Worksheets Chapter 1 Chemical Reactions and Equations Class 10 Science

Students should refer to Worksheets Class 10 Science Chemical Reactions and Equations Chapter 1 provided below with important questions and answers. These important questions with solutions for Chapter 1 Chemical Reactions and Equations have been prepared by expert teachers for Class 10 Science based on the expected pattern of questions in the class 10 exams. We have provided Worksheets for Class 10 Science for all chapters on our website. You should carefully learn all the important examinations questions provided below as they will help you to get better marks in your class tests and exams.

Chemical Reactions and Equations Worksheets Class 10 Science

The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.

Question. Which of the statements about the reaction below are incorrect?

(i) Lead is getting reduced.
(ii) Carbon dioxide is getting oxidised.
(iii) Carbon is getting oxidised.
(iv) Lead oxide is getting reduced.
(a) (i) and (ii)
(b) (i) and (iii)
(c) (i), (ii) and (iii)
(d) all 

Question. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct
(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.

Question. Which of the following is not a physical change?
(a) Boiling of water to give water vapour
(b) Melting of ice to give water
(c) Dissolution of salt water
(d) Combustion of liquefied Petroleum Gas (LPG) 

Question. The following reaction is used for preparation of oxygen gas in the laboratory
                Heat
2KClO₃(s)  →    2KCl (s) +3 O₂ (g)
Catalyst
Which of the following statement(s) is (are) correct about the reaction?
(a)It is a decomposition reaction and endothermic in nature.
(b) it is a combination reaction
(c)It is a decompostion reaction and is accompanied by release of heat.
(d) It is a photo chemical decomposition reaction and exothermic in nature. 

Question. Which among the following is following is (are) double displacement reaction(s)?
(i) Pb +CuCl₂ PbCl₂ + Cu
(ii) Na₂SO4 + BaCl₂ BaSO₄ + 2NaCl
(iii) C + O₂ CO₂
(iv) Zn+ 2HCl ZnCl₂ + H₂
(a) (i) and (iv)
(b) (ii) only
(c) (i) and (ii)
(d) (iii) and (iv) 

Question. Chemically the rust is
(a) Ferric sulphate
(c) ferric oxide
(b) Hydrated ferrous oxide
(d) hydrated ferric oxide 

Question. Which of the following observations help(s) us to determine that a chemical change has taken place?
(a) Change in temperature
(c) Change in colour
(b) Evolution of gas
(d) All of these 

Question. Assertion (A)- Calcium Carbonate when heated gives calcium oxide and water
Reason (R) – on heating CaCO₃, decomposition reaction takes place.
(a) Both A and R are true and R is correct explanation of A
(b) Both A and R are true and R is not correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Question. Assertion (A) – White silver chloride turns grey in sunlight.
Reason (R) – Decomposition of silver chloride in presence of sunlight takes place to form silver metal and chlorine gas.
(a) Both A and R are true and R is correct explanation of A
(b) Both A and R are true and R is not correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Very Short Answer Questions

Question. Which one is a chemical change: Rusting of iron or melting of iron?
Answer : Rusting of iron is a chemical change.

Question. State one basic difference between a physical change and a chemical change.
Answer : In a physical change, no new substance is formed.
In a chemical change, new substance(s) with new properties is/are formed.

Question. Identify the type of reaction in the following example:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Answer : Displacement reaction.

Question. Identify the type of reaction in the following example:
2H2(g) + O₂(g) → 2H₂O(l)
Answer :  Combination reaction.

Question. Name the oxidising and reducing agent in the following reaction:
CuO + H₂ → Cu + H₂O
Answer : CuO is oxidising agent H₂ is reducing agent.

Question. Identify in the following reaction:
2PbO  +  C → 2Pb + CO
(a) the substance oxidised and
(b) The substance reduced.
Answer: (a) Carbon is oxidized to CO.
(b)PbO is getting reduced to Pb.

Question. A shiny brown coloured element “x” on heating in air becomes black incolour . Name the element “x” and the black coloured compound formed.
Answer: Element ‘x’is Copper and the black coloured compound is cupric oxide Cuo

Question. What is meant by a chemical reaction?
Answer : Chemical reaction, a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.

