NCERT Solutions for Class 10 Chapter 1 Chemical Reactions & Equations

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Page No. 6

1) Why should a magnesium ribbon be cleaned before exposing to burn in the air ?
Ans: Magnesium ribbon gets covered with a layer of Magnesium Oxide (MgO) when it is kept in room in contact with air for a long time. This outermost extra layer prevents the burning of magnesium ribbon. Hence, it must be cleaned before burning.

2) Write down the balanced chemical equations for the following chemical reactions mentioned below.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Ans: Following are the balanced chemical equations mentioned below:

(i) H2 + Cl2 → 2HCl
(ii) 3 BaCl2 + Al2(SO4)3 → BaSO4 + 2 AlCl3
(iii) 2Na + 2H2O → 2NaOH + H2

3) Write down the balanced chemical equation with appropriate chemical state symbols for the following reactions mentioned below:
(i) Solutions of Barium Chloride & Sodium Sulphate in water reacts to give insoluble Barium Sulphate and the solution of Sodium Chloride.
(ii) Sodium Hydroxide solution (in water) reacts with Hydrochloric acid solution (in water) to produce Sodium Chloride solution and water.

Ans: (i) BaCl2 (aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl(aq) → NaCl(aq) + H2O(l)

Page No. 10

1) A solution of a substance named ‘X’ is used for the purpose of white washing.
(i) Name the substance ‘X’ and write its balanced chemical formula.
(ii) Write down the balanced chemical reaction of the substance ‘X’ named in (i) above with water.

Ans: (i) The substance named “X” whose solution in water is used for the purpose of white washing is called Calcium Oxide (or Quick Lime). Its chemical formula is CaO.
(ii) The substance named “CaO” is used in water to produce Calcium Hydroxide Ca(OH)2 solution. The following balanced chemical reaction takes place CaO (s) + H20 (l) → Ca(OH)2 (aq)

2) Why is the amount of gas collected in one of the test tubes in the textbook Activity 1.7 [i.e., electrolysis of water in two test tubes]) double of the amount collected in the other test tube? Name this gas. [CBSE 2015 (Delhi)]
Ans: In textbook Activity 1.7, we see that water is electrolysed to give Hydroge (H2) gas at one electrode and (O2) gas at the other electrode respectively.

The following balanced chemical reaction takes place:
2H2O(l) → 2H2(g) + O2(g)
Hence two molecules of water (H20) on electrolysis gives two molecules of hydrogen gas & one molecule of oxygen gas at the two electrodes respectively. Thus, the amount of hydrogen gas collected would be always double than that of the oxygen gas which proves hydrogen & oxygen ratio of 2:1.

Page no. 13:

1) State the changes in colour of Copper Sulphate solution change when an iron nail is immersed into the solution for 30 minutes? [CBSE 2015 (Delhi)]

Ans: When an iron nail is immersed in the Copper Sulphate solution for about 30 minutes, the chemical displacement reaction takes place which changes the blue colour of Copper Sulphate solution to light green due to the formation of Iron Sulphate. The following balanced chemical reactions takes place: Fe (s) + CuSo4 (aq) → FeSO4 (aq)+ Cu (s)

2) Give only one example of a chemical double displacement reaction other than the one mentioned in the textbook Activity 1.10 (NCERT Grade 10th Text Book).
Ans: The solutions of Sodium Hydroxide & Hydrochloric acid reacts together to form a solution of Sodium Chloride & water. The following balanced chemical reactions takes place: (NaOH) (aq) + HCl (aq) → NaCl (aq) + H2O (l)

3) Identify the elements that are oxidised/reduced in the following balanced chemical reactions mentioned below.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO (s) + H2(g) → Cu (s) + H2O(l)

Ans: (i) Elements oxidised is Sodium (Na) as it gains Oxygen & Oxygen is reduced respectively.
(ii) Elements reduced is Copper (Cu) & Hydrogen is Oxidised as it gains Oxygen respectively.

NCERT Solutions of Grade 10th Textbook Chemical Reactions & Equations

1) Which one of the following statements mentioned below about the balanced chemical reaction are false?
2PbO(s) + C(s) → 2Pb (s) + CO2(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.

