CBSE Class 10 Science HOTs Chemical Reactions and Equations Set 03

Question. Name the type of chemical reaction which takes place when quicklime is added to water.
Answer: Combination reaction (and also Exothermic reaction).

Question. What is observed after about 1 hour of adding the strips of copper and aluminium separately to ferrous sulphate solution filled in two beakers? Name the reaction if any change in colour is noticed. Also, write chemical equation for the reaction. 
Answer: In the beaker with copper strip, no change is observed because copper is less reactive than iron. In the beaker with aluminium strip, the green colour of ferrous sulphate fades and becomes colourless as aluminium is more reactive and displaces iron. This is a displacement reaction.
\( 2Al_{(s)} + 3FeSO_{4(aq)} \rightarrow Al_2(SO_4)_{3(aq)} + 3Fe_{(s)} \)

Question. A student wants to study a decomposition reaction by taking ferrous sulphate crystals. Write two precautions he must observe while performing the experiment. 
Answer: (1) Do not point the mouth of the boiling tube at your neighbors or yourself. (2) Waft the gas gently towards your nose rather than smelling it directly.

Question. Classify the following reactions into different types :
(i) \( AgNO_{3(aq)} + NaCl_{(aq)} \rightarrow AgCl_{(s)} + NaNO_{3(aq)} \)
(ii) \( CaO_{(s)} + H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)} \)
(iii) \( 2KClO_{3(s)} \xrightarrow{\Delta} 2KCl_{(aq)} + 3O_{2(g)} \)
(iv) \( Zn + CuSO_4 \rightarrow ZnSO_4 + Cu \) 
Answer: (i) Double displacement (Precipitation) reaction, (ii) Combination reaction, (iii) Decomposition reaction, (iv) Displacement reaction.

Question. A student mixes sodium sulphate powder in barium chloride powder. What change would the student observe on mixing the two powders? Justify your answer and explain how he can obtain the desired change. 
Answer: No change will be observed on mixing the powders as reactions between ionic compounds generally occur in aqueous solutions where ions are free to move. To obtain the desired change (formation of white precipitate of barium sulphate), he should prepare aqueous solutions of both salts and then mix them.

Question. Study the following equation of a chemical reaction :
\( H_2 + Cl_2 \rightarrow 2HCl \)
(i) Identify the type of reaction.
(ii) Write a balanced chemical equation of another example of this type of reaction. 
Answer: (i) Combination reaction. (ii) \( 2Mg + O_2 \rightarrow 2MgO \)

Question. State the type of chemical reactions, represented by the following equations :
(a) \( A + BC \rightarrow AC + B \)
(b) \( A + B \rightarrow C \)
(c) \( PQ + RS \rightarrow PS + RQ \)
(d) \( A_2O_3 + 2B \rightarrow B_2O_3 + 2A \) 
Answer: (a) Displacement reaction, (b) Combination reaction, (c) Double displacement reaction, (d) Displacement reaction.

Question. (i) While electrolysing water before passing the current some drops of an acid are added. Why? Name the gases liberated at cathode and anode. Write the relationship between the volume of gas collected at anode and the volume of gas collected at cathode.
(ii) What is observed when silver chloride is exposed to sunlight? Give the type of reaction involved. 
Answer: (i) Acid is added to make water a good conductor of electricity (pure water is a poor conductor). Gas at cathode: Hydrogen; Gas at anode: Oxygen. The volume of hydrogen at cathode is double the volume of oxygen at anode (2:1). (ii) White silver chloride turns grey due to its decomposition into silver and chlorine. It is a photodecomposition reaction.

Question. (a) Define a double displacement reaction.
(b) Write the chemical equation of a double displacement reaction which is also a (i) Neutralisation reaction and (ii) Precipitation reaction. Give justification for your answer. 
Answer: (a) A reaction in which there is an exchange of ions between the reactants to form new compounds is called a double displacement reaction. (b) (i) Neutralisation: \( NaOH + HCl \rightarrow NaCl + H_2O \) (Base + Acid gives Salt + Water). (ii) Precipitation: \( AgNO_3 + NaCl \rightarrow AgCl \downarrow + NaNO_3 \) (AgCl forms an insoluble solid called precipitate).

