CBSE Class 10 Science Chemical Reactions and Equations Assignment Set 03

Following questions consists of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below:
A. Both assertionand reason are true, and reason is the correct explanation of assertion.
B. Both assertion and reason are true, but reason is not the correct explanation of assertion.
C. Assertion is true but reason is false.
D. Assertion is false and reason is true.

Question. A ssertion(A): Decomposition of Calcium Carbonate is a type of chemical reaction.
Reason(R):Chemical reactions involve the breaking and making of bonds to produce new substances.

Answer: A

Question. A ssertion(A): Acidification of water decreases its conductivity during electrolysis.
Reason(R): H+ ions are released which hinder the flow of charges.

Answer: D

Question. A ssertion(A): Silver Chloride turns grey in sunlight.
Reason(R): This is an example of thermal decomposition

Answer: C

Question. A ssertion(A): Reaction between Barium Hydroxide and Ammonium Chloride makes the test tube cool to touch.
Reason(R): It is an example of endothermic reaction.

Answer: A

Question. A ssertion(A): The colour of salts does not depend on water of crystallisation.
Reason(R): Ferrous Sulphate crystals are blue in colour because of the seven water molecules of crystallisation.

Answer: D

Question. A ssertion(A): After white washing the walls, a shiny white finish on walls is obtained after three or four days.
Reason(R): Calcium hydroxide reacts with Carbon dioxide to form Calcium Carbonate which gives shiny white finish.

Answer: A

Question. A ssertion(A): The number of atoms of each element remains the same, before and after a chemical reaction.
Reason(R): Any chemical equation can be unbalanced because the mass may not be same on both sides of the equation.

Answer: C

Question. A ssertion(A): Brown fumes are obtained from the thermal decomposition of Lead Nitrate.
Reason(R): Nitrogen dioxide gas is released which is brown in colour.

Answer: A

Question. A ssertion(A): Silver Bromide is used in black and white photography.
Reason(R): Silver Bromide decomposes in the presence of sunlight.

Answer: B

Question. A ssertion(A): In double displacement reactions, no precipitate is formed.
Reason(R):Barium Sulphate is precipitated during the double displacement reaction between Sodium Sulphate and Barium Chloride.

Answer: D

Question. Assertion(A): Fatty foods taste the same even after long periods of exposure to air.
Reason(R): We should store oily foods in air tight containers.

Answer: D

Question. Assertion(A): The glucose combines with oxygen in the cells of our body and provides energy.
Reason(R): Respiration is an example of oxidation reaction.

Answer: B

Question. Assertion(A): Sulphur dioxide and Sulphur trioxides are released during the decomposition of iron sulphate.
Reason(R): This is an example of thermal decomposition.

Answer: B

Question. A ssertion(A): The surface of copper powder becomes black on heating.
Reason: Black colour copper oxide is formed.

Answer: A

Question. Assertion(A): Addition of oxygen is oxidation.
Reason(R): Removal of oxygen is reduction.

Answer: B

Question. Assertion(A): Oxidation and reduction take place simultaneously in a reaction.
Reason(R): All chemical reactions are redox reactions.

Answer: C

Question. Assertion(A): Iron nails turn blue colour copper sulphate solution to green colour.
Reason(R): Copper is more reactive than iron.

Answer: C

Very Short Answer Type Questions

Question. Identify the type of reaction in the following example:
2H₂ (g) + O₂ (g) ⎯⎯→ 2H₂O (l) 
Answer: It is a combination reaction because in this reaction two substances combine to form a single substance.

Question. Write the balanced chemical equation with the state symbols of the following reaction: Solutions of Barium chloride and Sodium sulphate in water react to give insoluble Barium sulphate and the solution of Sodium chloride. 
Answer: BaCl₂ (aq) + Na₂SO₄ (aq) ⎯⎯→ BaSO₄ (s) + 2 NaCl (aq)

Question. State one industrial application of reduction process.
Answer: It is used in the extraction of metals e.g.,
ZnO(s) + C(s) Heat Zn(s) + CO₂(g)

Question. Balance the following chemical reaction :
MnO₂ + HCl → MnCl₂ + Cl₂ + H₂O
Answer: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O

Question. Why is respiration considered as exothermic process?
Answer: Respiration is an exothermic process because energy is given out in respiration.

