Previous Year Questions: Chemical Reactions & Equations - Science Class 10

Very Short Answer Type Questions

Q1: What is a balanced chemical equation ? (2021 C)
Ans: The equation which contains an equal number of atoms of each element on both sides of the arrow is called a balanced chemical equation.

Q2: Name the type of chemical reaction which takes place when quicklime is added to water.   (2021 C)
Ans: The reaction between CaO and H₂O to form Ca(OH)₂ is an exothermic and combination reaction.

Q3: Translate the following statement into a balanced chemical equation:
"Barium chloride reacts with aluminium sulphate to give aluminium chloride and barium sulphate."
Ans: 3BaCI₂ + AI₂(SO₄)₃ → 2AICI₃ + 3BaSO₄

Short Answer Type Questions

Q1: (i) While electrolyzing water before passing the current some drops of an acid are added why? Name the gases liberated at the cathode and anode. Write the relationship between the volume of gas collected at the anode and the volume of gas collected at the cathode.
(ii) What is observed when silver chloride is exposed to sunlight? Give the type of reaction involved.   (2023 C)
Ans: (i) Acid is added to water before electrolysis to increase its conductivity. This allows the current to pass through the solution easily. Hydrogen gas is liberated at the cathode, while oxygen gas is liberated at the anode. The volume of gas collected at the anode is twice the volume of gas collected at the cathode. 2H₂O(l) → 2H₂(g) + O₂(g)
(ii) When silver chloride is exposed to sunlight, it decomposes to form silver metal and chlorine gas. 2AgCl(s) → 2Ag(s) + Cl₂(g) This is a photochemical decomposition reaction.

Q2: (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.  (2023)
Ans: (a) The chemical reaction in which two reactants exchange ions to form two new compounds is called a double displacement reaction.
(b) (i) When an aqueous solution of an acid reacts with a base (alkali) by exchanging their ions/radicals to form salt and water as the only products, the reaction which takes place is called neutralisation reaction.

(ii) When the aqueous solutions of two ionic compounds react by exchanging their ions/radicals, to form two or more new compounds such that one of the products formed is an insoluble salt, and hence forms precipitate, the double displacement reaction is said to be precipitation reaction. When lead nitrate solution is mixed with potassium iodide solution, a yellow precipitate is formed. This reaction is a precipitation reaction and can be expressed as follows:

Q3: Give the chemical name of the reactants as well as the products of the following chemical equation:   [2021 C]
HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
Ans: Reactants: Nitric acid, calcium hydroxide (slaked lime)
Products: Calcium nitrate, water

Q4: Mention with reason the color changes observed when:  (2022)
(A) Silver chloride is exposed to sunlight
(B) Copper powder is strongly heated in the presence of oxygen
(C) A piece of zinc is dropped in copper sulphate solution.
Ans: 

AgCI decomposes on absorbing light energy.

Copper metal undergoes oxidation.
Zn displaces Cu from CuSO₄ solution. Colour changes from blue to colourless.

Q5: 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.     (2022)
Ans: (a) 'M' is Mg and 'N' is MgO

(c) 'M' undergoes oxidation in this reaction because Mg gain oxygen to form MgO.

Q6: 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. H₂SO₄ is not added to water ?    (2020)
Ans: (a) At anode: Oxygen gas is liberated. At cathode: Hydrogen gas is liberated.
(b) In the test tube covering the cathode, the amount of gas collected is double than that of the gas collected in the test tube covering the anode due to stochiometry.
2H₂O → 2H₂ + O_{2
(c) Addition of a few drops of sulphuric acid make water a good conductor of electricity.}

_{Q7: 1 g of copper powder was taken in a China dish and heated. What change takes place on healing? 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. (2020)
Ans: When copper powder is heated in a China dish, the reddish brown surface of copper powder becomes coated with a black substance which is copper oxide.
 