Question. Which one of the following is a chemical change? Give reason also.
(a) Burning of wax (b) Melting of wax
Answer :Burning of gas is a chemical change because new products with new properties will be formed on burning.

Question. AgNO₃(aq) + NaCl(aq) → AgCl . + NaNO₃(aq) FeS + H₂SO₄ → FeSO₄ + H₂S –
Consider the above mentioned two chemical equations with two different kinds of arrows (- and . ) along with the product. What do these two different arrows indicate?
Answer : – shows evolution of gas, . represents a precipitate.

Question. Write a balanced chemical equation to represent the following reaction: carbon monoxide reacts with hydrogen gas at 340 atm to form methyl alcohol.
Answer : CO(g) + 2H₂(g) →340 atm CH₃OH(l)

Question. State the main difference between endothermic reaction and an exothermic reaction.
Answer : In endothermic reaction, heat is absorbed. In exothermic reaction, heat is evolved.

Question. Balance the following chemical equation:
Fe(s) + H₂O(g) → Fe₃O₄(s) + H2(g)
Answer : 3Fe(s) + 4H₂O(g) → Fe3O₄(s) + 4H₂(g)

Question. Which one is a chemical change: fermentation of fruit juice or diluting fruit juice?
Answer : Fermentation of fruit juice is a chemical change.

Short Answer Questions

Question. Write any two observations in an activity which may suggest that a chemical reaction has taken place. Give an example in support of your answer.
Answer : 
a. Change in colour:
AgNO (aq) + KI (aq) → AgI (s) + KNO (aq)
                              (Yellow ppt)
b. Evolution of gas:
Zn(s) +  H₂SO₄(aq) → ZnSO4 (aq) + H2(g)

Question. When the powder of common metal is heated in open china dish its colour turns black. However, when hydrogen is passed over the hot black substance so formed, it regains its original colour. Based on the above information answer the following questions:
a. What type of chemical reaction takes place in each of the two given steps?
b. Name the metal initially taken in powdered form.
Write balanced equations for both the reactions.
Answer : 
a. Oxidation reaction, Redox reaction
b. Copper metal was present initially

2Cu(s) + O₂ (g) →Heat CuO(s)
                                  (Black)
CuO(s) H2 (g) →Heat Cu(s) + H₂O(l)

Question. Why do we store silver chloride in dark coloured bottle? Explain in brief.
Answer :
It is done so as to cut off the exposure to sunlight.
AgCl is photosensitive, it will decompose to Ag and Cl₂ in the presence of sunlight.

2AgCl(s) →sunlight 2Ag(s) + Cl₂( g)

Question. “Oxidation and reduction processes occur simultaneously”. Justify this statement with the help of example.
Answer : 
Oxidation involves loss of electrons or addition of oxygen. Whereas reduction involves gam of electrons or addition of hydrogen e.g., 

H₂ is getting oxidised to H₂O, CuO is getting reduced to Cu.
It shows oxidation and reduction occur simultaneously.

Question. What is an oxidation reaction? Give an example of oxidation reaction. Is oxidation an exothermic or an endothermic reaction?
Answer : Oxidation reaction: The reaction in which O₂ is added or H₂ is removed or loss of electrons takes place is mcalled oxidation reaction. E.g.,
2Cu(s) + O₂(g) → 2CuO(s)
It is an exothermic reaction.

