CBSE Important Questions Class 10 Science Chapter 1

Important Questions Class 10 Science Chapter 1 – Chemical Reactions and Equations

Science is a fascinating subject, but at the same time, it has some concepts which are difficult to comprehend. Thus, students will have to make extra efforts to understand the chapters across Biology, Physics & Chemistry. 

The first chapter of Class 10 Science is about ‘Chemical Reactions and Equations’. The chapter introduces students to various topics such as physical changes, chemical reactions, types of chemical reactions, chemical equilibrium and equations, etc. It’s an introductory chapter to the vast field of Chemistry that will expose students to the world of chemical reactions and chemical equations. 

For Science, students are advised to practice many questions to score good marks in exams. At Extramarks, we recognise the importance of solving questions, and we have collated questions from various sources, including NCERT textbooks, NCERT exemplars, other reference books, past years’ exam papers, etc. Step-by-step solutions are prepared by our Science subject teachers to make it easy for students to understand the concepts. Students can register on the Extramarks website and access our full solutions for Important Questions Class 10 Science Chapter 1.

Regularly practising and solving questions from our question set of Science Class 10 Chapter 1 Important Questions will boost the student’s confidence. By referring to the given solutions, students will also understand the answer writing skills better. 

Students must register on the Extramarks website and get access to our Important Questions Class 10 Science Chapter 1 question bank. Apart from the solutions to important questions, the Extramarks website has an abundance of study materials like NCERT Solutions, CBSE revision notes, past year question papers, NCERT books and much more.

CBSE Class 10 Science Important Questions
Sr No. Chapters
1 Chemical Reactions and Equations
2 Acids, Bases and Salts
3 Metals and Non-metals
4 Carbon and Its Compounds
5 Periodic Classification of Elements
6 Life Processes
7 Control and Coordination
8 How do Organisms Reproduce?
9 Heredity and Evolution
10 Light Reflection and Refraction
11 Human Eye and Colourful World
12 Electricity
13 Magnetic Effects of Electric Current
14 Sources of Energy
15 Our Environment
16 Management of Natural Resources

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  +   Cl +   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. i) Solutions of Barium chloride and Sodium sulphate in water react to give insoluble Barium sulphate and a solution of Sodium chloride.
  2. ii) Sodium hydroxide solution in water interacts with hydrochloric acid to produce Sodium chloride solution and water.

iii) Hydrogen gas combines with nitrogen to form ammonia.

  1. iv) potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer 11:


  1. i) BaCl2 + Na2SO4 → BaSO4 + 2NaCl
  2. ii) NaOH + HCl → NaCl + H2O

iii) 3H2 + N2 2NH3

  1. 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.

C6​H12​O6​+ 6O2​ → 6CO2​ + H2​O + 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.


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.

Question 25: Balance the following chemical equations properly.

  1. i) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
  2. ii) NaOH + H2SO4 → Na2SO4 + H2O

iii) NaCl + AgNO3 → AgCl + NaNO3

  1. iv) BaCl2 + H2SO4 → BaSO4 + HCl

Answer 25– Balance the chemical reaction is shown below:

  1. i) 2HNO3 + Ca(OH)2   → Ca(NO3)2 + 2H2O
  2. ii) 2NaOH + H2SO4   → Na2SO4 + 2H2O

iii) NaCl + AgNO3         → AgCl + NaNO3

  1. iv) BaCl2 + H2SO4   → BaSO4 + 2HCl

Question 26: A shiny brown-coloured element ‘X’ on heating in the air becomes black. Name the element ‘X’ & the black-coloured compound formed.

Answer 26:

The shiny brown-coloured element is Copper metal (Cu). If the metal is heated in air, it interacts with atmospheric oxygen to form copper oxide. Therefore, the black-coloured compound is copper oxide.

2Cu(s) + O2(g) → 2CuO(s)

Explanations: copper is an element which has a shiny brown colour appearance. So, x is copper. If copper is heated in air, it becomes black due to the formation of copper oxide. It is an oxidation reaction in which the copper gains oxygen to form copper oxide.