When hydrogen gas is passed over CuO, the black coating on the surface turned reddish brown due to the formation of Cu.}

Q8: 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 the chemical equation for the reaction of A with water.
(iii) List two types of reactions in which this reaction may be classified. (2020)
Ans: (i) Compound A is calcium oxide (CaO) and compound B is calcium hydroxide (Ca(OH)₂).
(ii) The chemical equation for the reaction of A (calcium oxide) with water is:
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
(iii) The reaction can be classified as a combination reaction and an exothermic reaction. It is a combination reaction because two substances combine to form a new compound, and it is exothermic because it evolves a large amount of heat.

Q9: Identify the type of each of the following reactions. Also, write a 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. (2020)
Ans: (i) The type of reaction in which the reaction mixture becomes warm is an exothermic reaction. An example of such a reaction is the combustion of methane (CH₄):
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + heat
(ii) The type of reaction in which an insoluble substance is formed is a precipitation reaction. An example of such a reaction is the reaction between silver nitrate (AgNO₃) and sodium chloride (NaCl):
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
In this reaction, silver chloride (AgCl) is formed as an insoluble substance, which precipitates out of the solution.

Q10: Lead nitrate solution is added to a test tube containing potassium iodide solution.
(a) Write the name and color of the compound precipitated.
(b) Write the balanced chemical equation for the reaction involved.
(c) Name the type of this reaction justifying your answer.   (2020)
Ans: (a) The compound precipitated is lead iodide (PbI₂), which is yellow in color.
(b) The balanced chemical equation for the reaction is:
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
(c) The type of this reaction is a precipitation reaction. In this reaction, a solid (precipitate) is formed when two solutions are mixed together. In this case, lead iodide is formed as a yellow precipitate.

Q11: 2 g of silver chloride is taken in a China dish, and the China dish is placed in sunlight for some time. 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. (Delhi 2019)
Ans: When silver chloride (AgCl) is exposed to sunlight, it undergoes a decomposition reaction and gets converted into silver (Ag) and chlorine gas (Cl₂). The balanced chemical equation for this reaction is:
2AgCl(s) → 2Ag(s) + Cl₂(g)
The observation in this case would be that the white silver chloride gradually turns grayish-white or silver-colored due to the formation of silver, and the chlorine gas may be seen as a pale yellowish-green gas. This is an example of a photodecomposition reaction, where light energy is used to break down the compound into its elements.

Q12: 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. (Delhi 2019)
Ans: (a) The type of reaction taking place is a single displacement reaction. Zinc (Zn) displaces silver (Ag) from silver nitrate (AgNO₃) to form zinc nitrate (Zn(NO₃)₂) and silver (Ag). The balanced chemical equation for this reaction is:
Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
(b) The type of reaction taking place is a double displacement reaction or a precipitation reaction. Potassium iodide (KI) reacts with lead nitrate (Pb(NO₃)₂) to produce potassium nitrate (KNO₃) and lead iodide (PbI₂). The balanced chemical equation for this reaction is:
2KI(aq) + Pb(NO₃)₂(aq) → 2KNO₃(aq) + PbI₂(s)

Q13: 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 color 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. (2019)
Ans: (a) The color of the precipitate formed is yellow. The compound precipitated is lead iodide (PbI2)
(b) The balanced chemical equation for this reaction is:
2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3(aq)
(c) The two types of reactions in which this reaction can be placed are a double displacement reaction and a precipitation reaction. In a double displacement reaction, the positive and negative ions of two compounds switch places to form new compounds. In a precipitation reaction, a solid precipitate is formed when two solutions are mixed together.

Q14: 2 g of ferrous sulfate crystals are heated in a dry boiling tube. (a) List any two observations. (b) Name the type of chemical reaction taking place. (c) Write a balanced chemical equation for the reaction and name the products formed. (Al 2019, Board Term 1, 2017, 2016)
Ans: (a) Two observations during the heating of ferrous sulfate crystals are:
1. The crystals lose water and become anhydrous.
2. The color of the crystals changes from green to a reddish-brown color.
(b) The type of chemical reaction taking place is a thermal decomposition reaction.
(c) The balanced chemical equation for the reaction is:
FeSO₄⋅7H₂O(s) → Fe₂O₃(s) + SO₂(g) + H₂O(g)
The products formed are ferric oxide (Fe₂O₃), sulfur dioxide (SO₂), and water (H₂O).

