Extra 20 long-answer questions from Chapter:1 Chemical Reactions and Equations for Class 10 CBSE Science:
1-10: Basics & Types of Chemical Reactions
What is a chemical reaction? Explain with suitable examples.
Answer:
A chemical reaction is a process where reactants undergo a chemical change to form new substances (products) with different properties.
Example 1:
Burning of magnesium
2Mg+O2→2MgO2Mg + O_2 → 2MgO2Mg+O2→2MgO Magnesium reacts with oxygen to form magnesium oxide (a white powder).
Example 2:
Rusting of iron
4Fe+3O2+6H2O→4
Fe(
OH)34Fe + 3O_2 + 6H_2O → 4Fe(OH)_34Fe+3O2+6H2O→4Fe(OH)3 This is a slow reaction where iron reacts with oxygen and water to form hydrated iron oxide (rust).
Explain the Law of Conservation of Mass with an example.
Answer:
The
Law of Conservation of Mass
states that mass can neither be created nor destroyed in a chemical reaction.
Example: Consider the reaction between
barium chloride (
BaCl
₂) and sodium
sulphate
(
Na₂SO
₄)
. BaCl2+Na2SO4→BaSO4+2NaClBaCl_2 + Na_2SO_4 → BaSO_4 + 2NaClBaCl2+Na2SO4→BaSO4+2NaCl
Total mass of reactants = Total mass of products.
What is a balanced chemical equation? Why is it necessary to balance a chemical equation?
Answer:
A
balanced chemical equation
has an equal number of atoms of each element on both reactant and product sides.
Example (Balanced Equation): 2H2+O2→2H2O2H_2 + O_2 → 2H_2O2H2+O2→2H2O
It is necessary to balance an equation to satisfy the
Law of Conservation of Mass
, ensuring no atoms are lost or gained in the reaction.
What are combination reactions? Explain with two examples.
Answer:
A
combination reaction
is a reaction in which two or more substances combine to form a single product.
Example 1: 2Mg+O2→2MgO2Mg + O_2 → 2MgO2Mg+O2→2MgO Magnesium and oxygen combine to form magnesium oxide.
Example 2: CaO+H2O→
Ca(
OH)2CaO + H_2O → Ca(OH)_2CaO+H2
O→Ca
(OH)2 Quicklime reacts with water to form slaked lime.
What is a decomposition reaction? Explain its types with examples.
Answer:
Decomposition reaction
is a reaction in which a compound breaks down into two or more simpler substances.
Types:
Thermal Decomposition
: CaCO3→CaO+CO2CaCO_3 →
CaO
+ CO_2CaCO3→CaO+CO2 (Calcium carbonate decomposes into calcium oxide and carbon dioxide when heated).
Electrolytic Decomposition
: 2H2O→2H2+O22H_2O → 2H_2 + O_22H2O→2H2+O2 (Water decomposes into hydrogen and oxygen by electrolysis).
Photochemical Decomposition
: 2AgCl→2Ag+Cl22AgCl → 2Ag + Cl_22AgCl→2Ag+Cl2 (Silver chloride decomposes into silver and chlorine in sunlight).
Explain displacement reactions with examples.
Answer:
In a
displacement reaction
, a more reactive element displaces a less reactive element from its compound.
Example 1:
Reaction of iron with copper
sulphate
Fe+CuSO4→FeSO4+CuFe + CuSO_4 → FeSO_4 + CuFe+CuSO4→FeSO4+Cu (Iron displaces copper from copper
sulphate
).
Example 2:
Reaction of zinc with hydrochloric acid
Zn+2HCl→ZnCl2+H2Zn + 2HCl → ZnCl_2 + H_2Zn+2HCl→ZnCl2+H2 (Zinc displaces hydrogen from hydrochloric acid).
What is a double displacement reaction? Explain with examples.
Answer:
In a
double displacement reaction
, two compounds react by exchanging their ions to form two new compounds.
Example 1:
Reaction of sodium
sulphate
and barium chloride
Na2SO4+BaCl2→BaSO4+2NaClNa_2SO_4 + BaCl_2 → BaSO_4 + 2NaClNa2SO4+BaCl2→BaSO4+2NaCl (Barium
sulphate
forms as a white precipitate).
