Chapter – 1 Chemical Reactions and Equations

Class 10 Science NCERT Textbook – Page No. 6

Q1. Why should a magnesium ribbon be cleaned before burning in air?
Answer:
A magnesium ribbon is scrubbed to remove protective layer of magnesium oxide
from its surface so that it combines with the oxygen of air on heating.

Q2. Write the balanced equation for the following chemical reactions.
i) Hydrogen + chlorine ⟶ Hydrogen chloride
ii) Barium chloride + Aluminum sulphate ⟶ Barium sulphate +
Aluminum chloride
iii) Sodium + water ⟶ Sodium hydroxide + Hydrogen
Answer:
i) 𝐻2 + 𝐶𝑙2 ⟶ 2𝐻𝐶𝑙
ii) 3𝐵𝑎𝐶𝑙2 + 𝐴𝑙2 (𝑆𝑂4 )3 ⟶ 3𝐵𝑎𝑆𝑂4 + 2𝐴𝑙𝐶𝑙3
iii) 2𝑁𝑎 + 2𝐻2 𝑂 ⟶ 2𝑁𝑎𝑂𝐻 + 𝐻2

Q3. Write balanced chemical equations with state symbol for the following
reactions:
i) Solutions of barium chloride and sodium sulphate in water react 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.
Answer:
i) 𝐵𝑎𝐶𝑙2 (𝑎𝑞 ) + 𝑁𝑎2 𝑆𝑂4 (𝑎𝑞 ) ⟶ 𝐵𝑎𝑆𝑂4 (𝑠) + 2𝑁𝑎𝐶𝑙 (𝑎𝑞)
ii) 𝑁𝑎𝑜𝐻 (𝑎𝑞 ) + 𝐻𝐶𝑙 (𝑎𝑞 ) ⟶ 𝑁𝑎𝐶𝑙 (𝑎𝑞 ) + 𝐻2 𝑂 (𝑙)

Class 10 Science NCERT Textbook – Page No. 10

Q1. A solution of substance X is used for white washing.
i) Name the substance and write its formula.
ii) Write the reaction of the substance X named in (i) above with water.
Answer:
 i) The substance whose solution in water we use for white-washing is
calcium oxide. Hence, the substance X is calcium oxide, which is
represented by the formula 𝐶𝑎𝑂.
ii) The equation for the reaction of calcium oxide with water is:

𝐶𝑎𝑂 (𝑠) + 𝐻2 𝑂 ⟶ 𝐶𝑎(𝑂𝐻)2 (𝑠)

Q2. Why is the amount of gas collected in one of the test tubes in the
electrolysis is double of the amount collected in the other? Name the gas.
Answer:
The gas collected is double the amount in the electrolysis of water experiment
because water (𝐻2 𝑂) contain 2 parts of hydrogen element.

Class 10 Science NCERT Textbook – Page No. 13

Q1. Why does the color of copper sulphate change when an iron nail is dipped
in it?
Answer:
When an iron nail dipped in copper sulphate solution, then the blue color of copper
sulphate solution changes because iron displaces copper from copper sulphate
solution to form a light green solution of iron sulphate.

Q2. Give an example of a double displacement reaction.

Answer:
When a silver nitrate solution is added to sodium chloride solution, then a double
displacement reaction takes place in which a white precipitate of silver chloride is
formed along with sodium nitrate solution.
𝐴𝑔𝑁𝑂3 (𝑎𝑞) + 𝑁𝑎𝐶𝑙 (𝑎𝑞 ) ⟶ 𝐴𝑔𝐶𝑙 (𝑠) + 𝑁𝑎𝑁𝑂3 (𝑎𝑞)

Q3. Identify the substances that are oxidized and the substances that are
reduced in the following reactions:
i) 𝟒𝑵𝒂 (𝒔) + 𝑶𝟐 (𝒈) ⟶ 𝟐𝑵𝒂𝟐 𝑶 (𝒔)
ii) 𝑪𝒖𝑶 (𝒔) + 𝑯𝟐 (𝒈) ⟶ 𝑪𝒖 (𝒔) + 𝑯𝟐 𝑶 (𝒍)
Answer:
i) Substance oxidized : Na ; Substance reduced : 𝑂2
ii) Substance oxidized : 𝐻2 ; Substance reduced : 𝐶𝑢𝑂
Class 10 Science NCERT Textbook – Page No. 14, 15 and 16
Q1. Which of the statements about the reaction below are incorrect?
𝟐𝑷𝒃𝑶 (𝒔) + 𝑪 (𝒔) ⟶ 𝟐𝑷𝒃 (𝒔) + 𝑪𝑶𝟐 (𝒈)
a) Lead is getting reduced
b) Carbon dioxide is getting oxidized
c) Carbon is getting oxidized
d) Lead oxide is getting reduced

i) a) and b)
ii) a) and c)
iii) a), b) and c)
iv) all
Answer: Option i)
This is because the oxygen is being removed and the removed oxygen from the
Lead is added to the elemental Carbon.