Question. Mention with reason the colour changes observed when :
(i) silver chloride is exposed to sunlight.
(ii) copper powder is strongly heated in the presence of oxygen.
(iii) a piece of zinc is dropped in copper sulphate solution. 
Answer: (i) White to grey because silver chloride decomposes to form silver metal. (ii) Reddish brown to black because copper reacts with oxygen to form black copper(II) oxide. (iii) Blue solution turns colourless because zinc displaces copper to form colourless zinc sulphate solution.

Question. A shining metal ‘M’, on burning gives a dazzling white flame and changes to a white powder 'N'.
(a) Identify ‘M’ and ‘N’.
(b) Represent the above reaction in the form of a balanced chemical equation.
(c) Does ‘M’ undergo oxidation or reduction in this reaction ? Justify. 
Answer: (a) M = Magnesium (Mg), N = Magnesium oxide (MgO). (b) \( 2Mg + O_2 \rightarrow 2MgO \). (c) 'M' (Magnesium) undergoes oxidation because it gains oxygen to form MgO.

Question. In the electrolysis of water
(a) Name the gases liberated at anode and cathode.
(b) Why is it that the volume of gas collected on one electrode is two times that on the other electrode ?
(c) What would happen if dil. \( H_2SO_4 \) is not added to water ? 
Answer: (a) Anode: Oxygen (\( O_2 \)); Cathode: Hydrogen (\( H_2 \)). (b) Water molecule (\( H_2O \)) contains two atoms of hydrogen for every one atom of oxygen. During electrolysis, 2 volumes of hydrogen are released for every 1 volume of oxygen. (c) Electrolysis will proceed very slowly or not at all because pure water is a poor conductor of electricity.

Question. 1 g of copper powder was taken in a China dish and heated. What change takes place on heating? When hydrogen gas is passed over this heated substance, a visible change is seen in it. Give the chemical equations of reactions, the name and the colour of the products formed in each case. 
Answer: On heating, reddish brown copper turns black due to formation of copper(II) oxide: \( 2Cu + O_2 \xrightarrow{\Delta} 2CuO \) (Black). When \( H_2 \) is passed over hot CuO, the black surface turns reddish brown again: \( CuO + H_2 \xrightarrow{\Delta} Cu + H_2O \) (Reddish brown).

Question. A compound ‘A’ is used in the manufacture of cement. When dissolved in water, it evolves a large amount of heat and forms compound ‘B’.
(i) Identify A and B.
(ii) Write chemical equation for the reaction of A with water.
(iii) List two types of reaction in which this reaction may be classified. 
Answer: (i) A = Calcium oxide (Quick lime, CaO), B = Calcium hydroxide (Slaked lime, \( Ca(OH)_2 \)). (ii) \( CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat} \). (iii) Combination reaction and Exothermic reaction.

Question. Identify the type of each of the following reactions. Also write balanced chemical equation for each reaction.
(i) A reaction in which the reaction mixture becomes warm.
(ii) A reaction in which an insoluble substance is formed. 
Answer: (i) Exothermic reaction; \( CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat} \). (ii) Precipitation reaction; \( AgNO_3 + NaCl \rightarrow AgCl \downarrow + NaNO_3 \).

Question. Lead nitrate solution is added to a test tube containing potassium iodide solution.
(a) Write the name and colour of the compound precipitated.
(b) Write the balanced chemical equation for the reaction involved.
(c) Name the type of this reaction justifying your answer. 
Answer: (a) Lead iodide, yellow colour. (b) \( Pb(NO_3)_2 + 2KI \rightarrow PbI_2 \downarrow + 2KNO_3 \). (c) Double displacement (Precipitation) reaction because there is an exchange of ions between reactants and an insoluble solid is formed.

Question. 2 g of silver chloride is taken in a China dish and the China dish is placed in sunlight for sometime. What will be your observation in this case? Write the chemical reaction involved in the form of a balanced chemical equation. Identify the type of chemical reaction. 
Answer: White silver chloride turns grey. \( 2AgCl_{(s)} \xrightarrow{\text{Sunlight}} 2Ag_{(s)} + Cl_{2(g)} \). It is a Photodecomposition reaction.