Question 27: The following chemical reaction is an example of a:

             4NH3 (g) + 5O2 (g) → 4NO(g) + 6H2O(l)

(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 27: (C)  (i) and (iii)

The given reaction undergoes displacement and neutralisation reactions.

Displacement reaction: The oxygen atom displaces hydrogen from ammonia to form nitric acid and water.

Redox reaction: Ammonia interacts with oxygen atoms to undergo an oxidation reaction, and oxygen combines with hydrogen to undergo a reduction reaction.

Explanation: The chemical reaction provided is a mixture of displacement and redox reactions. Oxygen replaces hydrogen in the ammonia, making it a displacement reaction. Nitrogen gets oxidised, and oxygen is reduced, resulting in a redox reaction.

Question 28:  Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen & oxygen gases evolved during the electrolysis of water is

(a) 1:1

(b) 2:1

(c) 4:1

(d) 1:2

Answer 28: Correct option is (b) 2:1


On electrolysis of water, the water dissociates to liberate hydrogen and oxygen gas.

2H2O → H2 + O2

Generally, 1 Mole of water has 2 moles of Hydrogen and 1 mole of oxygen. Hence the mole ratio between hydrogen and oxygen is 2:1.

Question 29- Which of the following gases can be utilised for storing fresh samples of oil for a long time? 

(a) Carbon dioxide or oxygen 

(b) Nitrogen or oxygen 

(c) Carbon dioxide or Helium 

(d) Helium or nitrogen 

Answer 29: Correct option is d. Helium or nitrogen 

Oxygen molecules cannot be used as it is an oxidising agent. Helium can be utilised as it is an inert or noble gas. Nitrogen is less reactive, and it is less expensive than Helium. In most cases, nitrogen is used in packets to prevent rancidity.

Explanations: Helium is a noble gas that does not react with fats and oil and protects the oil from its oxidation or rancidity. Nitrogen gas has a triple bond between two nitrogen atoms; due to a triple bond, nitrogen acts as a noble gas and does not react with fats and oils and their rancidity.

Question 30:  Which of the following processes involves chemical reactions?

(i) Storing of oxygen gas under pressure in a gas cylinder

(ii) Liquefaction of air

(iii) Keeping petrol in a china dish in the open

(iv) Heating copper wire in the +nce of air at a high-temperature

Answer 30: The correct answer is (iv) Heating copper wire in the presence of air at a high-temperature


In the first three options shown here, there is no involvement of a chemical reaction. If copper is heated in the presence of air at a high temperature, copper undergoes an oxidation reaction to give out copper oxide.

Question 31 : Which of the following is(are) double displacement reaction(s)?

(i) Pb + CuCl2 → PbCl2 + Cu

(ii) Na2SO4 + BaCl2 → BaSO4 + 2NaCl

(iii) C + O2 → CO2

(iv) CH4 + 2O2 → CO2 + 2H2O

(a) (i) and (iv)

(b) (ii) only

(c) (i) and (ii)

(d) (iii) and (iv)

Answer 31: The correct answer is (b) (ii) only

Sodium Na and Barium Ba are displaced from each other’s salts; hence, it is a double displacement reaction.

Explanations: In a double displacement reaction, one compound exchanges its ions with the ions of another compound to develop two new compounds. In sodium sulphate Na2SO4, Na+ ion combines with Cl ion of BaCl2 whereas Barium ion combines with  SO42-ion to create BaSO4.

Question 32: The following reaction is used for the preparation of oxygen gas in the laboratory

2 KClO3 (s)2 KCl (s) + 3O2 (g) 

Which of the following statements is exactly correct about the reaction?

(a) It is a decomposition reaction & endothermic

(b) It is a combination reaction

(c) It is a decomposition reaction & accompanied by the release of heat

(d) It is a photochemical decomposition reaction & exothermic

Answer 32: Correct option is (a) It is a decomposition reaction and endothermic


Potassium chlorate decomposes to give potassium chloride KCl and oxygen. It is a decomposition reaction which is endothermic. The shown reaction is a thermal decomposition reaction, as the KClO3 decomposes to KCl salts and O2 gas on heating.

