CBSE Class 10 Science Chemical Reactions and Equations Assignment Set D

Very Short Answer Type Questions

Question. What change in colour is observed when white silver chloride is left exposed to sunlight? What type of chemical reaction is this? 
Answer: When white silver chloride is left exposed to sunlight, its colour changes to grey due to the formation of silver.
2AgCl_(s)sunlight → 2Ag_(s)+Cl_2(g)
White                             Grey
This type of reaction is called photodecomposition reaction.

Question. What happens chemically when quick lime is added to water?
Answer: Quick lime reacts vigorously with water to produce calcium hydroxide (slaked lime) releasing a large amount of heat (exothermic reaction).
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat
  (Quick lime)                (Slaked lime)

Question. In electrolysis of water, why is the volume of gas collected over one electrode double than that of gas collected over the other electrode?
Answer: In electrolysis of water, hydrogen (H₂) gas is liberated at cathode while oxygen (O₂) gas is liberated at anode. The overall reaction is
2H₂O(l) Electric current → 2H_2(g) + O_2(g)
From the above reaction, it is clear that amount of H2 liberated is twice that of O₂.

Question. Balance the following chemical equation :
Fe(s) + H₂O(g) → Fe₃O_4(s) + H_2(g) 
Answer: 3Fe_(s) + 4H₂O_(g) → Fe3O4(s) + 4H_2(g)

Question. Balance the following chemical equation :
FeSO₄  ^heat → Fe₂O₃ + SO₂ + SO₃
Answer: 2FeSO₄ heat → Fe O + SO + SO₃

Question. Write a balanced chemical equation for the reaction between sodium carbonate and hydrochloric acid indicating the physical state of the reactants and the products.
Answer: Na2CO_3(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l)
Sodium Hydrochloric Sodium
carbonate acid chloride + CO_{2 (g)}

Question. On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue green. Predict the new compound formed which imparts a blue green colour to the solution.
Answer: CuO + 2HCl → CuCl₂ + 2H₂O
Copper chloride solution imparts blue green colour to the solution.

Question. Hydrogen being a highly inflammable gas and oxygen being a supporter of combustion, yet water, a compound made up of hydrogen and oxygen is used to extinguish fire. Why?
Answer: H₂O is a compound constituted of hydrogen of oxygen elements and being a compound it has different properties as compared to its constituting elements.

Question. Oil and fat containing packed food items are flushed with nitrogen. Why?
Answer: Oil and fat containing food items are flushed with nitrogen to prevent them from spoiling. When such food items come in contact with air, they get oxidised. They become rancid and their smell and taste changes.

Question. Give one example of chemical reaction in which change in colour takes place.
Answer: Fe(s) + CuSO₄ (aq) ⎯⎯→ FeSO₄ (aq) + Cu (s)
Copper Sulphate Ferrous Sulphate Copper
(Blue colour) (Light-green colour) (Brown colour)

Question. What precautions would you take while handling quicklime?
Answer: (i) Quicklime can cause severe burns, therefore it should be handled with a spatula.
(ii) As reaction between quicklime and water is exothermic, water should be poured over quicklime slowly.

Question. Why does a moist blue litmus paper turn red when it is brought near the mouth of the test tube in which ferrous sulphate crystals are being heated?
Answer: On heating ferrous sulphate crystals (FeSO₄ . 7H₂O) it decomposes to ferric oxide, sulphur dioxide and sulphur trioxide. Ferric oxide is solid while sulphur dioxide and sulphur trioxide are gases. Both sulphur dioxide and sulphur trioxide are acidic and hence these gases turn the moist blue litmus red.

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: When copper is mixed in silver nitrate AgNO₃ solution, copper displaces the silver because copper is more reactive than silver.
2AgNO₃ (aq) + Cu (s) → Cu(NO₃)₂ (aq) + 2Ag (s)

Short Answer Type Questions

 Question. What is a 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: A reaction in which oxidation and reduction take place simultaneously is called redox reaction.
On heating in oxygen/air, magnesium burns with a dazzling white light to give magnesium oxide.
2Mg + O₂  heat → 2MgO
Here, magnesium is oxidised as addition of oxygen has taken place.