Question. Two reactions are given below:
a. 2KI + Cl₂ → 2KCl + I₂
b. 2K + Cl₂ → 2KCl
Identify the type of reaction, giving justification in each case.
Answer: a. Displacement reaction because Cl2 is displacing I2 from KI solution.
b. Combination reaction because K reacts with Cl₂ to form potassium chloride.

Question. When hydrogen gas is passed over heated copper (II) oxide, copper and steam are formed. Write the balanced chemical equation for this reaction and state (i) the substance oxidized and (ii) the substance reduced in the reaction. short (8)
Answer: (i) Substance oxidized = H₂ (Hydrogen gas)
(ii) Substance reduced = CuO (Copper oxide)

Question. Why do we apply paint on iron articles?
Answer: We apply paint on iron articles to prevent them from corrosion and rusting. Paint prevents the contact between iron and moist air.

Question. Balance the following chemical equation :
Pb(NO₃)_2(s) heat → PbO_(s) + NO_2(g) + O_2(g)
Answer: 2Pb(NO₃)_2(s) heat →  2PbO(s) + 4NO_2(g) + O_2(g)

Question. On what basis is a chemical reaction balanced?
Answer: Chemical equation is balanced on the basis of law of conservation of mass.

Question. What change in colour is observed when white silver chloride is left exposed to sun¬light? State the type of chemical reaction in this change.
Answer: Grey coloured silver metal is formed and pungent smelling chlorine gas is evolved.
2AgCl(s) sunlight 2Ag(s) + Cl₂(g)

Short Answer Type Questions

Question. What do you mean by exothermic and endothermic reactions? Give examples.
Answer: Exothermic reactions are those reactions in which heat energy is released during the reaction. Examples:
(i) N₂ + 3H₂ ⇌ 2NH₃ + Heat energy
(ii) CH₄ + 2O₂ → CO₂ + 2H₂O + Heat energy
Endothermic reactions are those reactions in which heat energy is absorbed (needed) during the reaction. Examples:
(i) CaCO₃ Heat⎯⎯→ CO₂ + CaO (ii) 2AgCl → sunlight⎯⎯→ 2Ag + Cl₂

Question. Why is respiration considered as an exothermic reaction? Explain.
Answer: During respiration, glucose is decomposed into carbon dioxide gas and water vapours using oxygen gas from air in the living cells of our body and heat energy is released during this process. So respiration is a type of exothermic reaction.
Example, C₆H₁₂O₆ + 6O₂ ^Respiration → 6CO₂ + 6H₂O + Heat energy
                Glucose

Question. An iron nail is dipped in the solution of copper sulphate for about 30 minutes, state the change in colour observed. Give the reason for the change.
Answer: Iron is more reactive than Cu. So it displaces Cu from copper sulphate solution. Thus, blue colour of the CuSO₄ solution fades and colour of the solution turns green due to the formation of ferrous sulphate Solution. This is a displacement reaction.
CuSO₄ (aq) + Fe (s) ⎯⎯→ FeSO₄ (aq) + Cu (s)
  (Blue)                              (Green)

Question. What happens chemically when quick lime is added to water? 
Answer: Quick lime reacts vigorously with water to produce slaked lime with the release of large amount of heat.
CaO (s)        +    H₂O (l) ⎯⎯→ Ca(OH)₂ (aq) + Heat energy
Calcium oxide                 Calcium hydroxide
(Quick lime)                      (Slaked lime)

Question. Write the balanced chemical equation for the following reaction and identify the type of reaction and define it.
‘Iron III oxide reacts with Aluminium and gives molten iron and aluminium oxide’. 
Answer: Fe₂O₃ (s) + 2Al (s) ⎯⎯⎯→ 2Fe (l) + Al₂ O₃ (l) + Heat
• It is a displacement reaction which is highly exothermic. The amount of heat evolved is so large that the metal is produced in the molten state.
• The displacement reaction of iron (III) oxide with aluminium is known as thermite reaction.