Q2. 𝑭𝒆𝟐 𝑶𝟑 + 𝟐𝑨𝒍 ⟶ 𝑨𝒍𝟐 𝑶𝟑 + 𝟐𝑭𝒆

The above reaction is an example of a:
a) combination reaction
b) double displacement reaction
c) decomposition reaction
d) displacement reaction
Answer: Option d)
Double Displacement Reaction

Q3. What happens when dilute hydrochloric acid is added to iron filings? Tick
the correct answer.
a) Hydrogen gas and iron chloride are produced
b) Chlorine gas and iron hydroxide are produced
c) No reaction takes place
d) Iron salt and water are produced
Answer: Option a)
Hydrogen gas and iron chloride are produced.

Q4. What is a balanced chemical equations? Why should chemical equations

be balanced?
Answer:
A chemical equation with equal number of atoms of different elements is balanced
chemical equation.
Example: Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen:
𝑍𝑛 + 𝐻2 𝑆𝑂4 ⟶ 𝑍𝑛𝑆𝑂4 + 𝐻2
The above equation has an equal number of Zn atoms (1 each), H atoms (2 each), S
atoms (1 each) and O atoms (4 each) in reactants and products, hence it is a
balanced chemical equation.
The chemical equations need to be a balanced one, so that it satisfies the law of
conservation of mass which says that ‘matter can neither be created nor destroyed in
a chemical reaction’. Hence, the total mass of all the elements present in the
products of a chemical reaction should be equal to the mass of all the elements
present in the reactant.

Q5. Translate the following statements into chemical equations and then
balance them:
a) Hydrogen gas combines with nitrogen to form ammonia.
b) Hydrogen sulphide gas burns in air to give water and Sulphur dioxide
c) Barium chloride reacts with aluminum sulphate to give aluminum
chloride and a precipitate of barium sulphate
d) Potassium metal reacts with water to give potassium hydroxide and
hydrogen gas
Answer:
a) 3𝐻2 + 𝑁2 ⟶ 2𝑁𝐻3
b) 2𝐻2 𝑆 + 3𝑂2 ⟶ 2𝐻2 𝑂 + 2𝑆𝑂2
c) 3𝐵𝑎𝐶𝑙2 + 𝐴𝑙2 (𝑆𝑂4 )3 ⟶ 2𝐴𝑙𝐶𝑙3 + 3𝐵𝑎𝑆𝑂4
d) 2𝐾 + 2𝐻2 𝑂 ⟶ 2𝐾𝑂𝐻 + 𝐻2

Q6. Balance the following chemical equations.