(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) All

Ans: The first option (i) (a) and (b)

2) The following balanced chemical reaction Fe2O3 + 2Al → Al2O3 + 2Fe is an example of:
(a) Combination reaction
(b) Double displacement reaction
(c) Decomposition reaction
(d) Displacement reaction

Ans: The fourth option (d) Displacement reaction.

3) What do you observe when you add dilute Hydrochloric acid to iron filings in the solution? Choose the correct answer below:
(a) Hydrogen gas & iron chloride are produced.
(b) Chlorine gas & iron hydroxide are produced.
(c) No chemical reaction takes place.
(d) Iron salt & water are produced.

Ans: The first option (a) Hydrogen gas & iron chloride are produced.

4) Define balanced chemical equation? Why must be balanced chemical equations balanced?
Ans: A balanced chemical equation has an equal number of atoms present of different substances in the reactants & products respectively. All the chemical equations must be balanced to satisfy the Newton’s law of conservation of mass.

5) Convert the following chemical statements into balanced chemical equations.
(a) Hydrogen gas reacts with Nitrogen to form Ammonia.
(b) Hydrogen Sulphide gas burns readily in air to produce Water and Sulphur Dioxide respectively.
(c) Barium Chloride reacts with Aluminium Sulphate to produce Aluminium Chloride & a precipitate of Barium Sulphate respectively.
(d) Potassium metal reacts with Water to produce Potassium Hydroxide & hydrogen gas respectively.

Ans: The balanced chemical reactions of the above statements is mentioned below:
(a) 3H2 (g) + N2 (g) → 2NH3 (g)
(b) H2S (g) + 3O2 (g) → SO2 (g) + 2H2O(l)
(c) 3BaCl2 (aq) + Al2(SO4)3 (aq) → 2AlCl3 (aq) + 3BaSO4 ↓(s)
(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)

6) Balance the following chemical reactions mentioned below:
(a) HNO3 + Ca (OH)2 → Ca (NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl

Ans: The balanced chemical reactions of the above equations is mentioned below:
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + 2HCl

7) Write down the balanced chemical equations for the following chemical reactions mentioned below:
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Ans: The balanced chemical equations for the reactions is mentioned below:
(a) Ca (OH)2 + CO2 → CaCO3 + H2O
(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag
(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu
(d) BaCl2 + K2SO4 → BaSO4 + 2KCl

8) Write down the balanced chemical equation & also identify the type of chemical reaction involved in each case mentioned below:
(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium
(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g) bromide(s)
(c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)

Ans: (a) 2KBr (aq) + Bal2(aq) → 2Kl(aq) + BaBr2(s)
It is a Double displacement reaction
(b) ZnCO3 (s) → ZnO (s) + CO2 (g)
It is a Decomposition reaction
(c) H2 (g) + Cl2 (g) → 2HCl(g)
It is a Combination reaction
(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
It is a Displacement reaction

9) What does one mean by exothermic and endothermic reactions ? Give examples. What is meant by endothermic & exothermic reactions? Explain by giving examples.
Ans: The two type of chemical reactions can be defined as follows:

Exothermic reactions: These are the chemical reactions in which heat is evolved are known as exothermic reactions. It is usually indicated by writing “+ Heat”on the right products side of a chemical equation. Following are the few examples mentioned below.
Ex: (i) C (s) + O2 (g) → CO2 (g) + Heat
(ii) N2 (g) + 3H2 (g) → 2NH3 (g) + Heat

Endothermic reactions: These are the chemical reactions in which heat is absorbed are known as endothermic reactions. It is usually indicated by writing “- Heat” on the right product side of a chemical equation. Following are the few examples mentioned below:
Ex: (i) C (s) + 2S (s) → CS2 (l) – Heat
(ii) N2 (g) + O2 (g) → 2NO(g) – Heat

10) Why is the process of respiration regarded as an exothermic chemical reaction ? Explain by giving suitable chemical reactions.
Ans: The process of respiration is an exothermic chemical process due to combination of glucose with oxygen in the human body cells to form Carbon Dioxide & Water respectively along with the production of energy. Following is the chemical reaction of human respiration mentioned below: C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + Energy

11) Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions. Why chemical reactions of decomposition are known as opposite of combination reactions. Explain using suitable chemical equations for such chemical reactions.
Ans: In a decomposition reaction, a single compound breaks down to produce two or more simpler substances. The decomposition chemical reaction is defined as a singled compound breaks apart into two or more substances. Ex: 2H20 (l) → 2H2 (g) + O2 The combination chemical reaction can be often defined as a two or more substances joins together to form a single compound. Ex: 2H (g) + O2 (g) → 2H2O (l)

12) Write one equation each for the decomposition reactions where energy is supplied in the form of heat, light or electricity. Write down one decomposition chemical equation where energy is provided in the form of heat, light or electricity to break down the combined reactants. [CBSE 2015 (Delhi)]
Ans: Decomposition reactions for various energy providers are mentioned below: Heat: CaCO3 (s) → CaO (s) + O2 (g) Light: 2AgBr (s) → 2Ag (s) + Br2 (g) Electricity: 2H2O (l) → 2H2 (g) + O2

13) Give the differences between double displacement & displacement chemical reactions by giving suitable chemical reactions.
Ans: In displacement reactions, a more reactive metal displaces a less reactive metal from its solution. For example, The two chemical reactions can be defined as: Displacement reaction: A more reactive metal displaces a less reactive metal from a chemical solution is called displacement chemical reaction. Ex: Fe(s) + CuSO4(aq) → Cu(s) + FeSO4(aq)
It is a displacement reaction where chemical reaction between Iron (Fe) & Copper Sulphate (CuSo4) leads to displacement of Cu from CuSO4 due to Fe in the chemical solution In double displacement ractions, two reactants always exchanges their ions is called double displacement chemical reaction.
Ex: AgNO3(aq) + NaCl (aq) → AgCl(s) + NaNO3 (aq) It is a double displacement reaction where chemical reaction between Silver Nitrate (AgNO3) & Sodium Chloride (NaCl) leads to double displacement of Ag from AgNO3 & Cl from NaCl respectively exchanging their ions.

14) The refinement of Silver from the Silver Nitrate solution involves displacement by Copper metal. Explain with the help of chemical equations.
Ans: 2AgNO3 (aq) + Cu (s) Cu(NO3)2 (aq) + 2Ag (s)
Copper (Cu) displaces Silver (Ag) from 2AgNO3 solution. Thus, this reaction is termed as displacement reaction.

15) What is meant by precipitation reactions. Explain with suitable examples.

Ans: Insoluble solid formed during chemical reaction apart from the solution is known as precipitate.
Ex: A Solution of Iron (III) chloride & Ammonium Hydroxide are mixed to react with each other which leads to the formation of brown precipitate known as Iron (III) hydroxide.
The chemical equation is as follows FeCl3 (aq) + 3NH4OH (aq) Fe(OH)3 + 3NH4Cl (aq)

16) Explain the following in terms of gain or loss of oxygen with two examples each: Explain the following mentioned below terms in gain/loss of oxygen atleast giving two examples for each.
(a) Oxidation (b) Reduction

Ans: (a) Oxidation: It is known as the addition of oxygen to a chemical substance is called oxidation. The chemical equations for oxidation/reduxtion reactions are mentioned below.
Ex: (i) S(s) + O2(g) → SO2(g) (Addition of Oxygen to Sulphur forming Sulphur Dioxide)
(ii) 2Mg(s) + O2 (g) → 2MgO(s) (Addition of Oxygen to Magnesium forming Magnesium Oxide)

17) A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed. Element “X” on heating in air turns its colour from shiny brown to black. Name the unknown element “X” & also the black coloured compound produced.
Ans: The unknown element “X” is Copper metal (Cu) & the black coloured compound produced during the chemical reaction is Copper Oxide (CuO). Following below is the chemical equation for the reaction 2Cu (s) + O2 (g) → 2CuO (s)

18) Why do we apply paint on iron articles ? Why do we coat iron materials using paint/grease? Ans: Applying paint/grease prevents ironic materials from coming in contact with the surroundings where water, oxygen, nitrogen dioxide, & other gases are present which saves ironic materials from rusting.

19) Food items which contains oil & fats are usually filled with nitrogen gas. Why?
Ans: To prevent food items from getting oxidised or unfit for consumption & preserve in its original state. Thus, they are filled or flushed with nitrogen gas.