Question. (a) What is the colour of ferrous sulphate crystals? How does this colour change after heating?
(b) Name the products formed on strongly heating ferrous sulphate crystals. What type of chemical reaction occurs in this change? 
Answer: (a) Ferrous sulphate crystals (FeSO₄.7H₂O) are light green in colour.
When it is heated, white colour solid is formed.
FeSO₄ .7H₂O heat → FeSO₄ + 7H₂O
(b) When anhydrous ferrous sulphate is further heated strongly, it decomposes to give ferrous oxide (Fe₂O₃) and oxides of sulphur.   

This type of chemical reaction is known as decomposition react

Question. (i) What is observed when a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube?
(ii) What type of reaction is this?
(iii) Write a balanced chemical equation to represent the above reaction. 
Answer: (i) When lead nitrate is added to potassium iodide then yellow precipitate of lead iodide is formed along with potassium nitrate.
(ii) This type of reaction is called precipitation reaction in which one of the products formed is an insoluble substance.
(iii) Chemical reaction will be as follows :
Pb(NO₃)_2(aq) + 2KI(aq) → PbI_2(s) ↓ + 2KNO_3(aq)
                                         (Yellow ppt.)

Question. Identify the type of reaction from the following equations: 
(i) CH₄ + 2O₂ → CO₂ + 2H₂O (ii) Pb (NO₃)₂ + 2KI → PbI₂ + 2KNO₃
(iii) CaO + H₂O → Ca(OH)₂ (iv) CuSO₄ + Zn → ZnSO₄ + Cu
Answer: (i) CH₄ + 2O₂ ⎯⎯⎯→ CO₂ + H2O
Redox reaction
(ii) Pb(NO₃)₂ + 2KI ⎯⎯⎯→ PbI₂ + 2KNO₃
Double displacement reaction
(iii) CaO + H₂O ⎯⎯⎯→ Ca(OH)₂
Combination reaction
(iv) CuSO₄ + Zn ⎯⎯⎯→ ZnSO₄ + Cu
Displacement reaction

Question. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of reaction.
Answer: AgNO₃(aq) + KCl(aq) → AgCl(s) + KNO₃(aq)
                                   (White ppt.)
It is a double displacement reaction.
b. Change in temperature:
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

Question. What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? State the physical conditions of reactants in which the reaction between them will not take place. Write the balanced chemical equation for the reaction and name the type of reaction.
Answer: When an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride then, white precipitate of barium sulphate (BaSO4) is formed.
Na₂SO₄(aq) + BaCl₂(aq)  → BaSO_4(s) + 2NaCl(aq)
If the reactants are present in solid state then no reaction will take place between them. This type of reaction is called double displacement reaction or precipitation reaction.

Question. What is an oxidation reaction? Identify in the following reaction : ZnO + C → Zn + CO
(i) the substance oxidised and
(ii) the substance reduced. 
Answer: The reaction in which oxygen is added or hydrogen is removed or loss of electrons takes place is called an oxidation reaction.
In the reaction,   

(i) Carbon is getting oxidised to carbon monoxide.
(ii) Zinc oxide is getting reduced to zinc.

Question. (a) A solution of substance X is used for white washing. What is substance X? Write the chemical reaction of X with water.
(b) Why does the colour of copper sulphate solution changes when iron nail is dipped in it?
Answer: a. X is CaO, calcium oxide
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
b. It is because Fe displaces Cu from CuSO₄ (blue)
solution to form FeSO₄ (pale green) and reddish brown Cu metal gets deposited.

