Question 1

Which of the following is an example of a physical change?

Accepted Answer

Melting of ice is a physical change because the substance changes state from solid to liquid, but the chemical composition (H₂O) remains unchanged. No new substance is formed.

Question 2

Which of the following is the main component of natural gas used in households for cooking?

Accepted Answer

Methane (CH₄) is the primary component of natural gas, typically comprising 70-90% of the gas mixture. It is a colorless, odorless hydrocarbon that burns to produce heat and light.

Question 3

What is the pH value of pure distilled water at 25°C?

Accepted Answer

Pure distilled water at 25°C has a pH of 7, which is considered neutral. At this pH, the concentration of hydrogen ions (H⁺) equals the concentration of hydroxide ions (OH⁻), both at 10⁻⁷ mol/L.

Question 4

Which gas is released when sodium carbonate reacts with dilute hydrochloric acid?

Accepted Answer

Carbon dioxide (CO₂) gas is released when sodium carbonate (Na₂CO₃) reacts with dilute hydrochloric acid (HCl). The reaction is: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑

Question 5

What is the chemical formula of common salt used in kitchens?

Accepted Answer

Sodium chloride (NaCl) is the chemical formula for common salt. It is an ionic compound composed of sodium cations (Na⁺) and chloride anions (Cl⁻) in a 1:1 ratio.

Question 6

Which of the following is the main component of natural gas used for cooking and heating in Indian households?

Accepted Answer

Methane (CH₄) is the primary component of natural gas, typically comprising 70-90% of its composition. It is a colorless, odorless hydrocarbon that burns to produce heat and energy. In India, natural gas supplied through pipelines for domestic use is predominantly methane.

Question 7

What is the chemical formula of the compound commonly known as 'washing soda' or 'soda ash'?

Accepted Answer

Washing soda is sodium carbonate (Na₂CO₃), a white crystalline solid used in detergents, glass manufacturing, and water softening.

Question 8

Which of the following reactions is an example of a neutralization reaction?

Accepted Answer

A neutralization reaction occurs when an acid reacts with a base to produce salt and water. For example: HCl + NaOH → NaCl + H₂O.

Question 9

What is the main purpose of adding 'quick lime' (calcium oxide) to water in water treatment plants?

Accepted Answer

Quick lime (CaO) is added to water to increase pH, precipitate impurities, and soften hard water by removing dissolved calcium and magnesium ions.

Question 10

Which gas is produced when a metal reacts with a dilute acid?

Accepted Answer

When a metal reacts with a dilute acid, hydrogen gas (H₂) is typically produced. For example: Zn + 2HCl → ZnCl₂ + H₂↑

Question 11

Which of the following is the primary reason why salt (NaCl) is added to ice during the preparation of ice cream?

Accepted Answer

Salt lowers the freezing point of ice through the colligative property of freezing point depression. When salt dissolves in the thin layer of water on ice, it creates a solution with a lower freezing point than pure water (0°C). This causes the ice to melt, and the resulting endothermic process absorbs heat from the ice cream mixture, cooling it below 0°C and allowing it to freeze.

Question 12

Which of the following is the correct definition of pH, and at what pH value is a solution considered neutral at 25°C?

Accepted Answer

pH is defined as the negative logarithm of hydrogen ion concentration (−log[H⁺]). At 25°C, a neutral solution has a pH of exactly 7.0, where [H⁺] = [OH⁻] = 10⁻⁷ mol/L.

Question 13

When sodium carbonate (Na₂CO₃) reacts with dilute hydrochloric acid (HCl), which gas is produced and what is the characteristic property of this gas?

Accepted Answer

Sodium carbonate reacts with dilute HCl to produce carbon dioxide (CO₂) gas, water, and sodium chloride. The reaction is: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑. CO₂ is colourless, odourless, and turns lime water milky white (due to formation of CaCO₃ precipitate).

Question 14

Which of the following statements about rusting of iron is INCORRECT?

Accepted Answer

Rusting of iron is an oxidation process that requires both oxygen and moisture (water). It is a chemical change, not a physical change, and produces iron oxide (Fe₂O₃·xH₂O), which is reddish-brown. Rusting is accelerated by salt (NaCl) and acidic conditions.

Question 15

In the process of water purification, which chemical is commonly used for coagulation and flocculation, and what is its primary function?

Accepted Answer

Aluminum sulfate [Al₂(SO₄)₃], commonly known as alum, is the most widely used coagulant in water purification. It neutralizes the negative charges on suspended particles (colloids), causing them to aggregate into larger flocs that can be easily filtered out. This process removes turbidity and suspended impurities.

Question 16

Which of the following correctly describes the difference between an exothermic and an endothermic reaction in terms of energy change?

Accepted Answer

In an exothermic reaction, energy is released to the surroundings, and the products have lower energy than the reactants (ΔH < 0). In an endothermic reaction, energy is absorbed from the surroundings, and the products have higher energy than the reactants (ΔH > 0). Combustion is exothermic; melting ice is endothermic.

Question 17

In chemistry, when a substance undergoes a chemical reaction and releases heat energy to the surroundings, the reaction is classified as which of the following?

Accepted Answer

An exothermic reaction is one in which heat energy is released to the surroundings, causing the surroundings to become warmer. Common examples include combustion (burning of fuel), neutralization of acids and bases, and rusting of iron. The opposite is an endothermic reaction, which absorbs heat from the surroundings.

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