a) 𝑯𝑵𝑶𝟑 + 𝑪𝒂(𝑶𝑯)𝟐 ⟶ 𝑪𝒂(𝑵𝑶𝟑 )𝟐 + 𝑯𝟐 𝑶
b) 𝑵𝒂𝑶𝑯 + 𝑯𝟐 𝑺𝑶𝟒 ⟶ 𝑵𝒂𝟐 𝑺𝑶𝟒 + 𝑯𝟐 𝑶
c) 𝑵𝒂𝑪𝒍 + 𝑨𝒈𝑵𝑶𝟑 ⟶ 𝑨𝒈𝑪𝒍 + 𝑵𝒂𝑵𝑶𝟑
d) 𝑩𝒂𝑪𝒍𝟐 + 𝑯𝟐 𝑺𝑶𝟒 ⟶ 𝑩𝒂𝑺𝑶𝟒 𝑯𝑪𝒍
Answer:
a) 2𝐻𝑁𝑂3 + 𝐶𝑎(𝑂𝐻)2 ⟶ 𝐶𝑎(𝑁𝑂3 )2 + 2𝐻2 𝑂
b) 2𝑁𝑎𝑂𝐻 + 𝐻2 𝑆𝑂4 ⟶ 𝑁𝑎2 𝑆𝑂4 + 2𝐻2 𝑂
c) 𝑁𝑎𝐶𝑙 + 𝐴𝑔𝑁𝑂3 ⟶ 𝐴𝑔𝐶𝑙 + 𝑁𝑎𝑁𝑂3
d) 𝐵𝑎𝐶𝑙2 + 𝐻2 𝑆𝑂4 ⟶ 𝐵𝑎𝑆𝑂4 + 2𝐻𝐶𝑙
Q7. Write the balanced chemical equation for the following reactions:
a) Calcium hydroxide + Carbon dioxide ⟶ Calcium carbonate + Water
b) Zinc + Silver Nitrate ⟶ Zinc nitrate + Silver
c) Aluminum + Copper chloride ⟶ Aluminum chloride + Copper
d) Barium chloride + Potassium sulphate ⟶ Barium sulphate + Potassium
chloride
Answer:
a) 𝐶𝑎(𝑂𝐻)2 + 𝐶𝑂2 ⟶ 𝐶𝑎𝐶𝑂3 + 𝐻2 𝑂
b) 𝑍𝑛 + 2𝐴𝑔𝑁𝑂3 ⟶ 𝑍𝑛(𝑁𝑂3 )2 + 2𝐴𝑔
c) 2𝐴𝑙 + 3𝐶𝑢𝐶𝑙2 ⟶ 2𝐴𝑙𝐶𝑙2 + 3𝐶𝑢
d) 𝐵𝑎𝐶𝑙2 + 𝐾2 𝑆𝑂4 ⟶ 𝐵𝑎𝑆𝑂4 + 2𝐾𝐶𝑙

Q8. Write balanced chemical equation for the following and identify the type of
reaction in each case.
a) Potassium bromide (aq) + Barium iodide (aq) ⟶ Potassium iodide (aq) +
Barium bromide (s)
b) Zinc carbonate (s) ⟶ Zinc oxide (s) + Carbon dioxide (g)
c) Hydrogen (g) + Chloride (g) ⟶ Hydrogen chloride (g)
d) Magnesium (s) + Hydrochloric acid (aq) ⟶ Magnesium chloride (aq) +
Hydrogen (g)
Answer:
a) 2𝐾𝐵𝑟 (𝑎𝑞 ) + 𝐵𝑎𝐼2 (𝑎𝑞 ) ⟶ 2𝐾𝐼 (𝑎𝑞 ) + 𝐵𝑎𝐵𝑟2 (𝑠)
𝑇ℎ𝑖𝑠 𝑖𝑠 𝑎 𝑑𝑜𝑢𝑏𝑙𝑒 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛.

b) 𝑍𝑛𝐶𝑂3 (𝑠) ⟶ 𝑍𝑛𝑂 (𝑠) + 𝐶𝑂2

𝑇ℎ𝑖𝑠 𝑖𝑠 𝑎 𝑑𝑒𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛.

c) 𝐻2 (𝑔) + 𝐶𝑙2 (𝑔) ⟶ 2𝐻𝐶𝑙 (𝑔)

𝑇ℎ𝑖𝑠 𝑖𝑠 𝑎 𝑐𝑜𝑚𝑏𝑖𝑛𝑎𝑡𝑖𝑜𝑛 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛.

d) 𝑀𝑔 (𝑠) + 2𝐻𝐶𝑙 (𝑎𝑞 ) ⟶ 𝑀𝑔𝐶𝑙2 (𝑎𝑞 ) + 𝐻2 (𝑔)

𝑇ℎ𝑖𝑠 𝑖𝑠 𝑎 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛.

Q9. What does one mean by exothermic and endothermic reactions? Give
examples.
Answer:
Exothermic reactions are those in which the heat is involved. Example; carbon burns
with oxygen forming carbon dioxide and large amount of heat is produced.
𝐶 (𝑠) + 𝑂2 (𝑔) ⟶ 𝐶𝑂2 (𝑔) + 𝐻𝑒𝑎𝑡

In Endothermic reactions is heat is absorbed. Example; when nitrogen and oxygen

are heated to high temperature (about 3000𝑜 𝐶) they combine to form nitrogen
monoxide and large amount of heat is absorbed.
𝑁2 (𝑔) + 𝑂2 (𝑔) + 𝐻𝑒𝑎𝑡 ⟶ 2𝑁𝑂(𝑔)

Q10. Why is respiration considered an exothermic reaction? Explain.

