Important Questions Class 10 Science Chapter 1 – With

Solutions
At Extramarks, we highlight crucial concepts and questions from each chapter which
help students with their studies right before their examinations. By solving the
Important Questions Class 10 Science Chapter 1, the students will be familiar with the
questions asked in final exams. Science is a subject which requires deep conceptual
understanding, so cramming answers won’t help especially in higher classes.
So, while solving important questions, students must understand every concept to
answer any question easily. This encourages the students to master the topic and
increases their confidence in achieving high grades. Our step-by-step solutions given for
all questions in our Class 10 Science Chapter 1 Important Questions help students
revise the chapter while solving the questions.
Below are a few questions and their answers from our questionnaire of Important
Questions Class 10 Science Chapter 1:

Question 1. A substance which oxidises itself and reduces

others is known as
1. Oxidising agent
2. reducing agent
3. Both (a) and (b)
4. None of these.
Answer 1: Correct option is (B) Reducing agent
Explanation: The reducing agent is HCl, and MnO2 is oxidised to MnCl2.
Increase of oxidation number
←——————-⟶
+4 -1 -2 0 +1 -2
MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O
←————————⟶
Decrease of oxidation number
In this given reaction, Mn is getting reduced from +4 to +2 oxidation state, and Cl is
getting oxidised from -1 to 0, so
The substance reduced is MnO2, and the substance oxidised is HCl.
The substance that is reduced shows as an oxidising agent, while a substance that is
oxidised shows as a reducing agent.
Hence Oxidising agent is MnO2, and the Reducing agent is HCl. The correct answer is
option B.
Question 2. Why are food particles preferably packed in
aluminium foil?
Answer 2: Food particles are mostly packed in an aluminium foil sheet because it does
not corrode in the atmosphere. A protective coating of aluminium oxide (Al2O3) is
formed on the surface of the foil, and it stops any further chemical reaction of the metal
with air and water so that even if it is kept for a longer time, food particles do not get
spoiled.

Question 3. Write balanced chemical equations for the

following chemical reactions-
(a) Hydrogen + Chlorine → Hydrogen chloride
(b) Lead + Copper chloride → Lead chloride + Copper
(c) Zinc oxide + Carbon → Zinc + Carbon monoxide
Answer 3:

1. H2(g) + Cl2 (g) → 2HCl (g)

2. Pb(s) + CuCl2 (aq) → PbCl2 (aq) + Cu(s)
3. ZnO(s) + C(s) → Zn (s) + CO(g)
Explanation: It is a balanced chemical reaction

Question 4: Two grams of ferrous sulphate crystals are heated

in a dry boiling tube.
1. a. explain two observations.
2. Name the type of chemical reaction taking place.
3. Write a balanced chemical equation for the reaction and name the
products formed.
Answer 4:
(a) If Ferrous sulphate crystals (FeSO4.7H2O) lose water when heated and the colour of
the crystals is modified. Then it decomposes to ferric oxide (Fe2O3), (SO2), sulphur
dioxide & sulphur trioxide (SO3) with an odour of burning sulphur.
(b) This is a thermal decomposition reaction.

1. c) 2FeSO4(s) Fe2O3(s) + SO2(g) +

SO3(g)
Ferrous sulphate Ferric oxide Sulphur dioxide Sulphur trioxide

Question 5: What happens if dilute hydrochloric acid is added

to iron filling? Tick the correct answer
 Hydrogen gas and iron chloride are produced.
  Chlorine gas and iron hydroxide are produced

 No reaction takes place

 Iron salt and water are produced

Answer 5: (A) Hydrogen gas and iron chloride are produced.
Explanations: option A is correct because hydrogen gas and iron chloride are
produced when treated with HCl and iron filling.
2HCl + 2Fe FeCl2 + H2

Question 6: Dissolving sugar is an example of which change-

1. Physical change
2. Chemical change
3. Redox Reaction
4. None of these.
Answer 6: correct option is (A) Physical change
Explanations: Option A is correct because the chemical composition is not changed in
a physical change, and we get it in its original form while heating.