Question. (a) A substance X, an oxide of a metal, is used extensively in the cement industry. This element is found in our bones also. On treatment with water it forms a solution which turns red litmus blue.
Identity X and also write the chemical reaction involved.
(b) Choose a metal from the following metals which reacts only with hot water: Sodium, magnesium, iron.
Mention the products formed during the reaction.
Answer: (a) ‘X’ is Calcium oxide (CaO). Calcium oxide reacts vigorously with water to form Calcium hydroxide
(slaked lime)
CaO (s) + H₂O (l) ⎯⎯⎯→ Ca(OH)2 + Heat
lime Slaked lime
(b) Magnesium reacts with hot water to form Magnesium hydroxide and hydrogen.
Mg (s) + 2H₂O (l) ⎯⎯⎯→ Mg(OH)2 (aq) + H2 (g)
(hot)    Magnesium Hydrogen
               hydroxide

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) æ æ æ Æ 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: (a) 2KBr (aq) + Ba I2 (aq) → 2K I (aq) + BaBr₂ (aq) [Double displacement reaction]
(b) ZnCO₃ (s) → ZnO (s) + CO₂ (g) [Decomposition reaction]
(c) H2 (g) + Cl2 (g) → 2HCl (g) [Combination reaction]
(d) Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g) [Displacement reaction]

Question. Define combination reaction. Give one example of a combination reaction which is also exothermic.
Answer: Combination reaction: The reaction in which two or more elements or compounds combine to form compound(s) e.g.,
C(s) + O₂(g) → CO₂(g) + Heat
It is an exothermic, combination reaction.

Question. What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? State the physical conditions of reactants in which the reaction between them will not take place.
Write the balanced chemical equation for the reaction and name the type of reaction.
Answer: White precipitate of BaSO4 is formed.
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl (aq)
It is a double displacement reaction. If reactants are taken in solid state, products will not be formed.

Question. What do you mean by a precipitation reaction? Explain by giving examples.
Answer: Any reaction that produces a solid insoluble product is known as precipitation reaction.
For example: Sodium sulphate (Na2SO₄) solution when mixed solution of barium chloride (BaCl₂). Then insoluble white precipitate of barium sulphate (BaSO4) is formed.
Na₂SO₄ (aq)    +    BaCl₂ (aq)  →   BaSO₄ (s)↓    +     2NaCl (aq)
  Sodium                Barium            Barium sulphate      Sodium
  sulphate              chloride          (precipitate)               chloride

Question. Write balanced chemical equations for the following reactions: 
(a) dilute sulphuric acid reacts with aluminium powder
(b) dilute hydrochloric acid reacts with sodium carbonate
(c) Carbon dioxide is passed through lime water.
Answer: (a) 2Al (s) + 3H₂SO₄ (aq) ⎯⎯⎯→ Al₂(SO₄)₃ (aq) + 3H₂ ↑
                                                   Aluminium sulphate
(b) Na2CO₃ (s) + 2HCl (aq) ⎯⎯⎯→ 2NaCl (aq) + CO₂ + H₂O
     Sodium carbonate
(c) CO₂ (g)  +  Ca(OH)₂ (aq) ⎯⎯⎯→ CaCO₃ (s) + H₂O
                    Lime water                (White ppt makes lime water milky)

Question. What type of material is formed when aqueous solutions of sodium sulphate and barium chloride are mixed. Give the balanced chemical equation involved. Name the type of reaction it is?
Answer: • A white precipitate of BaSO4 will be formed.
• Na₂SO4 (aq) + BaCl₂ (aq) ⎯⎯⎯→ BaSO₄ (s) + 2NaCl (aq)
• It is a double displacement reaction.

Question. Translate the following statements into chemical equations and then balance them:
(i) Hydrogen gas combines with nitrogen gas to form ammonia.
(ii) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(iii) Barium chloride reacts with aluminium sulphate to give aluminium chloride and precipitate of barium sulphate.
(iv) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer: (i) 3H₂ (g) + N₂ (g) → 2NH₃ (g)
(ii) 2H₂ S (g) + 3O₂ (g) → 2H₂O(l) + 2SO₂ (g) (air)
(iii) 3BaCl₂ (aq) + Al2 (SO₄)₃ (aq) → 2AlCl₃ (aq) + 3BaSO₄ (s)
(iv) 2K (s) + 2H₂ O (l) → 2KOH (s) + H₂ (g)