Question. Why is silver bromide stored in dark bottles in the laboratories? Write the chemical equation to justify your answer.
Answer: When silver bromide is exposed to light, it decomposes to form silver metal and bromine vapours.
2AgBr (s)    ^light⎯⎯⎯→_{Decomposition}   2Ag (s)     +      Br₂ (g)
Pale yellow                                Greyish white     Red brown
When light falls on pale yellow coloured silver bromide, it changes to greyish white due to the formation of silver metal. To prevent the decomposition of silver bromide, it is stored in dark bottles in the laboratories.

Question. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer: In a displacement reaction, less reactive element is displaced from its salt by a more reactive element.
For example: Copper (Cu) is displaced from CuSO₄ by iron because iron (Fe) is more reactive.
Fe (s) + CuSO₄ (aq)    →   FeSO₄ (aq) + Cu (s)
  Copper sulphate              Iron sulphate
In double displacement reaction exchange of ions takes place between two reactants to form two new products.
Na₂SO₄ (aq)    +   BaCl₂ (aq) →        BaSO₄ (s)        +     2NaCl (aq)
Sodium sulphate   Barium chloride    Barium sulphate    Sodium chloride

Question. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer: Balanced chemical equation. When the number of atoms of different elements on both sides of a chemical equation are equal, then it is called a balanced chemical equation.
A chemical equation needs to be balanced so that it follows “Law of Conservation of Mass” which states that mass can neither be created nor be destroyed.

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: The shiny brown coloured element ‘X’ is Copper (Cu).
When copper is heated in air, it becomes black due to the formation of copper oxide.
2Cu + O₂      ^Heat→  2CuO
(Reddish Brown)     (Black)
Copper Copper (II)  oxide

Question. Distinguish between a displacement reaction and a double displacement reaction. Identify the displacement and the double displacement reaction from the following reactions:
a. HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
b. Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Answer: Displacement Reaction: When a more reactive metal displaces a less reactive metal from its salt solution.
Double displacement reaction: When two compounds exchange their ions to form two new compounds.
a. Double displacement reaction.
b. Displacement reaction.

Question. Balance the following reactions:
a. BaCl₂ +  H₂SO₄ → BaSO₄ + HCl
b. Ca(OH)₂ + HNO₃ → Ca(NO₃)₂ + H₂O
c. Pb(NO₃)₂ → PbO + NO₂ + O₂
d. MnO₂ + HCl → MnCl₂ + Cl₂ + H₂O
Answer: a. BaCl₂(aq) +  H₂SO₄(dil) → BaSO₄(s) + 2HCl (aq) 
b. Ca(OH)₂(aq) + 2HNO₃ → Ca(NO₃)₂(aq) + 2H₂O(l) 
c. 2Pb(NO ) (s) → Heat 2PbO(s)  + 4NO (g) + O₂ (g) Heat
d. MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O

Question. Write the balanced equation for the following reactions and identify the type of reaction in each case:
a. Potassium bromide + Barium iodide → Barium bromide + Potassium Iodide
b. Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
Answer: a. 2KBr(aq) + BaI₂ → BaBr₂(aq) + 2KI(aq)
It is double displacement reaction.
b. H₂(g) +. Cl₂(g) →sunlight 2HCl(g)
It is combination reaction.

Question. Explain the following in terms of gain or loss of oxygen with two examples each:
(a) Oxidation (b) Reduction.
Answer: (a) Oxidation. The reactions in which the gain of oxygen takes place are called oxidation. Examples:
(i) 2Cu + O₂ → 2CuO (ii) 2H₂ + O₂ → 2H₂O
(b) Reduction. The reactions in which the loss of oxygen takes place are called reduction. Examples:
(i) ZnO + C → + CO (ii) CuO + H₂ Heat⎯⎯→ Cu + H₂O

Question. 2g of ferrous sulphate crystals are heated in a boiling tube.
(i) State the colour of ferrous sulphate crystals both before heating and after heating.
(ii) Name the gases produced during heating.
(iii) Write the chemical equation for the reaction. 
Answer: (i) The colour of the ferrous sulphate crystals is green before heating and changes to brown after heating.
The colour of the crystals changes due to the formation of ferric oxide.
(ii) During heating sulphur dioxide (SO₂) and sulphur trioxide (SO₃) are produced.
(iii) 2FeSO₄ (s) light⎯⎯⎯→ Fe2O₃ (s) + SO₂ (g) + SO₃ (g)
                                        Ferric oxide