The heat is passed to the reaction mixture in the above reaction, so it is also an endothermic reaction.

Question 33:  Solid calcium oxide interacts vigorously with water to form calcium hydroxide accompanied by the liberation of heat. This method 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 33: The correct option is (b) (ii) & (iii)

Explanations: When solid calcium oxide interacts vigorously with water, it forms calcium hydroxide, accompanied by heat generation. This effect proves the reaction is exothermic. The pH value of the solution will be more than seven because oxides and hydroxides of metals are alkaline.

Question 34: In the double displacement reaction between aqueous potassium iodide & aqueous lead nitrate, a yellow precipitate of lead iodide is developed. While performing the activity, when lead nitrate is unavailable, 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 34: The correct option is (b) Lead acetate

Explanations: To get lead iodide, we need a compound containing Lead because Ammonium nitrate and Potassium sulphate are ruled out. Lead sulphate is insoluble because it cannot be used, so the answer is (b) Lead acetate.

Question 35: Which of the following is not a physical change?

(i) Boiling of water to give water vapour

(ii) Melting of ice to give water

(iii) Dissolution of salt in water

(iv) Combustion of Liquefied Petroleum Gas (LPG)

Answer 35: Correct option is (iv) Combustion of Liquefied Petroleum Gas (LPG)

Explanation: The combustion process is always a chemical change because a new compound is formed after burning and is irreversible.

Question 36:  Complete the missing components/variables given as x and y in the following reactions-

(a) Pb(NO3 )2 (aq) + 2KI(aqueous) → PbI2 (x) + 2KNO3 (y)

(b) Cu(solid) + 2AgNO3 (aqueous)        → Cu(NO3)2 (aq) + x(s)

(c) Zn(s) + H2SO4 (aqueous)            → ZnSO4(x) + H2(y)

(d) CaCO3 (solid)                          →           x CaO(s) + CO2(g)

Answer 36:

(a) Pb(NO3 )2 (aq) + 2KI(aqueous) → PbI2 (s) + 2KNO3 (aq)

(b) Cu(s) + 2AgNO3 (aqueous) → Cu(NO3)2 (aqueous) + 2Ag(s)

(c) Zn(s) + H2SO4 (aqueous) → ZnSO4(aq) + H2(g)

(d) CaCO3(s) → heat → CaO(solid) + CO2(g)


  1. x(s), y (aq)
  2. x is 2Ag
  3. x-(aq) y(g)
  4. x is heat
  1. When lead nitrate is mixed with aqueous potassium iodide, it undergoes a precipitation reaction. A yellow precipitate of lead iodide occurred in an aqueous solution of potassium nitrate due to the displacement of nitrate by iodine.
  2. Copper metal is highly reactive than silver and replaces silver from silver nitrate aqueous solution, forming copper nitrate aqueous solution and silver metal.
  3. When zinc metal is combined with sulphuric acid, hydrogen gas is evolved, and the aqueous solution of zinc sulphate is obtained.
  4. Calcium carbonate undergoes a decomposition reaction on heating to form calcium oxide and carbon dioxide gas.

Question 37:  Identify the reducing agent in the following reactions

(a) 4NH3 + 5O2 → 4NO + 6H2O

(b) H2O + F2 → HF + HOF

(c) Fe2O3 + 3CO → 2Fe + 3CO2

(d) 2H2 + O2 → 2H2

Answer 37: Reducing agents are

  1. NH3– Ammonia
  2. H2O – Water
  3. CO – Carbon monoxide
  4. H2 – Hydrogen


  1. Ammonia molecule reduces the oxygen to water and the oxidation of oxygen changes from 0 to -2 state.
  2. Water (H2O) molecule reduces the fluorine to HF, and the oxidation state changes from 0 to -1.
  3. Carbon monoxide (CO) reduces iron (III) oxide to iron metal as the oxidation of iron (III) oxide changes from +3 to 0.
  4. Hydrogen (H2) gas reduces the oxygen to water and behaves as a reducing agent, changing the oxidation way of oxygen from 0 to -2.