Question. What happens when:
(a) solutions of sodium sulphate and barium chloride are mixed?
(b) an iron nail is dipped in copper sulphate solution for 20 minutes? 
Answer: (a) When solutions of sodium sulphate and barium chloride are mixed, a white precipitate of barium sulphate is formed. It is a double displacement reaction.
Na₂SO₄ + BaCl₂ ⎯⎯⎯→ BaSO₄ + 2NaCl
Sodium sulphate Barium chloride Barium sulphate Sodium chloride
(b) When an iron nail is dipped in copper sulphate solution, following reaction takes place
Fe(s) + CuSO4(aq) ⎯⎯⎯→ FeSO₄(aq) + Cu(s)
Iron Copper sulphate Ferrous sulphate Copper
In the above reaction, iron ions (Fe₂+) being more reactive displaces copper ions (Cu2+) and a new compound ferrous sulphate is formed.

Question. (i) While studying the combination reaction on adding water to quick lime, name the product formed and write its colour.
(ii) While studying the decomposition reaction by heating ferrous sulphate crystals in a test-tube, a product is formed in the test-tube. Name the product and write its colour.
Answer: (i) Quicklime (CaO) reacts vigorously with water to form slaked lime [Ca(OH)₂] which is white in colour.
CaO (s) + H₂O (l) Combination ⎯⎯→ Ca(OH)₂ (s)
Calcium oxide                              Calcium hydroxide
(Quick lime)                                (Slaked lime) (white)
(ii) When ferrous sulphate is heated strongly, it decomposes to form brown coloured ferric oxide and sulphur dioxide gas and sulphur trioxide gas.
2FeSO₄ (s)        ^Δ ⎯⎯→ _Decomposion Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)
Ferrous sulphate                           Ferric oxide
(green colour)                                    (brown)

Question. A student performed the experiment of heating ferrous sulphate crystals in a boiling tube. He smelt fumes of a pungent gas and saw colours of ferrous sulphate disappear.
(i) Write the chemical formula of the pungent gas.
(ii) Why does the colour of crystal disappear?
(iii) Identify the nature of this chemical reaction.
Answer: (i) SO₂ → Sulphur dioxide
SO₃ → Sulphur trioxide
(ii) Ferrous sulphate crystals contain 7 molecules water of crystallization (FeSO₄.7H₂O). These crystals are green in colour. When these crystals are heated, they first lose 7 molecules of water of crystallization to form anhydrous ferrous sulphate (FeSO4) which is white in colour.
(iii) It is a thermal decomposition reaction
2FeSO₄ ^Heat⎯⎯⎯→ Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)
Green                      (decomposition)
because in this reaction one substance is splitting up into three substances on heating.

Question. Balance the following chemical equations:
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
Answer: (a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

Question. (a) What is meant by balanced chemical equation? Why chemical equations are balanced?
(b) Balance the chemical equation given below:
Al₂O₃ + NaOH → NaAlO₂ + H₂O 
Answer: (a) A balanced chemical equation has an equal number of atoms of different elements in the reactants and products. It means a balanced chemical equation has equal masses of various elements in reactants and products.
According to law of conservation of mass, “Matter can neither be created nor destroyed in a chemical reaction”. Therefore the chemical equations are balanced to satisfy the law of conservation of mass in chemical reactions.
 (b) Al₂O₃ + 2NaOH ⎯⎯⎯→ 2NaAlO₂ + H₂O

Question. You are provided with a container made up of aluminium. You are also provided with solutions of dil HCl, dil HNO₃, ZnCl₂ and H₂O. Out of these solutions which solution, can be kept in the aluminium container? Name the type of reaction taking place. 
Answer: (i) Dilute HCl cannot be kept in aluminium container because aluminium metal reacts rapidly with dil HCl to form aluminium chloride and hydrogen gas.
2Al (s) + 6HCl (dil) ⎯⎯⎯→ 2AlCl3 (aq) + 3H₂ (g)
                  (Displacement reaction)
(ii) Dil HNO₃ cannot be kept in aluminium container because aluminium metal reacts with dil HNO3 to form aluminium nitrate and the hydrogen liberated in this reaction is oxidised to water and nitric acid itself is reduced to any of the oxides of nitrogen. (Displacement reaction)
(iii) ZnCl₂ solution also cannot be kept in aluminium container because Al is more reactive than Zn, therefore more reactive Al displaces less reactive Zn from its solution (ZnCl2).
2Al (s) + 3ZnCl2 (aq) ⎯⎯⎯→ 2AlCl₃ + 3Zn
                  (Displacement reaction)