Question. Describe an activity to show a decomposition reaction in which light is used to decompose a reactant.
Write chemical equation of the reaction and state its one use. 
Answer: Take about 2 g silver chloride in a china dish. It is first white in colour. Now place this china dish in sunlight for some time. After some time it is observed that white silver chloride turns grey. This is due to the decomposition of silver chloride into silver and chlorine by light.
2AgCl sunlight⎯⎯⎯→ 2Ag (s) + Cl₂ (g)
Silver bromide also decomposes to silver and bromine by light.
2AgBr sunlight⎯⎯⎯→ 2Ag (s) + Br₂ (g)
Use: This reaction is used in black and white photography.

Question. A zinc plate was put into solution of copper sulphate kept in a glass container. It was found that blue colour of the solution gets fader and fader with passage of time. After few days when zinc plate was taken out of the solution, a number of holes were observed on it.
a. State the reason for the changes observed on zinc plate.
b. Write the chemical equation for the reaction involved.
Answer: a. Zinc displaces copper from copper sulphate solution to form colourless ZnSO₄ and copper metal is deposited. Zinc gets consumed due to which holes are formed.
b. Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Question. Solid calcium oxide was taken in a container and water was added slowly to it.
(i) State two observations made in the experiment.
(ii) Write the balanced chemical equation of this reaction. 
Answer: (i) Solid calcium oxide was taken in a container and water was added to it.
Observations:
• The mixture produces a hissing sound.
• The beaker feels to be quite hot.
(ii) CaO (s) + H2O ⎯⎯⎯→ Ca(OH)₂ + Heat
Calcium oxide         Calcium hydroxide
 (lime)                   (slaked flame)

Question. Select (i) combination reaction (ii) decomposition reaction and (iii) displacement reaction from the following chemical equations: 
(i) ZnCO₃ (s) → ZnO (s) + CO₂ (g)
(ii) Pb (s) + CuCl₂ (aq) → PbCl₂ (aq) + Cu (s)
(iii) NaBr (aq) + AgNO₃ (aq) → AgBr (s) + NaNO3 (aq)
(iv) H₂ (g) + Cl2 (g) → 2HCl (g)
(v) Fe₂O₃ + 2Al → Al2O₃ + 2Fe
(vi) 3H₂ (g) + N₂ (g) → 2NH₃ (g)
(vii) CaCO₃ (s) Heat⎯⎯⎯→ CaO (s) + CO₂ (g)
Answer: (i) Decomposition reaction (ii) Displacement reaction
(iii) Double displacement reaction (iv) Combination reaction
(v) Displacement reaction (vi) Combination reaction
(vii) Decomposition reaction

Question. State the kind of chemical reactions in the following examples: 
(i) Digestion of food in stomach
(ii) Combustion of coal in air
(iii) Heating of limestone
Answer: (i) Digestion of food in stomach. During digestion, the complex food is broken into simpler form.
Therefore it is a type of decomposition reaction.
(ii) Combustion of coal is air. During combustion the coal burns in air to form CO₂, H₂O along with the evolution of heat. Thus, it is a type of exothermic decomposition reaction.
(iii) Heating of limestone. When limestone is heated strongly, it breaks into CO₂ and lime. Thus it is a type of thermal decomposition reaction.

Question. What is redox reaction? When a magnesium ribbon burns in air with a dazzling flame and forms a white ash, is magnesium oxidised or reduced. Why?
Answer: Redox reaction is a reaction in which oxidation and reduction takes place simultaneously.
Mg is getting oxidised because it is gaining oxygen to form magnesium oxide.