Question 38: 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 hydrogen carbonate.

(b) Sodium hydrogen carbonate on reaction with hydrochloric acid gives sodium chloride and water and escapes carbon dioxide.

(c) Copper sulphate interacts with potassium iodide, precipitates cuprous iodide (Cu2I2 ), liberates iodine gas, and forms potassium sulphate.

Answer 38 Solutions are:

(a) Na2CO3 + HCl → NaCl + NaHCO3

(b) NaHCO3 + HCl → NaCl + H2O + CO2

(c) 2CuSO4 + 4Kl → 2K2SO4 + CU2I2 + I2

 Question 39:  Ferrous sulphate compound decomposes with the evolution of a gas having a characteristic smell of burning sulphur. Write the chemical reaction and identify the various type of reaction.

Answer 39: The chemical reactions are 2FeSO4 + heat → Fe2O3 + SO2+ SO3

It is a decomposition reaction.

Explanations: Ferrous sulphate undergoes a decomposition reaction on heating to form a ferric oxide and release sulphur dioxide gas & sulphur trioxide gas, which have the characteristic odour of burning sulphur.

The given decomposition reaction is a thermal decomposition reaction as well as an endothermic reaction.

Question 40: 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 physical as well as a chemical change? 

Answer 40– When hanging on the plant, grapes do not ferment because of their active immune system. After grapes are plucked from the plant, they ferment as microbes start acting on the sugar +nt in grapes and cause fermentation. Because of the fermentation, the sugar in grapes is changed into ethanol & carbon dioxide. As the chemical constituents of sugar in grapes changes, it is a chemical change.

Explanations: Grapes on the plant do not ferment because of the defence chemical mechanism of plants. If grapes are plucked from the plant, grapes interact with yeast to carry out fermentation. And sugar changes to alcohol, and it is a chemical change.

Question 41: Balance the following chemical equations and identify the type of chemical reaction.

(a) Mg(solid) + Cl2 (g) → MgCl2 (s)

(b) HgO(solid) → heat Hg(l) + O2 (g)

(c) Na(solid) + S(s) → Fuse Na2S(s)

(d) TiCl4 (l) + Mg(solid) → Ti(s) + MgCl2 (s)

(e) CaO(solid) + SiO2 (s) → CaSiO3 (s)

(f) H2O2 (liquid) → U V H2O(l) + O2 (g)

Answer 41: Solutions are shown below:

(a) Mg(solid) + Cl2(g) → MgCl2(s)

This type of reaction is a combination or synthesis reaction 

(b) 2HgO(s) — (Heat) → 2 Hg(I) + O2(g)

It is the best example of a thermal decomposition reaction.

(c) 2Na(solid) + S(s) — (Fuse) → Na2S(s)

It is the best example of a Combination reaction.

(d) TiCI4(l) + Mg(s) → Ti(s) + 2MgCl2 (solid)

This reaction falls under the class of Displacement reactions

(e) CaO(s) + SiO2(s) + CaSiO3(s)

It is a  combination and synthesis reaction.

(f) 2H2O2(I) + UV → 2H2O(I) + O2 (g)

and this is a photodecomposition reaction.

Question 42: A substance X, an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with a water solution, it forms a solution which turns red litmus to blue. Identify X and further write the chemical reactions involved.

Answer 42: Compound X is Calcium oxide. CaO is extensively utilised in the cement industry. On treatment with water, CaO produces calcium hydroxide Ca(OH)2, which is alkaline and turns red litmus into blue colour.

CaO(s) +H2O(l)→ Ca(OH)2(aq)

Explanations: Calcium is an element of group 2 and is present here in bones, also. Calcium oxide, generally known as quicklime, is used intensively in the cement industry. Calcium, on treatment with water, forms alkaline calcium hydroxide, which turns the red litmus to blue.

Question 43: Why do fireflies glow at night?

Answer 43: Fireflies produce a chemical reaction inside their bodies, allowing them to glow at night. Oxygen interacts with calcium, ATP and luciferin in the presence of an enzyme called luciferase. This results in bioluminescence.