Question 7: Recognise a metal for each case:

(i) it does not interact with cold as well as hot water but reacts with any
additional physical state of water,
(ii) it does not interact with any physical state of water.
Answer 7:
(i) Aluminium (Al)
(ii) Copper (Cu)
Explanations: i) aluminium oxide is created on the surface of the metal. Still, the
reaction does not go further as the oxide formed isn’t porous, so water won’t be able to
penetrate through the metal to continue the reaction any further.

1. ii) Copper does not react with water because they lie below hydrogen in the
reactivity series. The conclusion is that they cannot replace hydrogen in water
molecules.
Question 8: Which of the gases is used to store fat and oil-
containing foods for a long time?
1. a) Carbon dioxide
2. b) Oxygen
3. c) Nitrogen
4. d) Neon
Answer: correct option is c) Nitrogen
Explanations: Nitrogen can be used for the storage of fresh samples of oil for a long
time. Due to the rancidification of oils and fats, you will observe the change in colour,
odour and test. The gases oxygen and carbon dioxide do not help in the rancidity of oil.
So the following option is incorrect.

Question 9: Why do we store silver chloride in dark-coloured

bottles?
Answer: Silver chloride is the best example of a light-sensitive chemical compound,
and reaction is an example of a photolytic decomposition reaction. It reacts with light
very fast and loses its property by forming chlorine gas and silver. So avoid this silver
chloride in dark-coloured bottles.
Explanations: 2AgCl 2Ag + Cl2
Silver chloride decomposes into silver & chlorine gas when exposed to light. Dark-
coloured bottles interrupt the path of light such that light cannot reach silver chloride in
the bottles, and its decomposition is prevented.
Question 10: Identify the type of reaction in the following example:
2H2 (g) + O2 (g) → 2H2O (I)

 Combination reaction

 Decomposition reaction

 Displacement reaction

 Double displacement reaction

Answer 10: correct option is (a) combination reaction
Explanations: It is a combination reaction because, in this reaction, two substances
combine to form a single substance.

Question 11: Translate a balanced chemical equation with

state symbols for the following reactions:
1. Solutions of Barium chloride and Sodium sulphate in water react to give insoluble
Barium sulphate and a solution of Sodium chloride.
2. Sodium hydroxide solution in water interacts with hydrochloric acid to produce
Sodium chloride solution and water.
3. Hydrogen gas combines with nitrogen to form ammonia.
4. Potassium metal reacts with water to give potassium hydroxide and hydrogen
gas.
Answer 11:
Solution:

1. BaCl2 + Na2SO4 → BaSO4 + 2NaCl

2. NaOH + HCl → NaCl + H2O
 3. 3H2 + N2 2NH3
4. iv) 2K + 2H2O 2KOH + H2
Explanations: all reactions show a combination reaction

Question 12: Which statements about the chemical reaction

below are incorrect?-
2PbO(solid) + C(s) → 2Pb(s) + CO2(g)
(a) Lead is getting reduced
(b) Carbon Dioxide CO2 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
Answer 12: option (i) (a) and (b)
Explanations: (a) because oxygen is being removed and (b) because the removed
oxygen from Lead is added to the elemental Carbon and forms carbon dioxide.

Question 13: What is a balanced chemical equation? Why

should chemical equations be balanced?
Answer 13: A chemical equation represents a chemical reaction. The presentation of a
chemical reaction in which the number of atoms of each element is equal on the
reactant and product sides is known as a balanced chemical equation. Chemical
reactions should be balanced because only a balanced equation explains the relative
quantities of different reactants and products involved in the reaction.
Explanations: A balanced chemical reaction is an equation with equal numbers of each
type of atom on both sides of the arrow. A chemical equation is a written symbolic
representation of a chemical reaction. The reactant chemicals are given on the left side
and the product chemical(s) on the right side. The law of conservation mass states that
no atoms or ions can be created and destroyed in chemical reactions; therefore, the
number of atoms present in the reactants has to balance the number of atoms in the
products.
Question 14: Why is respiration considered an exothermic
reaction? Explain.
Answer 14: Respiration is the process of burning food in the living body to produce
energy. Respiration is considered an exothermic chemical reaction because glucose
oxidation occurs in the respiration process, which creates a large amount of heat energy
consumed in the form of ATP. During respiration, we inhale oxygen from the
atmosphere, which reacts with glucose in our body cells to produce carbon dioxide and
water. It is explained in the following chemical equation.
C6H12O6+ 6O2 → 6CO2 + H2O + energy (ATP)
Explanations: For the survival of life, we require energy. We consume this energy from
the food we eat. The food molecules, through digestion, are broken down into simpler
molecules like glucose. These molecular substances come in contact with the oxygen in
our body cells and produce Carbon dioxide and water along with a limited amount of
ATP (adenosine triphosphate)energy (Respiration process). Hence the energy is in the
form of heat (which maintains our body temperature); respiration is considered an
exothermic reaction.

