Question 1

Which gas is primarily responsible for the rusting of iron?

Accepted Answer

Oxygen (O₂) is the primary gas responsible for rusting of iron. Rusting is an oxidation process where iron reacts with oxygen in the presence of moisture to form iron oxide (Fe₂O₃·xH₂O).

Question 2

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

Accepted Answer

A physical change is a change in the physical properties of a substance without altering its chemical composition. Melting of ice is a physical change because ice and water are the same substance (H₂O) in different physical states.

Question 3

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.

Question 4

Which gas is responsible for the pungent smell in onions and garlic?

Accepted Answer

Diethyl disulfide and other sulfur-containing compounds are responsible for the characteristic pungent smell in onions and garlic. These volatile sulfur compounds are released when the vegetables are cut or crushed, breaking down cell walls.

Question 5

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 widely used in cooking, cleaning, and as a mild antacid.

Question 6

Which of the following is the chemical formula for common table salt?

Accepted Answer

Common table salt has the chemical formula NaCl (sodium chloride). It is an ionic compound composed of sodium cations (Na⁺) and chloride anions (Cl⁻).

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 ions (Na⁺) and chloride ions (Cl⁻) in a 1:1 ratio.

Question 8

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

Accepted Answer

Pure distilled water at 25°C has a pH of exactly 7.0, 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 9

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 simple hydrocarbon and the most abundant component in household cooking and heating applications.

Question 10

Milk of magnesia is a common antacid used to neutralize stomach acid. What is the active chemical ingredient in milk of magnesia?

Accepted Answer

Milk of magnesia contains magnesium hydroxide, which neutralizes hydrochloric acid in the stomach.

Question 11

When a piece of copper is exposed to moist air for a long time, a green coating forms on its surface. What is this green coating called?

Accepted Answer

The green coating on copper is called verdigris, which is a mixture of copper carbonate and copper hydroxide.

Question 12

Which gas is primarily responsible for the rusting of iron in the presence of moisture?

Accepted Answer

Oxygen (O₂) from the air reacts with iron in the presence of water to form iron oxide (rust), a process called oxidation that requires both oxygen and moisture.

Question 13

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 14

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

Accepted Answer

Baking soda, chemically sodium bicarbonate, has the formula NaHCO₃ and is widely used as a leavening agent in baking and as a mild abrasive cleaner.

Question 15

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

Accepted Answer

Water's polar nature (with partial positive and negative charges) allows it to dissolve both ionic compounds and many polar molecules, making it the most versatile solvent in nature and industry.

Question 16

Which gas is responsible for the characteristic smell of rotten eggs?

Accepted Answer

Hydrogen Sulphide (H₂S) is the gas responsible for the foul, rotten egg smell. It is produced during the decomposition of organic matter containing sulphur. It is a colourless, toxic gas. Sulphur dioxide (SO₂) has a pungent, suffocating smell; ammonia (NH₃) has a sharp, irritating smell; and chlorine (Cl₂) has a bleach-like smell.

Question 17

Which of the following reactions is an example of a decomposition reaction commonly observed in everyday life?

Accepted Answer

Decomposition reactions break down a single compound into simpler substances. Heating calcium carbonate (in chalk or limestone) to produce calcium oxide and carbon dioxide is a common example.

Question 18

Baking soda (sodium bicarbonate) is commonly used in baking. Which gas is released when baking soda reacts with an acid like vinegar?

Accepted Answer

Sodium bicarbonate reacts with acids to produce carbon dioxide gas, which causes baked goods to rise.

Question 19

Rust is formed when iron reacts with oxygen and moisture. Which of the following best describes the chemical composition of rust?

Accepted Answer

Rust is primarily hydrated iron(III) oxide, formed through oxidation of iron in the presence of water and oxygen.

Question 20

In the Haber process for ammonia synthesis, why is a high pressure (around 150–300 atm) necessary, even though the reaction is exothermic and theoretically favored at lower temperatures?

Accepted Answer

The Haber process: N₂ + 3H₂ ⇌ 2NH₃ (ΔH = −92 kJ/mol). High pressure shifts equilibrium toward products (Le Chatelier's principle) because the forward reaction reduces the number of gas molecules (4 moles → 2 moles). Although lower temperature favors the reaction thermodynamically, it slows the reaction kinetically. High pressure compensates by driving equilibrium toward ammonia formation, allowing practical production rates at moderate temperatures (400–500°C).

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