Explanations: Fireflies glow at night due to chemical reactions inside their body to release light in the form of energy. Fireflies consume a luciferin protein, which is an organic compound. The oxygen interacts with calcium, adenosine triphosphate (ATP) and the chemical luciferin in the presence of luciferase, a bioluminescent enzyme, to create a new substance called oxyluciferin along with the evolution of energy in the form of light. This type of light production is termed bioluminescence.

Question 44: A silver article commonly turns black when kept in the open for a few days. The article, if rubbed with toothpaste, again starts shining. (a) Why do silver articles turn black when retained in the open for a few days? Name the phenomenon involved. (b) Name the black substance developed and give its chemical formula.

Answer 44: 

  1. Silver interacts with H2S present in the atmosphere to develop a black colour compound Silver Sulphide. This phenomenon is known as corrosion.
  2. The black colour compound developed is Silver Sulphide.

                 2Ag(solid)+ H2S(g) → Ag2S(s) + H2(g)

Explanations: a) Silver metal articles, when kept in the open air for a few days, interact with sulphur compounds present in the atmospheric oxygen and convert to black due to the formation of  Ag2S. This phenomenon is called corrosion.

                     2Ag(solid)    +      H2S(g)             →               Ag2S(s)         +          H2(g)

                     Silver      hydrogen sulphide              silver sulphide            hydrogen gas

Toothpaste contains calcium carbonate and aluminium hydroxide that can remove the black layer of silver sulphide, and silver shines again.

                          3Ag2S      +     2Al      →   6 Ag        +    Al2S3

            Silver Sulphide   Aluminium       Silver metal   Aluminium sulphide                

  1. b) Name the black substance developed and give its chemical formula. 

Ans: The black substance developed is silver sulphide and has a chemical formula of Ag2S.

Question 45:  Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?

Answer 45: Zinc is highly reactive than copper as Zinc is placed above Hydrogen & Copper is positioned below Hydrogen in the activity series of metals. Hence Zinc reacts with HCl, whereas copper will not react.

Explanations: Zinc metal is placed above hydrogen in the reactivity series and is more reactive to replace hydrogen from dilute hydrochloric acid and liberate hydrogen gas. 

                                Zn(s)     +     HCl(aq)           →                ZnCl2 (aq)      +             H2(g)

                                 Zinc      hydrogen chloride                  Zinc chloride            Hydrogen gas

Copper metal is positioned below hydrogen in the reactivity series, less reactive to displace hydrogen from dilute hydrochloric acid, and no reaction occurs. 

                                Cu(s)     +     HCl(aq)    →    No reaction

Question 46: On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas & a brown gas X are formed

(a) Write a balanced chemical equation of this reaction.

(b) Identify the brown gas (X) that evolved.

(c) Identify the reaction types.

(d) What could be the pH value range of the aqueous solution of the gas X?

Answer 46:

(a)        2 Cu(NO3)2(s)    →     2 CuO(s)     +    4 NO2                   +         O2   

      Copper Nitrate          Copper Oxide      Nitrogen Dioxide         oxygen gas

           (blue)                          (black)                     (brown)(X)

(b) The brown gas X is  NO2 nitrogen dioxide.

(c) The reaction type is Thermal decomposition.

(d) pH< 7, so NO2 dissolves in water to form an acidic solution (pH range below 7).

Explanations: a) The copper (II) nitrate decomposes on heating to synthesise black copper oxide, and oxygen gas is liberated along with brown nitrogen dioxide gas.

           2 Cu(NO3)2(s)  →  2 CuO(s)     +    4 NO2   +     O2   

      Copper Nitrate   Copper Oxide      Nitrogen Dioxide 

            (blue)                   (black)              (brown)(X)

(b) Identify the brown gas X evolved. 

Ans: The brown gas X evolved is nitrogen dioxide (NO2).

(c) The type of reaction is thermal decomposition as the single reactant decomposes on heating to give three products.

(d) The pH of the oxides of non-metal is acidic; nitrogen is a non-metal, so nitrogen dioxide gas, which is brown gas  X, is acidic, and the range of the aqueous solution of nitrogen dioxide is 0-7.