Question. Identify the type of reactions taking place in each of the following cases and write the balanced chemical equation for the reactions.
(a) Zinc reacts with silver nitrate to produce zinc nitrate and silver.
(b) Potassium iodide reacts with lead nitrate to produce potassium nitrate and lead iodide. 
Answer: (a) Displacement reaction; \( Zn + 2AgNO_3 \rightarrow Zn(NO_3)_2 + 2Ag \). (b) Double displacement reaction; \( 2KI + Pb(NO_3)_2 \rightarrow 2KNO_3 + PbI_2 \).

Question. When potassium iodide solution is added to a solution of lead (II) nitrate in a test tube, a precipitate is formed.
(a) What is the colour of this precipitate ? Name the compound precipitated.
(b) Write the balanced chemical equation for this reaction.
(c) List two types of reactions in which this reaction can be placed. 
Answer: (a) Yellow, Lead(II) iodide. (b) \( Pb(NO_3)_2 + 2KI \rightarrow PbI_2 + 2KNO_3 \). (c) Double displacement reaction and Precipitation reaction.

Question. 2 g of ferrous sulphate crystals are heated in a dry boiling tube.
(a) List any two observations.
(b) Name the type of chemical reaction taking place.
(c) Write balanced chemical equation for the reaction and name the products formed. 
Answer: (a) Green colour of crystals changes to reddish brown and a smell of burning sulphur is observed. (b) Thermal decomposition reaction. (c) \( 2FeSO_4 \xrightarrow{\Delta} Fe_2O_3 + SO_2 + SO_3 \). Products are Ferric oxide, Sulphur dioxide and Sulphur trioxide.

Question. You might have noted that when copper powder is heated in a China dish, the reddish brown surface of copper powder becomes coated with a black substance.
(a) Why has this black substance formed?
(b) What is the black substance?
(c) Write the chemical equation of the reaction that takes place.
(d) How can the black coating on the surface be turned reddish brown? 
Answer: (a) It is formed due to the oxidation of copper. (b) Copper(II) oxide (CuO). (c) \( 2Cu + O_2 \xrightarrow{\Delta} 2CuO \). (d) By passing hydrogen gas over hot CuO.

Question. Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity. 
Answer: Heat: \( CaCO_3 \xrightarrow{\Delta} CaO + CO_2 \); Light: \( 2AgCl \xrightarrow{\text{Sunlight}} 2Ag + Cl_2 \); Electricity: \( 2H_2O \xrightarrow{\text{Electricity}} 2H_2 + O_2 \).

Question. Take 3 g of barium hydroxide in a test tube, now add about 2 g of ammonium chloride and mix the contents with the help of a glass rod. Now touch the test tube from outside.
(i) What do you feel on touching the test tube?
(ii) State the inference about the type of reaction occurred.
(iii) Write the balanced chemical equation of the reaction involved. 
Answer: (i) The test tube feels cold. (ii) It is an endothermic reaction. (iii) \( Ba(OH)_2 + 2NH_4Cl \rightarrow BaCl_2 + 2NH_3 + 2H_2O \).

Question. (a) 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 the chemical reaction. 
(b) Ferrous sulphate when heated, decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction. 
Answer: (a) \( KCl + AgNO_3 \rightarrow AgCl \downarrow + KNO_3 \); Double displacement (Precipitation) reaction. (b) \( 2FeSO_4 \xrightarrow{\Delta} Fe_2O_3 + SO_2 + SO_3 \); Thermal decomposition reaction.

Question. Name the type of chemical reaction represented by the following equation :
(i) \( CaO + H_2O \rightarrow Ca(OH)_2 \)
(ii) \( 3BaCl_2 + Al_2(SO_4)_3 \rightarrow 2AlCl_3 + 3BaSO_4 \)
(iii) \( 2FeSO_4 \xrightarrow{\text{Heat}} Fe_2O_3 + SO_2 + SO_3 \) 
Answer: (i) Combination reaction, (ii) Double displacement (Precipitation) reaction, (iii) Thermal decomposition reaction.