Question 15: Write one equation each for decomposition

reactions in which energy is supplied in the form of heat, light
or electricity.
Answer 15:
(a) Thermal decomposition reaction (Thermolysis)
Decomposition of potassium chlorate: If heated strongly, potassium chlorate
decomposes into potassium chloride and oxygen molecules. This reaction is commonly
used for the synthesis of oxygen molecules.
2KClO3 + heat → 2KCl + 3O2
(b) Electrolytic decomposition reaction (Electrolysis)-
Decomposition of sodium chloride NaCl: On passing electricity through molten sodium
chloride NaCl, it decomposes into sodium and chlorine.
2NaCl—- 2Na + Cl2 (in Electrolysis)
(c) Photodecomposition reaction (Photolysis)
Decomposition of Hydrogen peroxide- In the presence of light, hydrogen peroxide
decomposes into water and oxygen molecules.
2H2O2 + light H2O + O2

Question 16: Why does the colour of copper sulphate solution

change when an iron nail is dipped in it?
Answer 16: The colour of the copper sulphate solution changes when an iron nail is
dipped in it because of the displacement of copper from the copper sulphate solution
and the formation of iron sulphate solution. The brown deposit is of copper. The
chemical reaction involved in this experiment is:
 Fe nail(s)+CuSO4(blue solution)→FeSO4(green)(aq) + Cu
Explanations: Iron displaces Cu from copper sulphate solution as iron is more reactive
than copper. Therefore this is a displacement reaction.

Question 17: Fe2O3 + 2Al → Al2O3 + 2Fe The above reaction

is an example of a
 Combination reaction.

 Double displacement reaction.

 Decomposition reaction.

 Displacement reaction.
Answer 17: Option is d Displacement reaction.
Explanation: The oxygen atom from ferrous oxide is displaced to the Aluminium metal
to form Aluminium Oxide. In this reaction, Aluminium is a more reactive metal atom
than Iron. Hence Al will displace Fe from its oxide. This type of chemical reaction in
which one of the elements substitutes another is called a displacement reaction. Here,
the less reactive metal is replaced by the more reactive metal. Since one-time
displacement occurs, it is called a single displacement reaction.

Question 18:Write the 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) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) +
Hydrogen(g)
Answer 18:
(a) 2KBr (aqueous) + Bal2(aq) → 2Kl(aq) + BaBr2(s)
Types- Double displacement reaction
(b) ZnCO3 (s) → ZnO (s) + CO2 (g)
Types- Decomposition reaction
(c) H2 (g) + Cl2 (g) → 2HCl(g)
Types – Combination or synthesis reaction
(d) Mg (s) + 2HCl (aqueous) → MgCl2 (aq) + H2 (g)
Types- Displacement reaction
 Question 19- Why are decomposition reactions called the opposite of
combination reactions? Write equations for decomposition reactions.
Answer 19: A combination reaction is said to be the reaction between two or more
molecules to form a larger molecule. A decomposition reaction is defined as splitting
large molecules into two or smaller molecules. It explains that the decomposition
reaction is the opposite of the combination reaction.
In most cases, the decomposition reaction is endothermic since the heat from the
surrounding or induced heat is used to diffuse the bonds of the larger molecule. Some
examples of decomposition reactions are
ZnCO3 → ZnO + CO2
CaCO3 + Energy → CaO + CO2
2HgO → 2Hg + O2
Explanations: In a decomposition reaction, a single substance breaks down into two or
more substances, while in a combination reaction, two or more substances react to
produce one substance. Therefore, decomposition reactions are called the opposite of
combination reactions.