Question. Explain the following terms with one example each:
(a) Corrosion (b) Rancidity.
Answer: (a) Corrosion. When a metal is attacked by substances around it such as moisture, acid, oxygen, etc. and forms an irreversible metallic compound, it is said to corrode and this process is called corrosion.
Example: Rusting of iron is a common example of corrosion.
Conditions of corrosion—
(i) presence of moisture (water); and
(ii) presence of air or acids.
(b) Rancidity. When fats and oils are oxidised in air they become rancid and their smell and taste changes. This phenomenon is called rancidity.
Prevention from rancidity. Antioxidants are added to food containing fats and oils to prevent them from oxidation.
Example: An inert gas such as nitrogen is added to prevent the packed chips from getting oxidised.

Question. Write the balanced chemical equation for the following reactions:
(i) Phosphorus burns in chlorine to form phosphorus pentachloride.
(ii) Burning of natural gas.
(iii) The process of respiration.
Answer: 

Question. A student adds a spoon full of powdered sodium hydrogen carbonate to a flask containing ethanoic acid. List two main observations, he must note in his note book, about the reaction that takes place. Also write chemical equation for the reaction.
Answer: When sodium hydrogen carbonate is added to a flask containing ethanoic acid, then — brisk effervescence will be formed because of CO2 gas escaping from the reaction mixture.— evolution of colourless and odourless gas. Some amount of heat is evolved during the reaction.
CH₃COOH + NaHCO₃ ⎯⎯⎯→ CH₃COONa + H₂O + CO₂

Long Answer Type Questions

Question. (a) What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? State the physical conditions of reactants in which reaction between them will not take place. Write the balanced chemical equation for the reaction and also mention the type of reaction.
(b) What changes in the colour of iron nails and copper sulphate solution do you observe after keeping the iron nails in copper sulphate solution for about half an hour? 
Answer: (a) When barium chloride solution is added to sodium sulphate solution, then a white precipitate of barium sulphate is formed along with sodium chloride solution:
BaCl₂ (aq)          +   Na2SO4 (aq) ⎯⎯⎯→  BaSO4 (s)         +   2NaCl (aq)
(Barium chloride)    (Sodium sulphate)  (Barium sulphate)   (Sodium chloride)
                                                            (white ppt.)
It is an example of a double displacement reaction. In this displacement reaction, two compounds— barium chloride and sodium sulphate react to form two new compounds—barium sulphate and sodium chloride. An exchange of ions takes place in this reaction.
For example, the barium ions (Ba2+) of barium chloride react with sulphate ions (SO4 2-) of sodium sulphate to form barium sulphate (Ba2+ SO4 2– or BaSO4). In this reaction, barium sulphate is formed as a white, insoluble solid which separates out suddenly from solution.
Reaction will not take place if the reactants are in a solid state.
(b) When iron nails are dipped in copper sulphate solution, then iron sulphate solution and copper metal are formed:
CuSO₄ (aq) + Fe (s) ⎯⎯⎯→ FeSO4 (aq) + Cu (s)
(Copper sulphate) Iron (Iron sulphate) (Copper)
(Blue solution) (Grey) (Greenish solution) (Red-brown)
In this reaction, iron displaces copper from copper sulphate solution. The deep blue colour of copper sulphate solution fades due to the formation of light green solution of iron sulphate. This displacement reaction occurs because iron is more reactive than copper. Also a brown substance will be deposited on the iron nail. This substance is nothing but copper metal.

