solution of nitric acid, an aqueous solution of zinc nitrate is formed along with water and nitrous oxide.

$4 \mathrm{Zn}(\mathrm{s}) \quad+10 \mathrm{HNO}_3(\mathrm{aq}) \quad \rightarrow 4 \mathrm{Zn}\left(\mathrm{NO}_3\right)_2(\mathrm{aq}) \quad+\mathrm{H}_2 \mathrm{O}(\mathrm{I}) \quad+\mathrm{N}_2 \mathrm{O}$

(iv) NaCl

Zinc granules on treatment with NaCl, do not undergoes any reaction as sodium is a reactive metal than zinc (sodium is placed above zinc in the reactivity series), and cannot displace sodium from its salt solution of sodium chloride.

$\mathrm{Zn}(\mathrm{s}) \quad+\mathrm{NaCl}(\mathrm{aq}) \quad \rightarrow \quad$ No Reaction

(v) NaOH:

Zinc metal reacts with alkali to form salt and evolve hydrogen gas.

$\mathrm{Zn}(\mathrm{s}) \quad+2 \mathrm{NaOH}(\mathrm{aq}) \quad \rightarrow \quad \mathrm{Na}_2 \mathrm{ZnO}_2(\mathrm{aq}) \quad+\mathrm{H}_2(\mathrm{~g})$

Question 44. You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute $HNO_3, ZnCl_2 \;and \;H_2O$. In which of the above containers these solutions can be kept?

Answer:

Dilute HCl
Copper does not react with dilute HCl, so it can be stored in a copper vessel
Dilute $HNO_3$
$3Cu(s)+8HNO_{3}(aq)\rightarrow 3Cu(NO_{3})_{2}(aq)+2NO(g)+4H_{2}O(l)$
Cannot be stored in copper vessel as above chemical reaction takes place.
Hence stored in aluminium vessel
$ZnCl_2$
No reaction takes place because copper is less active metal than zinc and can't displace it from $ZnCl_2$ solution.
Hence stored in copper vessel
$H_2O$
No reaction occurs between copper and water so water can be stored in copper vessel.