Question. What is a precipitationreaction? Give an example.
Ans. Reaction in which an insoluble substance or precipitate is formed
Na₂SO₄ + BaCl₂ ——–> BaSO₄  + 2NaCl

Question. Give an example, each for thermal decomposition and photochemical decomposition reactions. Write balanced equation for the same.
Answer:Thermal decomposition – Heating of lime stone.
CaCO₃→CaO +CO₂
Photochemical decomposition – Action of light on silver bromide.
2AgBr→2Ag +Br₂

Question. Classify the following reaction as combination,decomposition,displacementand double displacement reaction:-
(a) BaCl₂ +H₂SO₄ → BaSO₄ +2HCl
(b) 3CuSO₄+ 2Al→ Al₂ (SO₄)₃ +3Cu
(c) ZnCO₃ →ZnO +CO₂
(d) C +O₂→CO₂
Answer: (a) Double displacement reaction.
(b) Displacement reaction.
(c) Decomposition reaction
(d) Combination reaction

Question. Write chemical equations for the reactions taking place when
a. Iron reacts with steam
b. Magnesium reacts with dil. HCl
c. Copper is heated in air
Answer : 
a. 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
b. Mg(s) + 2HCl(dil) → MgCl₂(aq)
c. 2Cu(s) + O₂(g) → 2CuO(s)

Question. Define the term decomposition reaction. Give one example each of thermal decomposition and electrolytic decomposition reactions.
Answer : 
Decomposition reaction: The reaction in which a compound is broken down into simpler elements or compounds:
a. CaCO₃(s) →Heat CaO(s) + CO₂(g)
b. 2H₂O(l) →Electricity 2H2(g) + O₂(g)

Question. Name two salts that are used in black and white photography. Give equations for the reactions when these are exposed to sunlight.
Answer : 
AgBr (Silver bromide) and AgI (Silver iodide) are used in black and white photography.
2AgBr(s) →sunlight 2Ag(s) + Br₂(g)
2AgI(s) →sunlight 2Ag(s) + I₂(g)

Question. State one example each characterised by following along with suitable chemical equation.
a. Change in state,
b. Evolution of gas,
c. Change in temperature.
Answer : 
a. Change in state:
AgNO₃(aq) + HCl(aq) → AgCl(s) + HNO₃(aq)
b. Evolution of gas:

CaCO₃ s 2HCl dil → CaCl2 aq H₂O + CO₂ (g)
c. Change in temperature:
d. CH4(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) + Heat

Question. Name the type of reactions represented by the following equations:
a. CaO + H₂O → Ca(OH)₂
b. 3BaCl₂ + Al₂(SO₄) → 3BaSO4 + 2AlCl₃
c. 2FeSO₄ →Heat Fe₂O₃ + SO₂ + SO₃
Answer : 
a. Combination reaction,
b. Double displacement reaction,
c. Decomposition reaction.

Question. Write the balanced chemical equation for the following reaction and identify the type of reaction.
Thermite reaction, iron (III) oxide reacts with aluminium and give molten iron and aluminium oxide.
Answer : 
Fe₂O₃(s) + 2Al(s) → Al₂O₃(s) + 2Fe(l)
It is displacement as well as redox reaction.

Question. Write the chemical equation of the reaction in which the following changes take place with an example of  each.
a. Change in colour,
b. Change in tem-perature,
c. Formation of precipitate.
Answer : 
a. Change in colour:
Zn(s) FeSO₄ (aq) → ZnSO4 (aq) + Fe(s)
       (Pale green) (Colourless)

b. Change in Temperature:
2Mg + O₂(g) → 2MgO(s) + Heat
c. Formation of precipitate:
Pb(NO₃)2 (aq) + 2HCl (dil) → Pbcl (s) + 2HNO₃ (aq)
                                 White ppt

Question. Decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Write an equation each for decomposition reactions where energy is supplied in the form of heat,
light and electricity.
Answer : 
a. CaCO₃(s) →Heat CaO(s) + CO₂(g)
b. 2AgBr(s) →sunlight 2Ag(s) + Br2(g)
c. 2H₂O(l) →Electricity 2H2(g) + O₂(g)

Question. State the type of chemical reactions and chemical equations that take place in the following:
a. Magnesium wire is burnt in air.
b. Electric current is passed through water.
c. Ammonia and hydrogen chloride gases are mixed.
Answer : 
a. 2Mg(s) + O₂(g) → 2MgO(s);
b.                                   Oxidation reaction
c. 2H₂O(l) → 2H2(g) + O₂(g);
                                     Decomposition reaction
d. NH3(g) + HCl(g) → NH4Cl(s);
                                     Combination reaction