20) Define the following terms by giving one example each for each term. (a) Corrosion (b) Rancidity
Ans: (a) Corrosion: The process in which metals surfaces are gradually eaten/worn away by reacting with air or surroundings.
Ex: Whenever iron metal is allowed to react with moist air for a long interval of time, iron metal surface gradually acquires brown coating layer known as rust (Hydrated iron). The chemical equation for this reaction is 4Fe + 3O2 + 2H2O → 2Fe2O3.H20 where Iron reacts with oxygen along with water to produce hydrated Iron (III) oxide layer.

(b) Rancidity: Fat & Oil containing food items when exposed to air undergoes the process of oxidation producing often unpleasant smell/taste. This condition spoils the food items which eventually makes the them unfit for consumption. Thus, Rancidity can be stopped by flushing anti-oxidants such as Nitrogen gas before sealing.

1) Choose the incorrect statement about the chemical reaction below.

2PbO (s) + C (s)  2Pb (s) + CO2 (g)

a. Lead (Pb) is getting reduced

b. Carbon dioxide (CO2) is getting oxidised

c. Carbon (C) is getting oxidised

d. Lead oxide (PbO) is getting reduced

(i) (a) and (b)

(ii) (a) and (c)

(iii) (a), (b) and (c)

(iv) all

Ans: (i) (a) and (b)

2) Fe2O3 + 2Al  Al2O3 + 2Fe. The above mentioned chemical reaction equation is an example of

(i) Combination reaction

(ii) Double displacement reaction

(iii) Decomposition reaction

(iv) Displacement reaction

(iv) Displacement reaction


3) What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer. What changes do you observe when dilute Hydrochloric acid is reacted with iron fillings? Choose the correct answer below.

a. Hydrogen gas & iron chloride are produced

b. Chlorine gas & iron hydroxide are produced

c. No chemical reaction takes place

d Iron salt & water are produced


Ans: Hydrogen gas & iron chloride are produced

4) Define balanced chemical eqaution & why it should be balanced?

Ans: A chemical equation for any reaction is which contains equal number of atoms (moles) of each element on the reactants & products side respectively. These equations must be balanced to satisfy the Newton law of conservation of mass.

5) Give chemical equations for the statements mentioned below & also balance them.

a. Hydrogen gas combines with Nitrogen to form Ammonia.

b. Hydrogen Sulfide gas burns in air to give Water and Sulfur Dioxide.

c. Barium Chloride reacts with Aluminium Sulfate to give Aluminium Chloride & a precipitate of Barium Sulphate.

d. Potassium metal reacts with Water to give a Potassium Hydroxide & Hydrogen gas.


Ans: The following chemical reactions equations are mentioned below:

a. 3H2 + N2 → 2NH3

b. 2H2S + 3O2 → 2H2O + 2SO2

c. 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4

d. 2K + 2H2O → 2KOH + H2

6) Balance the chemical equations mentioned below:

(a)HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O

(b) NaOH + H2SO4 → Na2SO4 + H2O

(c) NaCl + AgNO3 → AgCl + NaNO3

(d) BaCl2 + H2SO4 → BaSO4 + HCl


Ans: (a) 2HNO3 + 2Ca(OH)2 → Ca(NO3)2 + 2H2O

(b) 6NaOH + 3H2SO4 → 3Na2SO4 + 6H2O

(c) NaCl + AgNO3 → AgCI + NaNO3

(d) BaCl2 + H2SO4 → BaSO4 + 2HCl

7) Write down the balanced chemical equations for the chemical reaction statements mentioned below:

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride


Ans: (a) Ca (OH)2 + CO2 → CaCO3 + H2O

(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag

(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu

(d) BaCl2 + K2SO4 → BaSO4 + 2KCl

8) Write down the balanced chemical equation for the unbalanced equations mentioned below. Also state the type of chemical reaction.

(a) KBr + BaI2 – KI + BaBr2

(b) ZnCO3 – ZnO CO2

(c) H2 + Cl – HCl

(d) Mg + HCl – MgCl2 + H2


Ans: Balanced chemical equations have been writen below with type for each equation.

(a) 2KBr + BaI2 → 2KI + BaBr2 — This is a Double Displacement reaction

(b) ZnCO3 → ZnO + CO2 — This is a Decomposition reaction

(c) H2 + Cl2 → 2HCl — This is a Combination reaction

(d) Mg + 2HCl → MgCl2 + H2 — This is a Displacement reaction

9) Define exothermic & endothermic chemical reactions by giving suitable examples.

