Question 1

Which gas is released when an acid reacts with a metal carbonate?

Accepted Answer

Carbon dioxide (CO₂) gas is released when an acid reacts with a metal carbonate. The general reaction is: Metal Carbonate + Acid → Salt + Water + CO₂↑

Question 2

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 H⁺ ions equals the concentration of OH⁻ ions.

Question 3

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 states.

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 its composition. It is a simple hydrocarbon and the most abundant constituent that provides the energy for combustion in household appliances.

Question 5

What is the chemical formula of common salt (table salt)?

Accepted Answer

Common salt, also known as table salt or sodium chloride, has the chemical formula NaCl. It is formed by the ionic bond between sodium (Na) and chlorine (Cl) ions. It is widely used in food preservation and cooking.

Question 6

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

Accepted Answer

Carbon dioxide (CO₂) gas is released when an acid reacts with a carbonate or bicarbonate compound. For example: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂↑. This reaction is commonly observed as fizzing or effervescence.

Question 7

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

Accepted Answer

Baking soda is sodium bicarbonate, with the chemical formula NaHCO₃. It is widely used in cooking and as a mild base.

Question 8

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 hydrocarbon and the simplest alkane.

Question 9

Which gas is primarily responsible for the greenhouse effect and is produced by burning fossil fuels?

Accepted Answer

Carbon dioxide (CO₂) is the primary greenhouse gas responsible for global warming. It is produced by combustion of fossil fuels (coal, oil, natural gas), deforestation, and industrial processes. CO₂ absorbs infrared radiation and traps heat in the atmosphere.

Question 10

Which of the following is the primary reason why water is used as a universal solvent in everyday life and laboratory settings?

Accepted Answer

Water is called the universal solvent because of its polar nature. The bent molecular structure of H₂O creates a partial positive charge on hydrogen atoms and a partial negative charge on oxygen atoms. This polarity allows water to dissolve a wide variety of ionic compounds, salts, and polar substances by surrounding and separating their ions or molecules. This property makes water essential for biological processes, industrial applications, and chemical reactions.

Question 11

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

Accepted Answer

A physical change is a change in the physical properties of matter (shape, size, state, color) without altering its chemical composition. Examples include melting, freezing, evaporation, condensation, and dissolving. The substance remains chemically the same.

Question 12

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⁻). It is essential for human health, food preservation, and industrial applications.

Question 13

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

Accepted Answer

A decomposition reaction is one in which a single compound breaks down into two or more simpler substances. The thermal decomposition of calcium carbonate (CaCO₃ → CaO + CO₂) is a classic example.

Question 14

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. 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 in water testing, food preservation, and medicine.

Question 15

Which gas is responsible for the green colour of leaves in plants?

Accepted Answer

Chlorophyll is the green pigment found in the chloroplasts of plant cells. It contains a magnesium ion at its centre and is responsible for the green colour of leaves. It absorbs sunlight (mainly red and blue wavelengths) and reflects green light, which is why leaves appear green.

Question 16

Which of the following is a non-metal that is liquid at room temperature (25°C)?

Accepted Answer

Bromine (Br₂) is the only non-metal that exists as a liquid at room temperature and atmospheric pressure. It is a reddish-brown liquid with a pungent odour and is highly toxic.

Question 17

Rusting of iron is an example of which type of chemical reaction?

Accepted Answer

Rusting of iron (formation of iron oxide) is a redox (oxidation-reduction) reaction where iron is oxidized and oxygen is reduced. It is also an exothermic reaction that occurs slowly at room temperature in the presence of moisture and oxygen.

Question 18

Which of the following statements about alloys is CORRECT?

Accepted Answer

An alloy is a mixture of two or more elements where at least one is a metal. Alloys are NOT chemical compounds because the constituent elements are not in fixed stoichiometric ratios and are not held together by chemical bonds in the traditional sense. Brass (Cu + Zn) and steel (Fe + C) are common alloys. Alloys typically have different physical and chemical properties compared to their constituent elements, often with improved hardness, strength, or corrosion resistance.

Question 19

Which of the following is the correct statement about the process of neutralization in acid-base chemistry?

Accepted Answer

Neutralization is a chemical reaction between an acid and a base that produces a salt and water. The general equation is: Acid + Base → Salt + Water. For example, HCl + NaOH → NaCl + H₂O. At the equivalence point, the moles of H⁺ ions from the acid equal the moles of OH⁻ ions from the base. The resulting salt may be acidic, basic, or neutral depending on the strength of the acid and base involved. The reaction is exothermic (releases heat).

Question 20

In the preparation of oxygen gas in the laboratory using hydrogen peroxide (H₂O₂) and manganese dioxide (MnO₂), what is the role of manganese dioxide, and what is the balanced chemical equation?

Accepted Answer

Manganese dioxide acts as a catalyst in the decomposition of hydrogen peroxide. It speeds up the reaction without being consumed. The balanced equation is: 2H₂O₂ → 2H₂O + O₂↑ (with MnO₂ as catalyst).

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