Question 1

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 2

Which gas is produced when an acid reacts with a carbonate or bicarbonate?

Accepted Answer

Carbon dioxide (CO₂) gas is produced when acids react with carbonates or bicarbonates. This is a characteristic reaction: Acid + Carbonate → Salt + Water + CO₂. For example, vinegar (acetic acid) + baking soda (NaHCO₃) → sodium acetate + water + CO₂.

Question 3

What is the chemical formula of the compound commonly known as 'baking soda'?

Accepted Answer

Baking soda's chemical formula is NaHCO₃ (sodium bicarbonate). It is a weak base used in cooking, cleaning, and as an antacid. It reacts with acids to produce carbon dioxide gas, which causes baked goods to rise.

Question 4

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

Accepted Answer

Methane (CH₄) is the primary component of natural gas, typically comprising 70-90% of the gas mixture. It is a simple hydrocarbon and the most abundant component in commercial natural gas supplied to homes.

Question 5

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 simple hydrocarbon and the most abundant constituent that provides the energy when burned.

Question 6

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

Accepted Answer

Melting of ice is a physical change because the water molecules remain the same; only the state of matter changes from solid to liquid. The chemical composition (H₂O) is unchanged. Physical changes are reversible and do not produce new substances.

Question 7

Rust formation on iron is an example of which type of chemical reaction, and what is the primary reactant besides iron?

Accepted Answer

Rusting is an oxidation reaction where iron reacts with oxygen from air in the presence of moisture to form iron oxide (Fe₂O₃·xH₂O). Oxygen is the primary reactant besides iron.

Question 8

Which of the following statements correctly describes the difference between a physical change and a chemical change?

Accepted Answer

A physical change alters the form or state of matter without changing its chemical composition (e.g., melting ice). A chemical change produces new substances with different chemical properties (e.g., burning paper).

Question 9

In everyday life, baking soda (sodium bicarbonate) is used to neutralize acidic spills. What is the chemical equation for this neutralization reaction?

Accepted Answer

Baking soda (NaHCO₃) reacts with acids to produce salt, water, and carbon dioxide gas. The general form is: NaHCO₃ + HCl → NaCl + H₂O + CO₂↑

Question 10

Which of the following is the primary reason why water is called a universal solvent in everyday chemistry?

Accepted Answer

Water dissolves more substances than any other liquid because of its polar nature—the bent molecular structure creates partial positive and negative charges that interact with ionic and polar compounds.

Question 11

Which of the following is the primary reason why hard water is unsuitable for washing clothes and making soap?

Accepted Answer

Hard water contains dissolved minerals, primarily calcium and magnesium ions. These ions react with soap molecules to form insoluble precipitates (soap scum), reducing the soap's cleaning efficiency. Additionally, hard water requires more soap to achieve the same cleaning effect as soft water.

Question 12

Why does salt (sodium chloride) dissolve readily in water but not in oil, even though both are liquids?

Accepted Answer

Salt is an ionic compound that dissolves in polar solvents like water due to ion-dipole interactions. Oil is nonpolar, so it cannot effectively interact with ions, following the principle 'like dissolves like.'

Question 13

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 14

In the context of everyday chemistry, which of the following correctly describes what happens when baking soda (sodium bicarbonate, NaHCO₃) is heated?

Accepted Answer

When sodium bicarbonate is heated, it decomposes into sodium carbonate (Na₂CO₃), water (H₂O), and carbon dioxide (CO₂). The reaction is: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. This is why baking soda is used as a leavening agent in baking—the CO₂ gas causes dough to rise.

Question 15

Which of the following statements about oxidation and reduction is correct?

Accepted Answer

Oxidation is the loss of electrons (or gain of oxygen/loss of hydrogen), and reduction is the gain of electrons (or loss of oxygen/gain of hydrogen). In any redox reaction, oxidation and reduction occur simultaneously. The substance that is oxidized is called the reducing agent, and the substance that is reduced is called the oxidizing agent.

Question 16

When calcium carbonate (CaCO₃) reacts with dilute hydrochloric acid, which of the following is NOT produced?

Accepted Answer

The reaction is: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑. The products are calcium chloride (soluble salt), water, and carbon dioxide gas. Calcium hydroxide is not formed in this reaction.

Question 17

Which of the following statements correctly explains why hard water requires more soap to produce lather compared to soft water?

Accepted Answer

Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions. When soap (sodium stearate or similar) is added to hard water, these metal ions react with the soap molecules to form insoluble precipitates (calcium stearate, magnesium stearate), which removes the soap from solution before it can form lather. Soft water lacks these ions, so soap remains in solution and produces lather readily.

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