Ans: Exothermic reactions are defined as the chemical reactions which evolve heat.

The chemical reaction between 1 mole of N2 & O2 molecule respectively. Heat of 184 KJ energy must be supplied to proceed the reaction & produce the products namely 1 mole of NO molecule. This chemical reaction is endothermic since 184 KJ energy was supplied. It also indicates that the chemical bonds of reactants (N≡N & (O=O) are too strong enough due to triple & doublr covalent bonding which do not breaks easily.

10) Why the process of human respirstion considered as an exothermic reaction?

Ans:  The process of respiration occurs in the human body cells where glucose combines with oxygen producing the products Carbon Dioxide & Water respectively beside releasing energy.

Glucose + Oxygen → Carbon Dioxide + Water + Energy

Thus, energy is released as a product rather than utilised so this reaction is known as Exothermic reaction.

11) Why decomposition chemical reactions are usually known as opposite of the combination chemical reactions. Also, write down decomposition chemical reactions equations.

Ans: Decompostion reactions are those where a chemical compound splits into two or more substances. This is opposite since Combination reaction involves those split substances to reform the original compound.

Ex: (i) Mercury (II) Oxide decomposes on heating into Mercury metal & Oxygen.

2HgO → 2Hg + O2 This chemical reaction is an endothermic since  heat energy is supplied to intiate the reaction.

(ii) Calcium Carbonate on heating decomposes into Calcium Oxide & Carbon Dioxide gas. CaCO3 → Cao + CO2

12) Energy supplied in the form of heat, light, or electricity of decomposition reactions. Write down their chemical equations.
Ans: One of the decomposition reaction is Calcium Carbonate on heating decomposes to produce the products Calcium Oxide & Carbon Dioxide gas respectively. CaCO3 (s) → CaO (aq) + CO2 (g)

Liquid water on electrolysis decomposes into Hydrogen & Oxygen gas respectively. 2H20 (l) → 2H2 (g) + O2 (g)

Silver Chloride on exposure to sunlight decomposes producing the products Silver metal & Chlorine gas respectively. 2AgCl (s) → 2Ag + 2Cl

13) State the differences between double displacement & displacement reactions by writing down suitable equations.
Ans: The difference between the displacement and double displacement reactions is that in a displacement reaction one element displaces another by virtue of it being more reactive whereas in a double displacement two anions and two cations switch places between two compounds respectively.

The differences between double displacement & displacement reactions are as follows:

Double Displacement: The chemical reaction in which a more reactive element displaces a less reactive element from its position. Ex: Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (pt) + 2NaCl(aq)

Displacement reaction: The chemical reaction in which two elements alternatively exchange corresponding ions (cation & anion) from its position. Ex: CuSO4 (aq) + Zn(s) → ZnSO4 + Cu(s)

14) The refinement of silver metal involves displacement of silver by Coper metal from the Silver Nitrate solution by giving suitable chemical equation.
Ans: The following chemical equation takes place Cu (s) + 2AgNO3 (aq) — Cu(NO3)2 (aq) + 2Ag (s)

15) What is meant by precipiation reaction. Also, give suitable chemical equation examples.
Ans: The chemical reaction where soluble ions (cation & anion) from different solutions are allowed to mix together to react producing an insoluble solid compound known as precipitate.

Ex: AgNO3 (aq) + KCl(aq) → AgCl(s) + KNO3(aq)

Silver Nitrate (AgNO3) on reacting with Potassium Chloride (KCl) tends to form a solution of a white solid precipitate (AgCl) along with Silver Chloride compounds respectively.

16) Element “X” on heating in air turns its shiny colour from brown to black. Name the element “X” along with the chemical equation.

Ans: The element “X” is a shiny brown colour Copper metal, this element on exposure to heat in air produces a black coloured compound known as Copper Oxide (CuO). The chemical equation is as follows 2Cu (s) + O2 — 2CuO (s).

17) Why paint/grease must usually be applied on iron materials/items?

Ans:  Iron materials on exposure to the surroundings/air reacts with Oxygen gas present in the environment. This chemical reaction of Iron (Fe) with Oxygen (O2) produces a brown soft & porous layer known as Rust. Corrosion of Iron is a continuous process until we apply some paint/grease to stop further rusting.

