Class 10 – Science Extra Questions – Ch. 1 – Chemical Reactions and Equations

Ans: B

Explanation: Electrolysis of water involves breaking down water molecules into their constituent elements, hydrogen and oxygen. The chemical formula for water is universally recognized as H2O, where two hydrogen atoms are bonded to one oxygen atom. When water (H2O) is electrolyzed, it decomposes into hydrogen gas (H2) and oxygen gas (O2). Options A, C, and D represent incorrect chemical formulas for water. H2O2 is hydrogen peroxide, HO is a radical, and H3O is the hydronium ion.
Correct Option: B

Ans: C

Explanation: A decomposition reaction is a reaction in which a single compound breaks down into two or more simpler substances. Option A is a synthesis reaction where two elements combine to form a compound. Option B is a single displacement reaction where a more reactive metal displaces a less reactive metal from its salt solution. Option D is a double displacement reaction where ions of two compounds are exchanged. Option C shows potassium chlorate (KClO₃), a single compound, breaking down into potassium chloride (KCl) and oxygen gas (O₂), which are simpler substances. Therefore, it represents a decomposition reaction.
Correct Option: C

Ans: C

Explanation: A physical change alters the form or appearance of a substance but does not change its chemical composition. Burning wood involves combustion, a chemical reaction that produces ash, smoke, and gases. Rusting of iron is also a chemical reaction where iron reacts with oxygen to form iron oxide. Cooking an egg involves denaturing proteins, which is a chemical change. Melting of ice, however, is a change of state from solid to liquid. The chemical composition of water (H2O) remains the same whether it is ice, liquid water, or steam.
Correct Option: C

Ans: D

Explanation: Complete combustion of a hydrocarbon (like methane, CH4) in the presence of sufficient oxygen (O2) always produces carbon dioxide (CO2) and water (H2O). Incomplete combustion would produce carbon monoxide (CO) and/or soot (C) in addition to or instead of carbon dioxide. Option C includes sulfur dioxide, which is formed from the combustion of sulfur-containing compounds, not methane. Option B incorrectly lists hydrogen as a product of complete combustion of methane.
Correct Option: D

Ans: D

Explanation: When aqueous solutions of barium chloride (\(BaCl_2\)) and potassium sulfate (\(K_2SO_4\)) are mixed, a double displacement reaction occurs. The ions present in the solutions are \(Ba^{2+}\), \(Cl^-\), \(K^+\), and \(SO_4^{2-}\). These ions can rearrange to form new compounds. The possible products are potassium chloride (\(KCl\)) and barium sulfate (\(BaSO_4\)). Potassium chloride is a soluble salt and will remain in aqueous solution. Barium sulfate, on the other hand, is an insoluble salt and will precipitate out of the solution as a solid. The balanced chemical equation for the reaction is: \(BaCl_2(aq) + K_2SO_4(aq) \rightarrow BaSO_4(s) + 2KCl(aq)\). Therefore, barium sulfate is the precipitate formed.
Correct Option: D

Ans: C

Explanation: Oxidation is a chemical reaction that involves the loss of electrons or an increase in oxidation state. While often associated with gaining oxygen, it can also occur through the loss of hydrogen or other electronegative elements, which is essentially taking oxygen away from a substance (or a substance losing oxygen). Reduction is the opposite process, gaining electrons or decreasing oxidation state, which often involves gaining oxygen. Corrosion is a broader term for the degradation of a material due to chemical or electrochemical reactions with its environment, often involving oxidation. Rancidity refers to the spoilage of fats and oils through oxidation. Therefore, reduction is the term for taking oxygen away from a substance.
Correct Option: C

Ans: B

Explanation: Decomposition reactions are reactions in which a single compound breaks down into two or more simpler substances.
Reaction (i) is a synthesis reaction where two substances combine to form a single compound.
Reaction (ii) shows calcium carbonate breaking down into calcium oxide and carbon dioxide upon heating, which is a decomposition reaction.
Reaction (iii) is a synthesis reaction where sulfur dioxide and oxygen combine to form sulfur trioxide.
Reaction (iv) shows potassium chlorate breaking down into potassium chloride and oxygen upon heating, which is a decomposition reaction.
Therefore, reactions (ii) and (iv) are decomposition reactions.
Correct Option: B

Ans: C

Explanation: Hydrochloric acid is a strong acid and is corrosive. Stainless steel, while resistant to many substances, can be corroded by strong acids over time, leading to contamination of the acid and degradation of the container. Glass and plastic are generally inert to dilute hydrochloric acid and are commonly used for its storage. Copper is a reactive metal and would readily react with hydrochloric acid, making it unsuitable for storage. Therefore, copper would be the most unsuitable.
Correct Option: C

Ans: C

Explanation: The equilibrium constant, Kc, is expressed as the ratio of the concentrations of products raised to their stoichiometric coefficients to the concentrations of reactants raised to their stoichiometric coefficients. For a reaction involving solids, their concentrations are considered constant and are not included in the expression for Kc. The decomposition of solid calcium carbonate is represented by the equation: CaCO3(s) <=> CaO(s) + CO2(g). Since CaCO3 and CaO are solids, they are excluded from the Kc expression. Therefore, Kc is equal to the concentration of the gaseous product, CO2.
Correct Option: C

Ans: C

Explanation: The reaction Mg(s) + CuO(s) → MgO(s) + Cu(s) shows magnesium (Mg) reacting with copper(II) oxide (CuO). In this reaction, magnesium displaces copper from copper(II) oxide. Magnesium is more reactive than copper, so it loses electrons and becomes oxidized (Mg → Mg²⁺ + 2e⁻). Copper ions in CuO gain electrons and are reduced (Cu²⁺ + 2e⁻ → Cu). Since there is a transfer of electrons, it is a redox reaction. Because magnesium replaces copper, it is also a displacement reaction. Therefore, the reaction exemplifies both a redox reaction and a displacement reaction.
Correct Option: C