Important Questions: Chemical Reactions and Equations | Science Class 10 PDF Download

Ans: (a)
1. Write the unbalanced equation
Al + CuSO₄ → Al₂(SO₄)₃ + Cu

2. Balance Aluminum (Al)

On the right, Al₂(SO₄)₃ has 2 Al atoms.

So, put 2 before Al on the left:
2Al + CuSO₄ → Al₂(SO₄)₃ + Cu

3. Balance Sulfate (SO₄)

4. Balance Copper (Cu)

On the left: 3 Cu atoms (from 3CuSO₄).

On the right: each Cu represents 1 atom, so put 3 before Cu:
2Al + 3CuSO₄ → Al₂(SO₄)₃ + 3Cu

5. Verify

Al: 2 (LHS) = 2 (RHS)

Cu: 3 (LHS) = 3 (RHS)

S: 3 (LHS) = 3 (RHS)

O: 12 (LHS) = 12 (RHS)

Balanced.

Final Answer

The coefficients are a = 2, b = 3, c = 1, d = 3 → Option (a).

Ans: (a)
Step 1: Write the unbalanced equation.
FeS₂ + O₂ → Fe₂O₃ + SO₂

Step 2: Balance iron (Fe).
On the right, Fe is in Fe₂O₃ (2 atoms).
So, put coefficient 4 before FeS₂ and 2 before Fe₂O₃.
4FeS₂ + O₂ → 2Fe₂O₃ + SO₂

Now Fe is balanced (4 on each side).

Step 3: Balance sulfur (S).
On the left: 4FeS₂ = 8 sulfur atoms.
So, put coefficient 8 before SO₂.
4FeS₂ + O₂ → 2Fe₂O₃ + 8SO₂

Now S is balanced (8 on each side).

Step 4: Balance oxygen (O).
On the right:

From 2Fe₂O₃ → 2 × 3 = 6 oxygen atoms

From 8SO₂ → 8 × 2 = 16 oxygen atoms
Total = 22 oxygen atoms.

On the left: O₂ molecule contains 2 oxygen atoms.
So, coefficient before O₂ must be 11.
4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂

Step 5: Verify.

Fe: 4 (LHS) = 4 (RHS)

S: 8 (LHS) = 8 (RHS)

O: 22 (LHS) = 22 (RHS)

Balanced.

Final Answer: (a) x = 4, y = 11, z = 2, w = 8

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.

Ans: (a)
In this question, both the Assertion (A) and the Reason (R) are true. 
The balanced chemical equation 3Fe (s) + 4H₂O (g) → Fe₃O₄ (s) + 4H₂ (g) correctly represents the reaction of steam with iron, forming iron(II,III) oxide (Fe₃O₄) and hydrogen gas (H₂). 
The Reason (R) states that the law of conservation of mass holds, meaning that mass is neither created nor destroyed in a chemical reaction, which is indeed reflected in the balanced equation. Therefore, option (a) is correct, as the Reason accurately explains the Assertion.

Ans: The balanced chemical equation for the reaction is:
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)

Ans: (b)
Burning of coal involves the reaction of carbon (coal) with oxygen to form carbon dioxide: C (s) + O₂(g) → CO₂(g). This is a combination reaction as two substances (carbon and oxygen) combine to form a single product (carbon dioxide).

Ans: (b)
In the reactions listed, options (a), (c), and (d) involve double displacement or exchange of ions between reactants, where compounds are formed by swapping partners. However, option (b) is a combination reaction, where calcium oxide (CaO) reacts with water (H₂O) to form calcium hydroxide (Ca(OH)₂) without exchanging ions. This makes (b) different from the others.

Ans: (d)
A redox reaction involves both oxidation and reduction, where electrons are transferred between substances. In option (d), iron(III) oxide (Fe₂O₃) is reduced to iron (Fe), while carbon monoxide (CO) is oxidized to carbon dioxide (CO₂). Unlike the other options, which are combination reactions, this one shows the reduction and oxidation of different substances, making it a redox reaction but not a combination reaction.

Ans: Combination reaction – Single product is formed (or any other)

Ans: 2 Mg + O₂ → 2MgO
Magnesium oxide
Type – Combination reaction
Reason: Two or more substances combine to form a single product.

Ans: The reaction between CaO and H₂O to form Ca(OH)₂ is an exothermic and combination reaction.

Ans: (c)
A decomposition reaction is when a compound breaks down into simpler substances. Option (c) 2AgBr → 2Ag + Br₂ is the correct answer because it requires light energy to break down silver bromide (AgBr) into silver (Ag) and bromine (Br₂). The other options do not use light for decomposition.

Ans: (i) A reactant breaks down to give two or more products. A reaction which requires energy to split a compound or reactant in two or more simple substances.
(I) Water splits into hydrogen gas and oxygen gas.
Type of Energy: Electrical energy
(II) Silver bromide decomposes into silver and bromine
Type of Energy: Light energy
(ii) (I) Formation of calcium oxide:
CaCO₃ + Heat → CaO + CO₂
It is an endothermic reaction/decomposition reaction.
(II) Formation of calcium hydroxide:
It is exothermic/combination reaction

Ans: (A) 

Sunlight is essential for the above reaction to take place.  This is a decomposition reaction. Such reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Silver chloride turns grey after its decomposition into silver and chlorine by sunlight.  This reaction is used in black and white photography.

(B) 

Ans:  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.

Ans: (a)
When lead nitrate is heated, it undergoes thermal decomposition, resulting in the formation of lead oxide,PbO (a yellow solid), nitrogen dioxide NO₂ (a brown gas), and oxygen, O₂. The brown fumes observed in this setup are due to the release of nitrogen dioxide gas.
The reaction is as follows:

Thus, the correct answer is (a) thermal decomposition of lead nitrate which produces brown fumes of nitrogen dioxide.