Question 47:  What happens when a piece of-

  1. a) Zinc metal added to copper sulphate solution?
  2. b) Aluminium metal is combined to dilute hydrochloric acid?
  3. c) silver metal is combined with copper sulphate solution?

Also, write the balanced chemical equation when the reaction occurs

Answer 47:

  1. a) If Zinc is combined with copper sulphate solution, Zinc replaces copper to form Zinc sulphate.

Zn(s) + CuSO4(aqueous)→ ZnSO4(aq) + Cu(s)

b)Aluminium metals interact with dilute HCl to form Aluminium chloride, and Hydrogen gas is evolved in this reaction.

2Al (s)+ 6HCl(aqueous)→ 2AlCl3(aq) + 3 H2

c)When silver metal is added to the Copper Sulphate solution, there will not be any reaction as silver is non-reactive metal.

Explanations: a) When zinc metal is added to blue copper sulphate solution, Zinc is highly reactive than copper (as Zinc is positioned above copper in the reactivity series). Therefore, it displaces copper metal from copper sulphate to form a colourless zinc sulphate solution and reddish brown copper metal steady down at the bottom surface.

  1. b) If aluminium metal is added to dilute hydrochloric acid, an aqueous solution of aluminium chloride is formed, and hydrogen gas is evolved.
  2. c) Silver is a less reactive metal than copper, so it will not displace copper from copper sulphate solution; therefore, no reaction occurs when silver is mixed with copper sulphate solution.

     Ag(s)        +       CuSO4(aq)        →   No Reaction

     Silver            Copper Sulphate                

Question 48:  You are provided with two containers of copper and Aluminium. You are also given dil HCl, dil HNO3, ZnCl2 and H2O solution. In which of the above containers solutions can be kept?

Answer 48: All these solutions can be kept in a copper container as copper is a noble metal which will not interact with HCl or not even HNO3. If we keep the aqueous solution in an aluminium container, Aluminium reacts with acids to form Zinc chloride. Water can be kept in either copper or aluminium containers as they do not react with copper.

Explanations: Copper is a less reactive metal placed below hydrogen and zinc metal in the reactivity series and hence cannot replace hydrogen from acids and water as well as Zinc from zinc chloride. Therefore, all the dilute HCl, HNO3, ZnCl2 and H2O solutions can be kept in copper vessels.

Aluminium is a reactive metal and placed above hydrogen in the reactivity series; hence reacts with the following:

  1. a) with dilute hydrochloric acid HCl:

      2 Al(s)        +      6  HCl(aq)           →                    AlCl3(aq)    +     H2(g)  

 Aluminium            Hydrochloric Acid                  Aluminium Chloride  

Aluminium interacts with dil HCl to develop aluminium chloride solution, and hydrogen gas is evolved. Therefore, dilute HCl solution cannot be stored in aluminium vessels.

(ii)With dilute nitric acid HNO3:

Aluminium interacts with a dilute solution of nitric acid to develop the protective layer of aluminium oxide (Al2O3), which prevents the further reaction of an acid with aluminium vessels; hence, dilute nitric acid solution can be stored in the aluminium vessel.

(iii) With zinc chloride ZnCl2 

      2 Al(s)        +      3 ZnCl2(aq)          →                   2 AlCl3(aq)    +     3 Zn(s)  

  Aluminium             Zinc chloride                      Aluminium Chloride   Zinc

Aluminium is a more reactive metal than Zinc (Aluminium is placed above Zinc in the reactivity series) and displaces Zinc from zinc chloride solution to form aluminium chloride and zinc metal. Hence Zinc chloride solution cannot be stored in aluminium vessels.

(iv) With Water H2O: 

Aluminium metal reacts with cold water but can react with steam to form a protective layer of aluminium oxide (Al2O3) which prevents further reaction with water; hence, water can be stored in aluminium vessels.