Question. Write the 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 : a. H₂(g) + Cl₂(g) → 2HCl(g)
b. Pb(s) + CuCl₂(aq) → PbCl₂(s) + Cu(s)
c. ZnO(s) + C(s) → Zn(s) + CO(g)

Question. In a schematic diagram for the preparation of hydrogen gas as shown in the figure. What would happen if the following changes are made  (Img 8)

a. In place of zinc granules, same amount of zinc dust is taken in the test tube?
b. Instead of dilute sulphuric, dilute hydrochloric acid is taken?
c. Sodium hydroxide is taken in place of dilute sulphuric acid and the flask is heated?
Answer :
a. Zinc dust will react faster, H2(g) will be liberated at a faster rate.
b. Same volume of H2(g) will be formed.
c. On heating Zn with NaOH, hydrogen gas will be formed at a faster rate.

Question. (a) Why is it necessary to balance a chemical equation?
(b) Write the balanced chemical equation for the following reactions:
(i) Natural gas burns in air to form carbon dioxide and water.
(ii) During respiration, glucose combines with oxygen and forms carbon dioxide and water along with the release of energy.

Answer : 
a. Chemical equation must be balanced so as to follow the law of conservation of mass.
b. (i) CH4(g) + 2O2(g) → CO₂(g) + 2H₂O(l)
(ii) C₆H₁₂O₆(s) + 6O2(g) → 6CO₂(g) + 6H₂O(l) + Heat 

Question. (i) Solid calcium oxide was taken in a container and water was added slowly to it.
(a) Write the observations.
(b) Write the chemical formula of the product formed.
(ii) What happens when carbon dioxide is bubbled through lime water (a) in small amount (b) in excess?
Answer : 
(i) (a) The container becomes hot and hissing sound is produced.
(b) Ca(OH)₂ is the formula of the product formed.
(ii) (a) Lime water turn milky when CO₂(g) is passed through it:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(b) If excess of CO₂ is passed milkiness disappears:
CaCO₃(s) + CO₂(g) + H₂O(s) → Ca(HCO3)2(aq)

Question. An aqueous solution of metal nitrate ‘P’ reacts with sodium bromide solution to form yellow precipitate ‘Q’ which is used in photography. ‘Q’ on exposure to sunlight undergoes decomposition to form metal present along with a reddish brown gas. Identify ‘P’ and ‘Q’ write the balanced chemical equation for the chemical reaction. List the two categories in which
reaction can be placed.
Answer :   ‘P’ is silver nitrate (AgNO₃)
AgNO₃(aq) + NaBr(ag) → AgBr(s) + NaNO₃(aq)
  (P)                            (Yellow ppt)
This reaction is categorised as double displacement as well as precipitation reaction.
2AgBr(s) → sunlight 2Ag(s) + Br2(g)
The above reaction is photochemical decomposition reaction.

Question. A green coloured hydrated metallic salt on heating loses water of crystallisation molecules and gives a gas with suffocating smell. Identify the salt and write the chemical equation for the reaction.
Answer :
The salt is FeSO₄ 7H₂O → (Hydrated ferrous sulphate)
FeSO 7H₂O →Heat FeSO₄ (s) + 7H₂O(l) 
2FeSO₄ (s) →Heat Fe₂O₃ (s) SO₂ (g) + SO₃ (g)

Question. a. Can combination reaction be an oxidation reaction?
b. How will you test whether the gas evolved in a reaction is hydrogen?
c. Why does copper not evolve hydrogen on reacting with dilute sulphuric acid?
Answer : 
a. Yes, combination reaction can be called a oxidation reaction.
b. Bring a burning splinter near the gas, if it burns with pop sound, it is hydrogen gas.
c. It is because copper is less reactive than hydrogen.