Ans: (a)

When lead nitrate (Pb(NO₃)₂) is heated, it decomposes and produces brown fumes of nitrogen dioxide (NO₂). This happens because the lead nitrate breaks down into lead oxide (PbO), nitrogen dioxide (NO₂), and oxygen (O₂). The brown color of the fumes is characteristic of nitrogen dioxide, making option (a) the correct answer.

Ans: (c)
Sol: Silver salt (AgCl) are used in black and white photography. Silver salt (AgCl) is photosensitive compound, it decomposes into elemental chlorine (Cl₂) and Ag(metal).
AgBr is also used as black and white photography.

Ans: (A) Electrolytic decomposition of water/ electrolysis of water. 
(B) The gas collected at cathode is hydrogen which is double the volume of oxygen collected at anode. 
(C) The balanced chemical equation for the reaction is:
(D) The reaction does not occur without dilute sulphuric acid because:

• Water is a poor conductor of electricity.
• Adding sulphuric acid improves conductivity, allowing the reaction to proceed.

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 + O₂

_{(c)Without adding dilute sulphuric acid, water would not conduct electricity effectively, which would hinder the electrolysis process. As addition of a few drops of sulphuric acid make water a good conductor of electricity.} 

Ans: (b)
In the reactions listed, options (a), (c), and (d) involve double displacement or exchange of ions between reactants, where compounds are formed by swapping partners. However, option (b) is a combination reaction, where calcium oxide (CaO) reacts with water (H₂O) to form calcium hydroxide (Ca(OH)₂) without exchanging ions. This makes (b) different from the others.

Ans: (b)
In the given reaction, zinc (Zn) replaces hydrogen in acetic acid (CH₃COOH) to form zinc acetate (Zn(CH₃COO)₂) and hydrogen gas (H₂). This type of reaction, where one element displaces another from a compound, is called a displacement reaction. Therefore, option (b) is correct.

Ans: Fe(s) + CuSO₄ (aq) → FeSO₄(aq) + Cu(s)
Displacement reaction: A reaction in which a more reactive metal displaces a less reactive metal from its salt solution.
Other metals that can be used in place of iron to show the same type of reaction with copper sulphate solution: Zinc, Aluminium, Calcium, Magnesium

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:

Ans: (d)

When hydrogen sulfide (H₂S) is passed through a blue solution of copper sulfate (CuSO₄), it forms a black precipitate of copper sulfide (CuS) while sulfuric acid (H₂SO₄) remains in the solution. This process involves the exchange of ions between the reactants, characteristic of a double displacement reaction. In this type of reaction, the ions from both compounds swap partners, which makes option (d) the correct answer.

Ans: (d)
In a double displacement reaction, like the one between sodium sulfate and barium chloride, the ions in the reactants swap places. This results in the formation of an insoluble salt (barium sulfate), which is a solid that separates out (precipitate) from the solution. So, options (B), (C), and (D) are all correct because they describe the ion exchange and the formation of a precipitate.

Ans: 

Cu(s) + FeSO₄(aq) → No change will take place

Copper is less reactive than Fe, so Cu cannot displace iron from a ferrous sulphate solution. Hence, No reaction will take place.

2 Al(s) + 3 FeSO₄(aq) → Al₂(SO₄)₃ + 3 Fe(s) (Displacement reaction)

When Al is added to a FeSO₄(aq) solution, the green colour of FeSO₄(aq) disappears and the Fe is seen setting down as the reaction occurs. Al being higher in the reactivity series displaces the Fe in FeSO₄.

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)

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.

Ans: (a) Consider the following displacement reaction:
Zn(s)+ CuSO₄(aq) → ZnSO₄(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.

Ans: The reaction in which one element gets oxidised or addition of oxygen occurs and other element gets reduced or removal of oxygen occurs in other element is called redox reaction. 
Example:

Ans: (a)
Sol: 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.
Carbon is a reducing agent that reduces ZnO to Zn.

Ans: 

(a) 'M' is (Mg) Magnesium and 'N' is (MgO) Magnesium Oxide

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

Ans: 

Copper metal undergoes oxidation.

_{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.}

Ans: A reduction reaction is a reaction in which hydrogen is added to a substance or oxygen is removed from a substance. 
(A) In this reaction, Fe₂O₃ is losing oxygen and forming Fe, whereas Al is gaining oxygen and forming Al₂O₃. Therefore, Fe₂O₃ is getting reduced and Al is getting oxidised. 
(B) In this reaction, PbO is losing oxygen and forming Pb whereas C is gaining oxygen and forming CO. Therefore, PbO is getting reduced and C is getting oxidised.

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.

Ans: (a)
Sol: Reaction of quick lime (CaO) with water is an exothermic reaction because CaO reacts vigorously with water releasing a large amount of heat.
CaO(s) + H₂O(I) → Ca(OH)₂(ag) + Heat

_{Ans: (c)
Sol: In the process of respiration, glucose combines with oxygen in cells of our body and provides energy. Thus, respiration is an exothermic process.}
C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(I) + Energy

Ans: (d)
Sol: During sublimation of dry ice, heat is absorbed, so, it is an endothermic process. Dissolution of NH₄CI in water is also an endothermic process.

Ans: (d)
Assertion (A): Burning of natural gas is an endothermic process. This is incorrect. Burning (or combustion) of natural gas, which mainly consists of methane, is an exothermic process, meaning it releases heat.
Reason (R): Methane gas combines with oxygen to produce carbon dioxide and water. This statement is correct. During combustion, methane reacts with oxygen to produce carbon dioxide and water as products.
Since the assertion is false, but the reason is true, the correct answer is (d) (A) is false but (R) is true.

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.

Ans: 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