Answer:
Respiration is considered an exothermic reaction because energy is released in this
process. In respiration, glucose reacts with oxygen in the cells of body to form
carbon dioxide and water releasing energy.
𝐶6 𝐻12 𝑂6 (𝑎𝑞 ) + 6𝑂2 (𝑔) ⟶ 6𝐶𝑂2 (𝑔) + 6𝐻2 𝑂(𝑙 ) + 𝐸𝑛𝑒𝑟𝑔𝑦

Q11. Why are decomposition reactions called the opposite of combination

reactions? Write equations for these reactions.
Answer:
The decomposition reactions are opposite of combination reactions because in a
combination reaction two or more substances combine to form product whereas in
decomposition reaction, a substance splits up to form two or more simpler
substances.
a) When hydrogen burns in oxygen, it forms water:

2𝐻2 (𝑔) + 𝑂2 (𝑔) ⟶ 2𝐻2 𝑂 (𝑙)

In this reaction, two substances hydrogen and oxygen combine to form a

single substance water, so this is a combination reaction.

b) When acidified water is electrolyzed, hydrogen and oxygen are formed:

2𝐻2 𝑂(𝑙 ) ⟶ 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 ⟶ 2𝐻2 (𝑔) + 𝑂2 (𝑔)

Here a single substance water breaks up into simpler substance, hydrogen

and oxygen. Hence, this is a decomposition reaction.
Q12. Write one equation each for decomposition reactions where energy is
supplied in the form of heat, light or electricity.
Answer:
a) When calcium carbonate is heated, it decomposes to form calcium oxide and
carbon dioxide:

𝐶𝑎𝐶𝑂3 (𝑠) ⟶ 𝐻𝑒𝑎𝑡 ⟶ 𝐶𝑎𝑂(𝑠) + 𝐶𝑂2 (𝑔)

𝐻𝑒𝑟𝑒, 𝑡ℎ𝑒 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑠 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑓𝑜𝑟𝑚 𝑜𝑓 𝐻𝑒𝑎𝑡.

b) When silver chloride is exposed to sunlight, it decomposes to form silver

metal and chlorine gas:

2𝐴𝑔𝐶𝑙(𝑠) ⟶ 𝑆𝑢𝑛𝑙𝑖𝑔ℎ𝑡 ⟶ 2𝐴𝑔(𝑠) + 𝐶𝑙2 (𝑔)

𝑇ℎ𝑒 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑠 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑓𝑜𝑟𝑚𝑚 𝑜𝑓 𝑠𝑢𝑛𝑙𝑖𝑔ℎ𝑡.

c) When acidified water is electrolyzed, it decomposes to form hydrogen and

oxygen:

2𝐻2 𝑂(𝑙 ) ⟶ 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 ⟶ 2𝐻2 (𝑔) + 𝑂2 (𝑔)

𝑇ℎ𝑒 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑠 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑓𝑜𝑟𝑚 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦.

Q13. What is the difference between displacement and double displacement

reactions? Write equations for these reactions.
Answer:
In a displacement reaction, a more reactive element displaces a less reactive
element from its compound whereas in a double displacement reaction, two
compounds combine to form two new compounds.
i) A displacement reaction takes place between copper sulphate solution
and zinc to form zinc sulphate solution and copper:

𝐶𝑢𝑆𝑂4 (𝑎𝑞 ) + 𝑍𝑛(𝑠) ⟶ 𝑍𝑛𝑆𝑂4 (𝑎𝑞 ) + 𝐶𝑢(𝑠)

ii) A double displacement reaction takes place between barium chloride and
sodium sulpahte solution to form a white precipitate of barium sulphate
and sodium chloride solution:

𝐵𝑎𝐶𝑙2 (𝑎𝑞 ) + 𝑁𝑎2 𝑆𝑂4 (𝑎𝑞 ) ⟶ 𝐵𝑎𝑆𝑂4 (𝑠) + 2𝑁𝑎𝐶𝑙 (𝑎𝑞)

Q14. In the refining of silver, the recovery of silver from silver nitrate solution
involved displacement by copper metal. Write down the reaction involved.
Answer:
2𝐴𝑔𝑁𝑂3 (𝑎𝑞 ) + 𝐶𝑢 (𝑠) ⟶ 𝐶𝑢(𝑁𝑂3 )2 (𝑎𝑞 ) + 2𝐴𝑔 (𝑠)

Q15. What do you mean by precipitation reaction? Explain by giving example.