Question. Write balanced equations for the following reactions:
a. Aluminium + Bromine → Aluminium bromide
b. Calcium carbonate Heat
Calcium oxide + Carbon dioxide
c. Silver chloride sunlight Silver + Chlorine
Answer : 
a. 2Al(s) + 3Br₂(g) → 2AlBr₃(s)
b. CaCO₃(s) →Heat CaO(s) + CO₂(g)
c. 2AgCl(s) →sunlight 2Ag(s) + Cl2(g)

Question. a. Why is respiration considered an exothermic reaction?
b. Define the terms oxidation and reduction,
c. Identify the substance that is oxidised and reduced in the following reaction:

CuO(s) + Zn(s) →heat Cu(s) + ZnO(s)

Answer : 
a. In respiration, energy is released, therefore, it is considered an exothermic reaction.
b. Oxidation involves addition of O₂ or loss of electrons. Reduction involves addition of H2 or gain of electrons.
c. Zn is getting oxidised, CuO is getting reduced.

Question. Write balanced chemical equations for the following reactions:
a. Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
b. Barium chloride reacts with zinc sulphate to give zinc chloride and barium sulphate.
c. Natural gas bums in air to form carbon dioxide and water.
Answer : 
a. 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
b. BaCl₂(aq) + ZnSO4(aq) → BaSO₄(s) + ZnCl₂(aq)
c. CH₄(g) + 2O₂ → CO₂(g) + 2H₂O(l)

Long Answer Questions

Question. a. Define a balanced chemical equation. Why should an equation be balanced?
b. Write a balanced chemical equation for the following reactions:
(i) Phosphorus burns in the presence of chlorine to form phosphorus pentachloride.
(ii) Burning of natural gas.
(iii) The process of respiration.
Answer :  
a. Balanced chemical equation is a equation in which number of atoms of various elements are equal on both sides of the equation.
The equation should be balanced due to law of conservation of mass.
b. (i) 2P(s) + 5Cl₂(g) → 2PCl₅(S)
(ii) CH₄(g) + O₂(g) → CO₂(g) + 2H₂O(l)
(iii) C₆H₁₂O₆(s) + 6O₂(g) → 6CO₂(g) + 6H₂O(l)

Question. (a) Write one example for each of decomposition reaction carried out with the help of (i) Electricity, (ii) Heat, (iii) Light.
(b) Which of the following statement is correct and why?
Copper can displace silver from silver nitrate solution and silver can displace copper from copper sulphate solution.
Answer : 
a. (i) CaCO₃(s) →Heat CaO(s) + CO₂(g)
(ii) 2AgBr(s) →sunlight 2Ag(s) + Br2(g)
(iii) 2H₂O(l) →Electricity 2H2(g) + O₂(g)
b. Copper can displace Ag from AgNO₃(aq) solution because Cu is more reactive than Ag.
Cu(s) + 2AgNO₃(aq) → Cu(NO₃)2(aq) + 2Ag(s) 

Question. In the electrolysis of water:
a. Name the gas collected at the cathode and anode respectively.
b. Why is volume of gas collected at one electrode double than that at the other? Name this gas.
c. How will you test this gas?
Answer : 
a. Hydrogen is collected at the cathode, oxygen is collected at the anode.
b. It is because H₂O contains hydrogen and oxygen in the ratio 2 : 1.
c. Bring a burning matchstick near the gas, if the gas burns with ‘pop’ sound, the gas is H2.

Question. Identify the type of reactions taking place in each of the following:
a. Barium chloride solution is mixed with copper sulphate solution and white precipitate is formed.
b. On heating copper powder in china dish, the surface of copper powder turns black.
c. On heating green coloured ferrous sulphate crystals, raddish brown solid is left and smell of a gas having odour of burning sulphur is experienced.
d. Iron nails when left dipped in blue copper sulphate solution become reddish brown in colour and the blue colour of copper sulphate fades away.
e. Quick lime reacts vigorously with water releasing a large amount of heat.
Answer : 
a. Double displacement reaction,
b. Oxidation,
c. Decomposition reaction,
d. Displacement reaction,
e. Combination reaction.

Question. What does one mean by exothermic and endothermic reactions? Give examples. 
Answer. Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.
Example. Mixture of sodium and chlorine to yield table salt

In other words, combination reactions are exothermic.
Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.
For example. In the process of photosynthesis, plants use the energy from the sun to convert carbon dioxide and water to glucose and oxygen.