Q15: 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? (Al 2019)
Ans: (a) The black substance is formed because copper reacts with oxygen in the air to form copper oxide.
(b) The black substance is copper oxide (CuO).
(c) The chemical equation for the reaction is:
2Cu(s) + O₂(g) → 2CuO(s)
(d) The black coating on the surface can be turned reddish-brown by reducing it back to copper. This can be done by passing hydrogen gas over the hot copper oxide. The chemical equation for this reaction is:
CuO(s) + H₂(g) → Cu(s) + H₂O(g)
The reduction reaction converts the black copper oxide back to reddish-brown copper.

Q16: 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. (2018)
Ans: Decomposition reaction with heat energy supplied:
ZnCO₃(s) → ZnO(s) + CO₂(g)
Decomposition reaction with light energy supplied:
2H₂O₂(l) → 2H₂O(l) + O₂(g)
Decomposition reaction with electrical energy supplied:
2AgCl(s) → 2Ag(s) + Cl₂(g)

Q17: 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. (Board Term 1, 2017)
Ans: (i) On touching the test tube from outside, you will feel the test tube becoming cold.
(ii) The inference about the type of reaction that occurred is that it is an endothermic reaction. In endothermic reactions, heat is absorbed from the surroundings, resulting in a decrease in temperature
(iii) The balanced chemical equation for the reaction is:
Ba(OH)₂(s) + 2NH₄Cl(s) → BaCl₂(aq) + 2NH₃(g) + 2H₂O(l)

Q18: (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. (NCERT Exemplar)
(b) Ferrous sulfate, when heated, decomposes with the evolution of a gas having a characteristic odor of burning sulfur. Write the chemical reaction involved and identify the type of reaction. (Board Term 1, 2016)
Ans: (a) The chemical reaction involved is a double displacement reaction or a precipitation reaction. The reaction between potassium chloride (KCl) and silver nitrate (AgNO₃) produces silver chloride (AgCl), which is an insoluble white substance. The balanced chemical equation for this reaction is:
KCl(aq) + AgNO₃(aq) → AgCl(s) + KNO₃(aq)
(b) The chemical reaction involved is a thermal decomposition reaction. Ferrous sulfate (FeSO₄) when heated decomposes to form ferric oxide (Fe₂O₃), sulfur dioxide (SO₂), and water (H₂O). The balanced chemical equation for this reaction is:
FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + H₂O(g)

Q19: Name the type of chemical reaction represented by the following equation: 
(i) CaO + H₂O → Ca(OH)₂ 
(ii) 3BaCl₂ + Al₂(SO₄)3 → 2AlCl₃ + 3BaSO₄ 
(iii) 2FeSO₄ + Heat → Fe₂O₃ + SO₂ + SO₃   (Board Term 1, 2016)
Ans: (i) The type of chemical reaction represented by the equation CaO + H₂O → Ca(OH)₂ is a combination or synthesis reaction.
(ii) The type of chemical reaction represented by the equation 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄ is a double displacement or precipitation reaction.
(iii) The type of chemical reaction represented by the equation 2FeSO₄ + Heat → Fe₂O3 + SO₂ + SO₃ is a thermal decomposition reaction.

Long Answer Type Questions

Q1: (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. (Board Term I, 2017)
Ans: (a) Consider the following displacement reaction:
Zn_(s)+ CuSO_4(aq) → ZnSO_4(aq) + Cu_(s)
Here, Zn has changed into ZnSO₄ (i.e., Zn²⁺ ions) by loss of electrons. Hence, Zn has been oxidised. CuSO₄ (i.e., Cu²⁺) has changed into Cu by gain of electrons. Hence, CuSO₄ has been reduced. Thus, the above reaction is a displacement reaction as well as a redox reaction.
(b) (i) Neutralisation reaction
(ii) Oxidation reaction.