Test: Chemical Reactions & Equations - Class 10 MCQ

An oxidizing agent oxidizes other substances and gains electrons; therefore, its oxidation state decreases. Hence, it gets reduced.

In a combination reaction, multiple reactants combine to form a single product. An example is the reaction between calcium oxide and water to produce calcium hydroxide. This type of reaction involves the synthesis of a new compound from the combining of elements or compounds.

• Let's analyze each option to determine the correctly matched pair:
• Option A: (a) Burning of coal - C(s) + O₂(g) → CO(g)
• This is a incorrect chemical equation for the burning of coal, which is a combination reaction. 
• Option B: (b) Formation of water - 2H₂(g) + O₂(g) → 2H₂O₂(l)
• This is also a incorrect chemical equation for the formation of water from hydrogen and oxygen, which is another example of a combination reaction.
• Option C: (c) Respiration - C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O₂(l) + energy
• This equation incorrectly represents the process of respiration, an exothermic reaction where glucose reacts with oxygen to produce carbon dioxide, water, and energy.
• Option D: (d) Whitewashing reaction - Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
• This is the correct chemical equation for the whitewashing reaction. When calcium hydroxide reacts with carbon dioxide, it forms calcium carbonate (which gives a shiny finish on walls) and water.

2H₂ + O₂ → 2H₂O

The balanced chemical equation is:

• 2 molecules of hydrogen gas (H₂) react with 1 molecule of oxygen gas (O₂).
• This produces 2 molecules of water (H₂O).
• Each side of the equation has 4 hydrogen atoms and 2 oxygen atoms, ensuring mass conservation.

Let's analyze the statements:

Assertion (A): Burning of natural gas is an example of an exothermic reaction.

• This is true because burning natural gas (methane, CH4) releases heat energy.

Reason (R): Exothermic reactions release heat energy along with the formation of products.

• This is also true because exothermic reactions are characterized by the release of heat energy during the reaction process.

Since both statements are true and the Reason correctly explains the Assertion: If both Assertion and Reason are true and Reason is the correct explanation of Assertion.

The reaction C(s) + O2(g) → CO2(g) represents an exothermic reaction. In exothermic reactions, energy is released from the reaction system into the surroundings, usually in the form of heat, light, or sound. This particular reaction involves the formation of carbon dioxide (CO2) from carbon (C) and oxygen (O2) with the release of heat.

• Option A: CaO + H₂O → Ca(OH)₂ + Heat - This is correctly matched as a Combination Reaction. In this reaction, calcium oxide (CaO) reacts with water (H₂O) to form calcium hydroxide (Ca(OH)₂) and releases heat. This is a classic example of a combination reaction where two or more substances combine to form a single product.
• Option B: Fe + 4H₂O → Fe₃O₄ + 4H₂ - This reaction is incorrectly labeled as a Decomposition Reaction. It is actually a redox reaction involving the oxidation of iron and the reduction of water.
• Option C: 3Fe + 4H₂O → Fe₃O₄ + 4H₂ - This is also incorrectly labeled. Although there is a combination of elements involved, it is best described as a redox reaction .This reaction is a redox reaction (oxidation-reduction reaction), where iron (Fe) is oxidized and water (H₂O) is reduced.
• Option D: CO + 2H₂ → CH₃OH - This is incorrectly matched as a Displacement Reaction. It is actually a synthesis reaction where carbon monoxide (CO) and hydrogen (H₂) combine to form methanol (CH₃OH).

Fe(s) + CuSO_4​ ⟶ Cu(s) + FeSO_4​

When an iron nail is dipped in copper sulphate solution, a brown coating of copper is formed on the surface of iron and the colour of copper sulphate solution changes from blue to light green. This reaction shows that iron is more reactive than copper as it displaces copper from its solution and iron passes into solution as Fe²⁺ ions and ferrous sulphate solution is formed.