Question 49:  Give the characteristic tests for the following gases

(a) CO2

(b) SO2

(c) O2

(d) H2

Answer 49:

  1. Pass CO2 into limewater which will turn water into milky. It is the confirmation test for the presence of Carbon-dioxide.
  2. The smell is the characteristic feature of SO2, which smells like a rotten egg.
  3. Testing for oxygen involves burning matchstick near oxygen makes it burn even more brightly.
  4. When a burning matchstick is brought near H2 gas, the flame burns with a pop sound. It is the test to confirm Hydrogen gas.


(a) CO2: Carbon dioxide gas represents the characteristic of turning limewater milky due to the formation of an insoluble precipitate of calcium carbonate. 

                    Ca(OH)2(aqueous)    +     CO2(g)      →          CaCO3(s)    +     H2O(g)

                 Lime Water           Carbon Dioxide        Calcium Carbonate  

(b) SO2: Sulphur dioxide gas turns the purple colour acidic solution of potassium permanganate colourless. The SO2 acts as a reducing agent and forms colourless potassium sulphate and colourless manganese sulphate.

2 KMnO4(aq)   + 2 H2O(l)  + 5 SO2(g)  →  K2SO4(aq)    +   2 H2SO4   +     2MnSO4 

Potassium Permanganate                    Potassium Sulphate             Manganese sulphate

     (Purple)                                                       (Colourless)                                (Colourless)

(c) O2: When a matchstick is brought near the oxygen gas, it burns with more intensity and with bright flames as the oxygen gas supports the burning.

(d) H2: When a matchstick is brought near the hydrogen gas, it burns with a ‘popup’ sound. 

Question 50: On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, a white ppt is obtained.

(i) Write balanced chemical equations of the reaction involved?

(ii) What other name can be provided for this precipitation reaction?

(iii) On combined dilute hydrochloric acid HCl to the reaction mixture, white ppt disappears. Why?

Answer 50:

i)On mixing a drop of Barium Chloride solution with an aqueous solution of sodium sulphite, barium sulphite is formed, which is a white colour precipitate.

            BaCl2 + Na2SO3——>BaSO3 + 2NaCl

  1. ii) In this case, the precipitation reaction is a double displacement.

iii) When we add dilute HCl to this reaction mixture, Barium chloride, Sulphur dioxide and water are formed. Barium chloride is a soluble substance which will make the white precipitate disappear.

         BaSO3 + HCl→ BaCl3 + SO2 + H2O.

Benefits of Solving Important Questions Class 10 Science Chapter 1

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Q.1 [7210740]


Q.2 [ 7212138 ]


Q.3 [ 7210681 ]


Q.4 The reaction shown in the given figure is an example of

(a) combination reaction

(b) displacement reaction

(c) oxidation reaction

(d) neutralisation reaction


In this reaction, iron displaces copper from its salt (copper sulphate). Therefore, it is a displacement reaction.

Q.5 The balanced chemical equation that represents the formation of barium sulphate from barium chloride solution and aluminum sulphate solution is

(a) BaCl2l+Al2SO43lBaSO4s+AlCl3l

(b) BaCl2 + Al2(SO4)3 BaSO4 + AlCl3

(c) 3BaCl2(aq)+Al2(SO4)3(aq)3BaSO



The chemical equation represented in option (c) correctly represents the reaction as it clearly indicates the physical states of all reactants and products and is also balanced.

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  • Chapter 1 Chemical Reactions and Equations
  • Chapter 2 Acids, Bases and Salts
  • Chapter 3 Metals and Non-metals
  • Chapter 4 Carbon and its Compounds
  • Chapter 5 Periodic Classification of Elements
  • Chapter 6 Life Processes
  • Chapter 7 Control and Coordination
  • Chapter 8 How do Organisms Reproduce?
  • Chapter 9 Heredity and Evolution
  • Chapter 10 Light Reflection and Refraction
  • Chapter 11 The Human Eye and The Colorful World
  • Chapter 12 Electricity
  • Chapter 13 Magnetic Effects of Electric Current
  • Chapter 14 Sources of Energy
  • Chapter 15 Our Environment

Chapter 16 Sustainable Management of Natural Resources