Answer:
Any reaction in which an insoluble solid (called precipitate) is formed that suddenly
separates from the solution, is called a precipitation reaction. The reaction between
silver nitrate solution and sodium chloride solution to form silver chloride precipitate
is an example of a precipitation reaction:
𝐴𝑔𝑁𝑂3 (𝑎𝑞) + 𝑁𝑎𝐶𝑙 (𝑎𝑞 ) ⟶ 𝐴𝑔𝐶𝑙 (𝑠) + 𝑁𝑎𝑁𝑂3 (𝑎𝑞)
In this reaction, silver chloride is formed as a white, insoluble solid which separates
out suddenly from the solution.

Q16. Explain the following in terms of gain or loss of oxygen with two
examples each:
a) Oxidation
b) Reduction
Answer:
a) Oxidation is gain of oxygen by a substance in a reaction.

i) When magnesium is burned in air, then magnesium is formed:

2𝑀𝑔 + 𝑂2 ⟶ 2𝑀𝑔𝑂

In the reaction, magnesium (Mg) has gained oxygen to form

magnesium oxide (MgO), so magnesium oxidized to magnesium oxide.

ii) When copper oxide react with hydrogen, copper metal with water are
formed:

𝐶𝑢𝑂 + 𝐻2 ⟶ 𝐶𝑢 + 𝐻2 𝑂

In this reaction, hydrogen is gaining oxygen to form water, so hydrogen

is getting oxidized to water.
 b) Reduction is loss of oxygen by a substance in a reaction.

i) When zinc oxide is heated with carbon, then zinc metal and carbon
monoxide are formed:

𝑍𝑛𝑂 + 𝐶 ⟶ 𝑍𝑛 + 𝐶𝑂

In this reaction, Zinc oxide (𝑍𝑛𝑂) is losing oxygen to form zinc metal
(𝑍𝑛), therefore, zinc oxide is reduced to zinc.

ii) When iron (III) oxide is heated with aluminum powder, then aluminum
oxide and iron metal are formed:

𝐹𝑒2 𝑂3 + 2𝐴𝑙 ⟶ 𝐴𝑙2 𝑂3 + 2𝐹𝑒

In this reaction, iron (III) oxide (𝐹𝑒2 𝑂3 ) is losing oxygen to form iron
metal (𝐹𝑒), therefore, iron (III) oxide is reduced to iron.

Q17. A shiny brown colored element X on heating in air becomes black in

color. Name the element X and the black colored compound formed.
Answer:
i) The brown element X is copper metal (𝐶𝑢).
ii) When copper metal is heated in air, it forms a black compound copper
oxide or copper (II) oxide, 𝐶𝑢𝑂.

Q18. Why do we apply paint on iron articles?

Answer:
Paint is applied on iron articles to prevent their rusting as air and moisture cannot
come in contact with the iron metal of the article.

Q19. Oil and fat containing food items are flushed with nitrogen. Why?
Answer:
The plastic bags containing oil and fat containing food item are flushed with an
unreactive gas nitrogen so as to prevent them from getting oxidized and turn rancid,
because in the presence of oxygen of air, the fats and oils present in food item get
oxidizing forming products having unpleasant smell and taste which turn the foods
rancid. When air containing oxygen is replaced by unreactive nitrogen gas, the
packed food item do not gets spoiled. They remain fresh for a much longer time.
Q20. Explain the following terms with one example each:
a) Corrosion
b) Rancidity
Answer:
a) Corrosion is the process in which metals are eaten up gradually by the action
of air, moisture or a chemical on their surface. For example: Rusting of iron
Rusting involves unwanted oxidation of iron metal which occurs in nature on
its own. When an iron object is exposed to damp air, it gets covered into red-
brown substance called ‘rust’. The corrosion of iron is a continuous process
which, if not prevented intime, eats up the whole iron object.
b) Rancidity: oxidation has damaging effect on foods containing fats and oils.
When the food materials prepared in fats and oil are kept for a long time, they
give unpleasant smell and taste is called rancidity. Rancidity spoils the food
materials prepared in fats and oil which makes them unfit for eating. Example:
if potatoes chips prepared in oil are kept exposed to air for a long time, they
start giving unpleasant smell and taste due to the oxidation of oil present in
them. The potato chips turn rancid and become unfit for eating.