18) Oil & fat containing food items are flushed with nitrogen. Why?

Ans: To prevent food items from getting oxidised or unfit for consumption & preserve in its original state. Thus, they are filled or flushed with nitrogen gas.

19) Explain the following terms with one example each. Define the following terms mentioned below by giving suitable examples.

(a) Corrosion
(b) Rancidity


Ans: (a) Corrosion: The process in which metals surfaces are gradually eaten/worn away by reacting with air or surroundings.
Ex: Whenever iron metal is allowed to react with moist air for a long interval of time, iron metal surface gradually acquires brown coating layer known as rust (Hydrated iron). The chemical equation for this reaction is 4Fe + 3O2 + 2H2O → 2Fe2O3.H20 where Iron reacts with oxygen along with water to produce hydrated Iron (III) oxide layer.

(b) Rancidity: Fat & Oil containing food items when exposed to air undergoes the process of oxidation producing often unpleasant smell/taste. This condition spoils the food items which eventually makes the them unfit for consumption. Thus, Rancidity can be stopped by flushing anti-oxidants such as Nitrogen gas before sealing.


20) Why should a magnesium ribbon be cleaned before burning in air?

Ans: Magnesium ribbon gets covered with a layer of Magnesium Oxide (MgO) when it is kept in room in contact with air for a long time. This outermost extra layer prevents the burning of magnesium ribbon. Hence, it must be cleaned before burning.

21) Write down the balanced chemical equations for the chemical reactions mentioned below.
(i) Hydrogen + Chloride → Hydrogen chloride
(ii) Barium Chloride + Aluminium Sulphate → Barium Sulphate + Aluminium Chloride
(iii) Sodium + Water → Sodium Hydroxide + Hydrogen

Ans: The chemical equations of the above chemical reactions are:
(i) H2 + Cl2 → 2HCl
(ii) 3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3
(iii) 2Na + 2H2O → 2NaOH + H2

22) Write down the balanced chemical equations with appropriate symbols for the chemical reactions below.
(a) Solutions of Barium Chloride & Sodium Sulphate in Water react to give insoluble Barium Sulphate & the solution of Sodium Chloride.
(b) Sodium Hydroxide aqueous solution reacts with aqueous Hydrochloric acid solution to produce Sodium Chloride solution and Water.


Ans: The chemical equations for the above chemical reactions are:
(a) BaCl2 (aq) + Na2SO4 (aq) — BaSO4 (aq) + 2NaCl (aq)
(b) NaOH (aq) + HCl (aq) — NaCl (aq) + H2O (aq)

23) Name the element “X” used for the purpose of white-washing. Also,
(i) Write down its chemical formula.
(ii) Write down the chemical reaction of element X” with water.


Ans: The element “X” which is used for the purpose of white-washing is known as Calcium Oxide.
(i) Calcium Oxide chemical formula is (CaO) which is also known as “Quick lime”.
(ii) Calcium Oxide (CaO) on reacting with water (H2O) readily forms Calcium Hydroxide (CaOH2). The chemical equation is as follows: CaO (s) + H2O (l) — CaOH2 (s)

24) Copper Sulphate solution changes its colour after iron nail is immersed into it?
Ans: When an iron nail is immersed in the Copper Sulphate solution for about 30 minutes, the chemical displacement reaction takes place which changes the blue colour of Copper Sulphate solution to light green due to the formation of Iron Sulphate. The following balanced chemical reactions takes place: Fe (s) + CuSo4 (aq) → FeSO4 (aq)+ Cu (s)

25) Identify the elements oxidised/reduced in the chemical equations given below.

(i) 4Na(s) + O2 (g) → 2Na2O(s)

(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)

Ans: The elements oxidised/reduced in the following equations are:
(i) This reaction is oxidixation since Oxygen is added to Sodium upon reacting forming Sodiun Oxide & oxidising the element “Sodium”.
(ii) This reaction is redox reaction since Copper Oxide (CuO) gets reduced giving Oxygen required for Hydrogen (H2) oxidation. Hydrogen (H2) is getting oxidised since it accepts Oxygen from Copper Oxide (CuO). Thus, Copper Oxide (CuO) acts as an oxidising agent & Hydrogen (H2) as a reducing agent.

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