Question. What is a reduction reaction? Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(a) \( Fe_2O_3 + 2Al \rightarrow Al_2O_3 + 2Fe \)
(b) \( 2PbO + C \rightarrow 2Pb + CO_2 \) 
Answer: A reduction reaction is a reaction that involves the removal of oxygen or addition of hydrogen. (a) Oxidised: Al; Reduced: \( Fe_2O_3 \). (b) Oxidised: C; Reduced: PbO.

Question. (a) Can a displacement reaction be a redox reaction? Explain with the help of an example.
(b) Write the type of chemical reaction in the following :
(i) Reaction between an acid and a base
(ii) Rusting of iron. 
Answer: (a) Yes, displacement reactions are generally redox reactions. Example: \( Zn + CuSO_4 \rightarrow ZnSO_4 + Cu \). Here, Zn is oxidised to \( Zn^{2+} \) and \( Cu^{2+} \) (in \( CuSO_4 \)) is reduced to Cu. (b) (i) Double displacement (Neutralisation) reaction. (ii) Redox reaction (specifically Oxidation and Corrosion).

Have You Observed the Effects of Oxidation Reactions in Everyday Life?

Question. Copper utensils slowly lose their shiny brown surface and gain a green coat on prolonged exposure to atmospheric air. This is due to the formation of a coating of
(a) copper sulphate
(b) copper carbonate
(c) cupric oxide
(d) cuprous oxide. 
Answer: (b) copper carbonate

Question. What happens when food materials containing fats and oils are left for a long time? List two observable changes and suggest three ways by which this phenomenon can be prevented. 
Answer: They become rancid. Observable changes: (1) Unpleasant smell (2) Change in taste. Prevention: (1) Use of antioxidants (2) Vacuum packing (3) Flushing with nitrogen gas.

Question. (a) In the following reaction :
\( MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2 \)
Identify the oxidant and reductant.
(b) Give reasons :
(i) Antioxidants are added to foods containing fats and oils.
(ii) White silver chloride turns grey in sunlight. 
Answer: (a) Oxidant: \( MnO_2 \); Reductant: HCl. (b) (i) To prevent oxidation of fats and oils (rancidity). (ii) Due to photodecomposition of silver chloride into metallic silver and chlorine gas.

Question. (i) Why is respiration considered as an exothermic reaction? 
(ii) Write chemical name and the formula of the brown gas produced during thermal decomposition of lead nitrate.
(iii) Why do chips manufactures flush bags of chips with gas such as nitrogen? 
Answer: (i) Because energy is released in the form of heat during the breakdown of glucose. (ii) Nitrogen dioxide (\( NO_2 \)). (iii) To provide an inert atmosphere and prevent the oxidation of fats and oils in chips (prevent rancidity).

Questions

Question. Which of the following correctly represents a balanced chemical equation ?
(a) \( Fe_{(s)} + 4H_2O_{(g)} \rightarrow Fe_3O_{4(s)} + 4H_{2(g)} \)
(b) \( 3Fe_{(s)} + 4H_2O_{(g)} \rightarrow Fe_3O_{4(s)} + 4H_{2(g)} \)
(c) \( 3Fe_{(s)} + H_2O_{(g)} \rightarrow Fe_3O_{4(s)} + H_{2(g)} \)
(d) \( 3Fe_{(s)} + 4H_2O_{(g)} \rightarrow Fe_3O_{4(s)} + H_{2(g)} \)
Answer: (b) \( 3Fe_{(s)} + 4H_2O_{(g)} \rightarrow Fe_3O_{4(s)} + 4H_{2(g)} \)

Question. Why is it important to balance a skeletal chemical equation?
(a) To verify law of conservation of energy.
(b) To verify the law of constant proportion.
(c) To verify the law of conservation of mass.
(d) To verify the law of conservation of momentum. 
Answer: (c) To verify the law of conservation of mass.

Question. In the redox reaction \( MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2 \)
(a) \( MnO_2 \) is reduced to \( MnCl_2 \) & HCl is oxidized to \( H_2O \)
(b) \( MnO_2 \) is reduced to \( MnCl_2 \) & HCl is oxidized to \( Cl_2 \)
(c) \( MnO_2 \) is oxidized to \( MnCl_2 \) & HCl is reduced to \( Cl_2 \)
(d) \( MnO_2 \) is oxidized to \( MnCl_2 \) & HCl is reduced to \( H_2O \)
Answer: (b) \( MnO_2 \) is reduced to \( MnCl_2 \) & HCl is oxidized to \( Cl_2 \)

Question. Assertion (A) : Silver bromide decomposition is used in black and white photography.
Reason (R) : Light provides energy for this exothermic reaction.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (c) A is true but R is false.