Example 2:
Reaction of lead nitrate and potassium iodide
Pb(
NO3)2+2KI→PbI2+2KNO3Pb(NO_3)_2 + 2KI → PbI_2 +
2KNO_3Pb(NO3)2+2KI→PbI2+2KNO3 (Lead iodide forms as a yellow precipitate).
Explain oxidation and reduction reactions with examples.
Answer:
Oxidation
: Gain of oxygen or loss of electrons.
Reduction
: Loss of oxygen or gain of electrons.
Example:
Reaction of copper oxide and hydrogen
CuO+H2→Cu+H2OCuO + H_2 → Cu + H_2OCuO+H2→Cu+H2O (Copper oxide is reduced to copper, and hydrogen is oxidized to water).
What is a redox reaction? Give an example.
Answer:
A
redox reaction
is a reaction where oxidation and reduction occur simultaneously.
Example:
Reaction of zinc with hydrochloric acid
Zn+2HCl→ZnCl2+H2Zn + 2HCl → ZnCl_2 + H_2Zn+2HCl→ZnCl2+H2
Zinc is
oxidized
to Zn²⁺.
Hydrogen is
reduced
to H₂ gas.
Explain exothermic and endothermic reactions with examples.
Answer:
Exothermic reaction
: Releases heat.
Example:
Combustion of methane
CH4+2O2→CO2+2H2O+HeatCH_4 + 2O_2 → CO_2 + 2H_2O + HeatCH4+2O2→CO2+2H2
O+Heat
Endothermic reaction
: Absorbs heat.
Example:
Decomposition of calcium carbonate
CaCO3→CaO+CO2CaCO_3 →
CaO
+ CO_2CaCO3→CaO+CO2
11. What is corrosion? How can it be prevented?
Answer:
Corrosion
is the slow deterioration of metals due to chemical reactions with the environment.
Example:
Rusting of iron
4Fe+3O2+6H2O→4
Fe(
OH)34Fe + 3O_2 + 6H_2O → 4Fe(OH)_34Fe+3O2+6H2O→4Fe(OH)3 (Iron reacts with oxygen and water to form hydrated iron oxide, commonly known as rust).
Prevention of Corrosion:
Painting
: Forms a protective layer to prevent air and moisture contact.
Galvanization
: Coating iron with zinc to prevent rusting.
Oiling/Greasing
: Protects from moisture.
Alloying
: Stainless steel (iron mixed with chromium and nickel) resists rust.
Electroplating
: Coating metal with another metal to prevent corrosion.
12. What is rancidity? How can it be prevented?
Answer:
Rancidity
is the spoilage of food due to oxidation of fats and oils, giving an unpleasant taste and smell.
Example: Spoiling of chips or fried food when exposed to air for a long time.
Prevention of Rancidity:
Using Antioxidants
: Chemicals like BHA (Butylated
Hydroxyanisole
) and BHT (Butylated Hydroxytoluene) prevent oxidation.
Airtight Packaging
: Reduces contact with oxygen.
Refrigeration
: Slows down oxidation.
Storing in Nitrogen Atmosphere
: Food packets (e.g., chips) are filled with nitrogen gas to prevent oxidation.
Using Dark Containers
: Prevents oxidation caused by light.
13. Explain the importance of balancing a chemical equation.
Answer:
A
balanced chemical equation
follows the
Law of Conservation of Mass
, which states that mass can neither be created nor destroyed.
Example (Unbalanced Equation): H2+O2→H2OH_2 + O_2 → H_2OH2+O2→H2O
Balanced form: 2H2+O2→2H2O2H_2 + O_2 → 2H_2O2H2+O2→2H2O
Importance of Balancing Chemical Equations:
Ensures the same number of atoms on both sides of the reaction.
Helps in understanding the proportion of reactants and products.
Provides information about the energy changes in the reaction.
Helps in calculating the amount of substances required or produced.
Essential in industrial applications for accurate chemical production.
14. What happens when iron nails are placed in copper sulfate solution? Explain with an equation.
Answer:
When an iron nail is placed in a
copper
sulphate
(
CuSO
₄) solution
, the blue
colour
of the solution fades, and a reddish-brown deposit of
copper
forms on the nail.
Reaction:
Fe+CuSO4→FeSO4+CuFe + CuSO_4 → FeSO_4 + CuFe+CuSO4→FeSO4+Cu
Explanation:
Iron is more reactive
than copper, so it displaces copper from the solution, forming
iron
sulphate
(
FeSO
₄)
which is pale green in
colour
.