Question. What is the difference between burning of magnesium ribbon in air and heating of ferrous sulphate crystals? Write equations for these reactions.
Answer: 

Question. Identify the type of chemical reactions in the following processes: 
(i) Barium chloride solution is mixed with copper sulphate and a white precipitate is formed.
(ii) On heating copper powder in a china dish, the surface of copper powder becomes black.
(iii) On heating green ferrous sulphate crystals, reddish brown solid is left as residue and a gas having smell of burning sulphur is evolved.
(iv) Iron nails when left dipped in blue copper sulphate solution become brownish in colour and blue colour of copper sulphate solution fades away.
(v) Quicklime reacts vigorously with water releasing large amount of heat.
(vi) Silver nitrate solution reacts with sodium chloride solution and a white precipitate is formed.
Answer: (i) BaCl₂ (aq) + CuSO₄ (aq) ⎯⎯⎯→ BaSO4 (s)↓ + CuCl₂ (aq)
         (white ppt)
• Double displacement reaction.
• Characterised by the formation of precipitate.
(ii) 2Cu + O₂ Heat⎯⎯⎯→ 2CuO
      (Black)
In this reaction, Cu is changing into CuO. This is the addition of oxygen. So this is an oxidation reaction.
(iii) When green coloured ferrous sulphate is heated strongly, it decomposes to form brown coloured ferric oxide. A smell of burning sulphur is also obtained due to the formation of sulphur dioxide. In this reaction, one substance is splitting up into three substances so this is a decomposition reaction.

Iron displaces Cu from copper sulphate solution as iron is more reactive than copper. Therefore this is a displacement reaction.
(v) CaO (s) + H₂O (l) ⎯⎯⎯→ Ca(OH)₂
This is a combination reaction as two compounds quick lime and water combine to form a single compound slaked lime.
(vi) When silver nitrate solution is added to NaCl solution, then a white precipitate of silver chloride is formed.
AgNO₃ (aq) + NaCl (aq) ⎯⎯⎯→ AgCl (s) + NaNo₃ (aq)
   (white ppt)
This is a double displacement reaction as two compounds react to form two new compounds.

Question. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: In a decomposition reaction, one substance breaks up into two or more chemical substances, while in a combination reaction two or more substances combine to form one single substance. So these two reactions are called opposites of each other.
Examples of decomposition reaction:
(i) CaCO₃ (s) Heat→ CaO (s) + CO₂ (g)
Lime stone Quick lime
(Calcium carbonate) (Calcium oxide)
(ii) 2Pb(NO₃)₂ (s) Heat⎯⎯→ 2PbO (s) + 4NO₂ (g) + O₂ (g)
Lead nitrate Lead oxide Nitrogen dioxide Oxygen gas
Examples of combination reaction:
(i) N2 (g) + 3H2 (g) Heat⎯⎯→ 2NH₃ (g) + energy
Nitrogen Hydrogen Ammonia
(ii) Burning of hydrogen in oxygen of air to make water
2H₂ (g) + O2 (g) Burning→ 2H₂O (l)

Question. (a) In electrolysis of water, why is the volume of gas collected over one electrode double that of gas collected over the other electrode?
(b) (i) What is observed when a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube?
(ii) What type of reaction is this?
(iii) Write a balanced chemical equation to represent the above reaction. 
Answer: (a) In electrolysis of water (H₂O), the hydrogen goes to one test tube and oxygen goes to another. The two electrodes collect H and O separately.
Since water (H₂O) consists of 2 parts of hydrogen and 1 part of oxygen, so, the volume of hydrogen gas (H2) collected over cathode (negative electrode) is double the volume of oxygen gas (O₂) collected over anode (positive electrode).
(b) (i) When potassium iodide solution is added to lead nitrate solution, then a yellow precipitate of lead iodide is produced along with potassium nitrate solution.
(ii) This is a double displacement reaction.
(iii) Pb(NO₃)2 (aq) + 2KI (aq) ⎯⎯⎯→ PbI₂ (s) + 2KNO₃ (aq)
Lead nitrate Potassium Lead iodide
iodide (yellow ppt)