Question. Reema took 5 mL of lead nitrate solution in a beaker and added approximately 4 mL of potassium iodide solution to it. What would she observe?
(a) The solution turned red.
(b) Yellow precipitate was formed.
(c) White precipitate was formed.
(d) The reaction mixture became hot.
Answer: (b) Yellow precipitate was formed.

Question. In the reaction of iron with copper sulphate solution :
\( CuSO_4 + Fe \rightarrow Cu + FeSO_4 \)
Which option in the given table correctly represents the substance oxidised and the reducing agent?
(a) Substance Oxidized: Fe, Reducing Agent: Fe
(b) Substance Oxidized: Fe, Reducing Agent: \( FeSO_4 \)
(c) Substance Oxidized: Cu, Reducing Agent: Fe
(d) Substance Oxidized: \( CuSO_4 \), Reducing Agent: Fe
Answer: (a) Substance Oxidized: Fe, Reducing Agent: Fe

Question. The chemical reaction between copper and oxygen can be categorized as
(a) displacement reaction
(b) decomposition reaction
(c) combination reaction
(d) double displacement reaction.
Answer: (c) combination reaction

Question. Heated Limestone \( \xrightarrow{\text{Step 1}} X + CO_2 \)
\( X + H_2O \xrightarrow{\text{Step 2}} \) Slaked lime
Identify the correct option from the given table which represents the type of reactions occurring in step 1 and step 2.
(a) Endothermic: x, Exothermic: ✓
(b) Endothermic: ✓, Exothermic: x
(c) Endothermic: ✓, Exothermic: ✓
(d) Endothermic: x, Exothermic: x
Answer: (c) Endothermic: ✓, Exothermic: ✓

Question. Assertion (A) : Decomposition of vegetable matter into compost is an endothermic reaction.
Reason (R) : Decomposition reaction involves breakdown of a single reactant into simpler products.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (d) A is false but R is true.

Question. Assertion (A) : After white washing the walls, a shiny white finish on walls is obtained after two to three days.
Reason (R) : Calcium oxide reacts with carbon dioxide to form calcium hydrogen carbonate which gives shiny white finish.
(a) Both A and R are true, and R is correct explanation of the assertion.
(b) Both A and R are true, but R is not the correct explanation of the assertion.
(c) A is true, but R is false.
(d) A is false, but R is true.
Answer: (c) A is true, but R is false.

Question. List any two observations when ferrous sulphate is heated in a dry test tube.
Answer: (i) The green colour of crystals changes to brownish black.
(ii) Smell of burning sulphur is observed due to the evolution of \( SO_2 \) and \( SO_3 \) gases.

Question. Identify the products formed when 1 mL of dil. hydrochloric acid is added to 1 g of sodium metal.
Answer: The products formed are sodium chloride and hydrogen gas.
\( 2Na(s) + 2HCl(dil) \rightarrow 2NaCl(aq) + H_2(g) \)

Question. (i) \( A + BC \rightarrow AC + B \)
(ii) \( AB + CD \rightarrow AC + BD \)
Identify the types of reaction mentioned above in (i) and (ii). Give one example for each type in the form of a balanced chemical equation.
Answer: (i) Displacement reaction.
Example: \( Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s) \)
(ii) Double displacement reaction.
Example: \( Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq) \)

Question. (a) Which of the following reactions is/are an endothermic reaction(s) where decomposition also happens?
• Respiration
• Heating of lead nitrate
• Decomposition of organic matter
• Electrolysis of acidified water
(b) Silver chloride when kept in the open turns grey. Illustrate this with a balanced chemical equation.
Answer: (a) The endothermic decomposition reactions are:
• Heating of lead nitrate
• Electrolysis of acidified water
(b) When silver chloride is exposed to sunlight, it decomposes to form silver metal (grey) and chlorine gas.
\( 2AgCl(s) \xrightarrow{Sunlight} 2Ag(s) + Cl_2(g) \)