Question 20– What is the difference between displacement

and double displacement reactions? Write relevant equations
for the above.
Answer 20: A displacement reaction occurs when a more reactive substance replaces
a less reactive substance from its salt solutions. A double displacement reaction occurs
when a mutual exchange of metal ions happens between 2 compounds.
In this displacement reaction, only a single displacement occurs, whereas in the double
displacement reaction, as the name suggests, two displacements occur between the
molecules.
Example:
Displacement reaction
Mg + 2HCl → MgCl2 + H2
Double displacement reaction
2KBr + BaI2 → 2KI + BaBr2
Explanations: A displacement reaction occurs if a more reactive substance replaces a
less reactive one from its salt solution. A double displacement reaction occurs when a
mutual exchange of metal ions happens between two compounds. In this reaction, only
a single displacement occurs, whereas in the double displacement reaction, as the
name explains, two displacements occur between the molecules.

Question 21: Why do we apply paint on iron articles?

Answer 21: Iron articles are painted to prevent them from rusting. If left unpainted, the
metal surface comes in contact with the atmospheric oxygen and, in the presence of
moisture, it forms (FeO)Iron(III) oxide. Therefore if painted, the surface does not come in
contact with moisture and air, thus preventing Rusting.
Explanations: Paint is always applied to the iron articles to prevent them from
corrosion by rust formation. Rust is an iron oxide, commonly red oxide, developed by
the redox reactions of iron Fe and oxygen O in the presence of water or atmospheric air
moisture. Paint always prevents iron from getting exposed to air and humidity.

Question 22: Explain the following topics with one example

each.
(a) Corrosion (b) Rancidity
Answer 22:
(a) Corrosion is a slow process where a refined metal atom is oxidised by atmospheric
oxygen to create a more stable compound, like oxides. The metal atom gradually
degrades during the corrosion process. Rusting of iron is an important example of
corrosion where the iron is converted to Iron oxide. Millions of pounds are spent
annually to prevent bridges and other monuments from rusting.
(b) Rancidity: The condition produced by the aerial oxidation of the oil and fat in the
food material has an unpleasant taste and odour. The rancidity is retarded if the food is
kept inside the refrigerator since the low temperature does not promote the oxidation
reaction.
Explanations: Corrosion is a reaction where a metal reacts with water, air or acid to
form oxides and carbonates. It is also known as rust. For example, black coating on
silver in the presence of air or atmosphere. Rancidity is the oxidation process of fats
and oils when kept in the open or in the presence of oxygen for a long time. Due to this,
changes in taste and odour of food can be observed. To prevent rancidity, food items
are flushed with nitrogen or kept in airtight containers. For example, the taste and smell
of butter change when held for a long time.

Question 23: In the refining of silver, the recovery of silver

from Silver nitrate solution involves displacement reaction by
Copper metal. Write down the reaction involved.
Answer 23:
So Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
Explanations: The silver nitrate is in the solution, and the metallic copper will dissolve
to form copper nitrate. The silver in the aqueous solution will be precipitated out as
metallic silver. The silver in the solution is exchanged for copper, and the copper not in
the solution is substituted for silver.

Question 24: Explain the following in terms of the gain of

oxygen with two examples each.
(a) Oxidation (b) Reduction
Answer 24:
(a) In a chemical reaction when oxygen is added to the element to form its respective
oxide, it is the element being oxidised. For Example:
4Na(s) + O2(g) → 2Na2O(s)
H2S + O2 → H2O + SO2
(b) In a chemical reaction, it is said to be reduced when oxygen is removed from the
compound. For Ex.,:
so CuO(s) + H2(g) → Cu(s) + H2O(l)
2HgO → 2Hg + O2(g)
Explanations: The process of adding oxygen or removing hydrogen in a chemical
reaction is called an oxidation reaction. The method of adding hydrogen or removing
oxygen in a chemical reaction is called a reduction reaction.