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⁻).

Question 2

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

Accepted Answer

Melting of ice is a physical change because the substance remains water (H₂O) in both solid and liquid states; only its physical state changes, not its chemical composition.

Question 3

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

Accepted Answer

Melting of ice is a physical change because the substance (water) remains chemically the same; only its state changes from solid to liquid. No new substance is formed.

Question 4

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

Accepted Answer

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

Question 5

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

Accepted Answer

Methane (CH₄) is the primary component of natural gas, typically comprising 70-90% of the gas mixture. It is a hydrocarbon and the simplest alkane.

Question 6

Which of the following is a non-metal that is liquid at room temperature?

Accepted Answer

Bromine (Br₂) is a non-metal that exists as a liquid at room temperature (25°C). It is reddish-brown in colour and has a pungent odour. Mercury is a metal (not a non-metal), so it is excluded.

Question 7

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

Accepted Answer

Common salt, also known as table salt or rock salt, has the chemical formula NaCl (sodium chloride). It is an ionic compound composed of sodium (Na⁺) and chloride (Cl⁻) ions.

Question 8

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 its composition. It is a simple hydrocarbon and the most abundant component in commercial natural gas supplied to homes.

Question 9

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

Accepted Answer

Common salt, also known as table salt or rock salt, has the chemical formula NaCl (sodium chloride). It is an ionic compound composed of sodium and chloride ions.

Question 10

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.

Question 11

What is the chemical formula of the compound commonly known as 'baking soda' used in Indian kitchens?

Accepted Answer

Baking soda, scientifically known as sodium bicarbonate, has the chemical formula NaHCO₃. It is widely used in cooking, cleaning, and as a leavening agent in baked goods.

Question 12

Which gas is primarily responsible for the rusting of iron when exposed to air and moisture?

Accepted Answer

Oxygen (O₂) is the primary gas responsible for rusting of iron. When iron is exposed to both oxygen and moisture, it undergoes oxidation to form iron oxide (rust), a reddish-brown compound.

Question 13

Which gas is primarily responsible for the rusting of iron when exposed to air and moisture?

Accepted Answer

Oxygen in air, combined with moisture, causes iron to oxidize and form rust (iron oxide).

Question 14

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

Accepted Answer

Carbon dioxide (CO₂) gas is released when acids react with carbonates or bicarbonates. This is a characteristic reaction: Acid + Carbonate → Salt + Water + CO₂. The CO₂ gas causes effervescence (bubbling).

Question 15

Which of the following is the primary reason why salt (NaCl) is added to icy roads during winter?

Accepted Answer

Salt lowers the freezing point of water through colligative properties, preventing ice formation and improving road safety.

Question 16

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 its composition. It is a hydrocarbon and the simplest alkane, making it ideal for combustion in domestic appliances.

Question 17

Which of the following is the correct definition of pH in chemistry?

Accepted Answer

pH is defined as the negative logarithm (base 10) of hydrogen ion concentration. pH = -log₁₀[H⁺]. A solution with pH < 7 is acidic, pH = 7 is neutral, and pH > 7 is basic. This is a fundamental concept in everyday chemistry used to measure acidity and alkalinity of substances like soil, water, and food.

Question 18

Which of the following is the primary reason why salt (NaCl) is added to icy roads during winter?

Accepted Answer

Salt lowers the freezing point of water through colligative properties, preventing ice formation and improving road safety.

Question 19

What is the chemical formula of the compound commonly known as 'baking soda,' which is used in cooking and cleaning?

Accepted Answer

Baking soda is sodium bicarbonate (NaHCO₃), a weak base used in baking and household cleaning.

Question 20

Which of the following is the correct definition of pH in chemistry?

Accepted Answer

pH is the negative logarithm of hydrogen ion concentration, measuring acidity or alkalinity on a scale of 0–14.

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