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: (a) Ca(OH)₂ + CO2 → CaCO₃ + H₂O
(b) Zn + 2AgNO₃ → Zn(NO3)2 + 2Ag
(c) 2Al + 3CuCl2 → 2AlCl₃ + 3Cu
(d) BaCl₂ + K2SO4 → BaSO₄ + 2KCl

Question. What will be the nature of solution formed when calcium oxide is dissolved in water? Name the substance formed when carbon dioxide gas is passed through this solution.
Answer: On dissolving calcium oxide in water, calcium hydroxide is formed. When a drop of this liquid is put on a red litmus paper, it turns blue. This shows calcium hydroxide is basic in nature. When carbon dioxide gas is passed through calcium hydroxide (lime water), it turns milky due to formation of calcium carbonate.
Ca(OH)₂ + CO₂ ⎯⎯⎯→ CaCO₃(s)↓ + H₂O(l)
Lime water                White ppt.
                               (Milkiness)

Question. Write an equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer: (i) Energy supplied in the form of heat. This reaction is also known as endothermic reaction. 
CaCO₃ (s) ^Heat⎯⎯→ CaO (s) + CO₂ (g)
Lime stone             Quick lime
(ii) Energy supplied in the form of light. This reaction is also known as photochemical reaction.
2AgBr (s) ^sunlight → 2Ag (s) + Br₂ (g)
Silver bromide
(iii) Energy supplied in the form of electricity. This reaction is also known as electrolytic reaction.
2H₂O ^Electricity → 2H2   +   O₂
Water          Hydrogen    Oxygen

Question. A student took a small piece of solid quick lime in a china dish and poured over it a small amount of water. List two changes he is likely to observe in the china dish immediately after pouring water.
Answer: When a small amount of water is poured on a piece of quick lime—
• it reacts vigorously with water and a hissing sound is produced and slaked lime is formed.
• the reaction mixture becomes hot as it is an exothermic reaction.

Question. While studying the double displacement reaction, the solutions of barium chloride and sodium sulphate are mixed together.
(i) What do you observe as soon as the two solutions are mixed together?
(ii) What will happen in the above observation made by you after ten minutes?
Answer: (i) The reaction mixture becomes white in colour and a precipitate is formed.
(ii) White precipitate settles down after 10 minutes.

Long Answer Type Questions

Question. Give an example of decomposition reaction.
Describe an activity to illustrate such a reaction by heating.
Answer: Those reactions in which a compound breaks down into two or more simpler substances are known as decomposition reactions.
Aim : To show thermal decomposition reaction of ferrous sulphate.
Material required : Ferrous sulphate crystals, dry test tube, burner 

Procedure : 1. Take 2 g of ferrous sulphate crystals in a dry test tube.
2. Observe the colour of ferrous sulphate crystals.
3. Heat the crystals of ferrous sulphate over the flame of a burner for some time.
4. Observe the crystals after heating for 5 minutes.   

Observation : The pale green colour of ferrous sulphate crystals changes to reddish brown ferric oxide and smell of burning sulphur is observed.

Question. Write balanced chemical equations for the following statements:   
(i) NaOH solution is heated with zinc granules.
(ii) Excess of carbon dioxide gas is passed through lime water.
(iii) Dilute sulphuric acid reacts with sodium carbonate.
(iv) Egg shells are dropped in hydrochloric acid.
(v) Copper (II) oxide reacts with dilute hydrochloric acid.
Answer: 

Question. Name the substance oxidised and the substance reduced, and also identify the oxidising agent and reducing agents in the following reaction:
(a) 3MnO₂ + 4Al → 3Mn + 2Al2O₃
(b) Fe₂O₃ + 3CO → 2Fe + 3CO₂
(c) SO₂ + 2H₂S → 3S + 2H₂O
Answer: 

Substance oxidised = H₂S
Substance reduced = SO₂
Oxidising agent = SO₂
Reducing agent = H₂S

Question. Mention the rules for writing a chemical equation illustrating with the help of example. 
Answer: Rules for writing a chemical equation:
(i) The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.
(ii) The substances which combine or react are known as reactants.
(iii) The new substances produced in a reaction are known as products.
(iv) The arrow sign (→) pointing towards the right hand side is put between the reactants and products.
This arrow indicates that the substances written on the left hand side are combining to give the substances written on the right hand side in the equation.
For example:
Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.
Zinc + Sulphuric acid ⎯⎯⎯→ Zinc sulphate + Hydrogen
Putting the symbols and formulae of all the substances in the above words gives equation,
Zn + H₂SO₄ ⎯⎯⎯→ ZnSO₄ + H₂
      ↓                           ↓
Reactants           Products