Question. Why do we apply paint on iron articles?
Answer.  Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Hence, rusting is prevented their presence is essential for rusting to take place.

Question. Oil and fat containing food items are flushed with nitrogen. Why?
Answer.  Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

Question. A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Answer. (i) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).

Question. Why should a magnesium ribbon be cleaned before burning in air?
Answer. Magnesium is very reactive metal. When stored it reacts with oxygen to form a layer magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be exposed into air. 

Question. Give an example of a double displacement reaction.
Answer. Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium  chloride.

In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence, it is a double displacement reaction.

Question. Identify the substances that are oxidised and the substances that are reduced in the following reactions.

Answer. (i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
(ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H₂) gets oxidised to water (H₂O).

Question. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Answer. (a) Corrosion: Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc. 
For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide

This hydrated iron oxide is rust.
(b) Rancidity: The process of oxidation of fats and oils that can be easily noticed by the change in taste
and smell is known as rancidity.
For example, the taste and smell of butter changes when kept for long. Rancidity can be avoided by.
1. Storing food in air tight containers
2. Storing food in refrigerators
3. Adding antioxidants
4. Storing food in an environment of nitrogen 

Question. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer. A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation. The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this reason that chemical equations should be balanced.

Question. 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 aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer.

Question. Balance the following chemical equations

Question. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer. When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour. 

Therefore, the blue colour of copper sulphate solution fades and green colour appears.

Question. Why is respiration considered an exothermic reaction? Explain.
Answer. Energy is required to support life. Energy in our body is obtained from the food we eat. 
During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration. Since energy is released in the whole process, it is an exothermic process.

Question. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer. Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.

Question.Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer. (a) Thermal decomposition.

Question. What is the difference between displacement and double displacement reactions? Write equations for these reactions. 
Answer.  In a displacement reaction, a more reactive element replaces a less reactive element from a compound.
A  +  BX  →  AX  +  B ;
where A is more reactive than B
In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.
AB  +  CD  →  AD + CB  
For example.
Displacement reaction.
CuSO_4(aq)  +  Zn_(s) → ZnSO_4(aq)  +  Cu_(s)
Double displacement reaction.
Na₂SO_4(aq)  +   BaCl_2(aq)  →  BaSO_4(s)   +   2NaCl_(aq)

Question.  In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer.

Question. What do you mean by a precipitation reaction? Explain by giving examples. 
Answer. A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.
For example.

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.
Another example of precipitation reaction is.

In this reaction, barium sulphate is obtained as a precipitate.

Question. Write a balanced chemical equation with state symbols 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.

Question. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction
Answer.  (a) Oxidation is the gain of oxygen.
For example .

In equation (i), H₂ is oxidized to H₂O and in equation (ii), Cu is oxidised to CuO.
(b) Reduction is the loss of oxygen.
For example.

In equation (i), CO₂ is reduced to CO and in equation (ii), CuO is reduced to Cu.

Question. (i) Write chemical equations for the following and balance them.
(a) Zinc carbonate(s) →Zinc oxide+ Carbon dioxide
(b) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide + Barium bromide.
(c) Nitrogen + Hydrogen → Ammonia
(ii) What happens when electricity is passed through acidified water?
Answer.(i) (a) ZnCO₃ →ZnO +CO₂
(b) 2KBr + BaI₂ → 2KI + BaBr₂
(c) N2 + 3H2 → 2NH3
(ii) Decomposition of water takes place resulting in the formation of hydrogen and oxygen.

Question. A shiny brown-coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Answer.  ‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.

Question. Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer.

Question. Write the balanced chemical equations for the following reactions.
(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
Answer.

Question. 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.

Question. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Answer. Water (H₂O) contains two parts hydrogen and one part oxygen. Therefore, the amount of hydrogen and oxygen produced during electrolysis of water is in a 2. 1 ratio. During electrolysis, since hydrogen goes to one test tube and oxygen goes to another, the amount of gas collected in one of the test tubes is double of the amount collected in the other.