Copper metal gets deposited
on the iron nail.
15. Write the reaction of quicklime with water. Why is it an exothermic reaction?
Answer:
Quicklime (
Calcium oxide,
CaO
) reacts with water to form
slaked lime (Calcium hydroxide,
Ca(
OH)₂)
and releases a large amount of heat.
Reaction:
CaO+H2O→
Ca(
OH)2+HeatCaO + H_2O → Ca(OH)_2 + HeatCaO+H2
O→Ca
(OH)2+Heat
Why is it an Exothermic Reaction?
Heat is
released
in this reaction, making the solution
hot
.
Exothermic reactions
increase the temperature
of the surroundings.
Uses of Slaked Lime:
Used in
whitewashing walls
.
Used to
neutralize acidic soil
.
Used in the
manufacturing of cement
.
16. Describe the reaction between baking soda and vinegar. Write its equation.
Answer:
Baking soda (
Sodium bicarbonate,
NaHCO
₃
) reacts with vinegar (
Acetic acid, CH₃COOH
) to form
carbon dioxide gas, water, and sodium acetate
.
Reaction:
NaHCO3+CH3COOH→CO2+H2O+CH3COONaNaHCO_3 + CH_3COOH → CO_2 + H_2O + CH_3COONaNaHCO3+CH3COOH→CO2+H2O+CH3
COONa
Observations:
Fizzing or bubbling
occurs due to the evolution of
CO₂ gas
.
This reaction is commonly used in
baking
to make cakes fluffy.
17. What is the role of a catalyst in a chemical reaction? Give examples.
Answer:
A
catalyst
is a substance that
speeds up a reaction
without undergoing permanent change.
Examples of Catalysts:
Decomposition of Hydrogen Peroxide
2H2O2→2H2O+O22H_2O_2 → 2H_2O + O_22H2O2→2H2O+O2
Manganese dioxide (
MnO
₂)
acts as a catalyst.
Manufacture of Ammonia (Haber Process)
N2+3H2→2NH3N_2 + 3H_2 → 2NH_3N2+3H2→2NH3
Iron (Fe) catalyst
is used.
Conversion of
Sulphur
Dioxide to
Sulphur
Trioxide (Contact Process)
2SO2+O2→2SO32SO_2 + O_2 → 2SO_32SO2+O2→2SO3
Vanadium pentoxide (V₂O₅) catalyst
is used.
18. Why does silver chloride turn black when exposed to sunlight? Explain with an equation.
Answer:
Silver chloride (
AgCl
) turns
black in sunlight
due to a
photochemical decomposition reaction
.
Reaction:
2AgCl→2Ag+Cl22AgCl → 2Ag + Cl_22AgCl→2Ag+Cl2 (Silver chloride decomposes into
silver (Ag) and chlorine (Cl₂)
in sunlight).
Reason for Black
Colour
:
The
formation of silver metal
(Ag), which is
black in
colour
, causes the
colour
change.
19. What happens when ferrous sulfate is heated? Explain with an equation.
Answer:
When
ferrous
sulphate
(
FeSO
₄)
is heated, it decomposes to form
ferric oxide (
Fe₂O
₃),
sulphur
dioxide (SO₂), and
sulphur
trioxide (SO₃)
.
Reaction:
2FeSO4→Fe2O3+SO2+SO32FeSO_4 → Fe_2O_3 + SO_2 + SO_32FeSO4→Fe2O3+SO2+SO3
Observations:
A
reddish-brown solid
(
Fe₂O
₃) forms.
SO₂ and SO₃ gases
have a strong smell like
burning
sulfur
.
20. Describe the reaction of sodium bicarbonate with hydrochloric acid. Write its balanced equation.
Answer:
Sodium bicarbonate (
NaHCO
₃)
reacts with
hydrochloric acid (HCl)
to form
sodium chloride (NaCl), carbon dioxide (CO₂), and water (H₂O)
.
Reaction:
NaHCO3+HCl→NaCl+CO2+H2ONaHCO_3 + HCl → NaCl + CO_2 + H_2ONaHCO3+HCl→NaCl+CO2+H2O
Observations:
Effervescence
due to CO₂ gas formation.
Used in
baking
and
fire extinguishers
.
