Answer: (A) Proton + Neutron

Explanation

The mass of an atom is determined mainly by the protons and neutrons present in its nucleus. Electrons have a negligible mass (about 1/1836 of the mass of a proton), so they do not significantly contribute to the atomic mass. Therefore, the mass number = number of protons + number of neutrons.

Exam Facts

• Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N).
• Atomic Number (Z) is the number of protons and determines the identity of an element.
• Nearly 99.9% of an atom’s mass is concentrated in the nucleus.
• Proton: Positive charge (+1), mass ≈ 1 amu.
• Neutron: No charge, mass ≈ 1 amu.
• Electron: Negative charge (−1), mass ≈ 1/1836 amu; its contribution to atomic mass is negligible.
• Ernest Rutherford discovered the atomic nucleus (1911), while James Chadwick discovered the neutron (1932).
• Isotopes have the same atomic number but different mass numbers due to different numbers of neutrons.

Answer: (C) Hydrogen bonding

Explanation

Hydrogen bonding is present in H₂O and HF because hydrogen is directly bonded to highly electronegative atoms (oxygen and fluorine). In H₂S, sulfur is less electronegative and larger in size, so it cannot form significant hydrogen bonds. Hence, H₂S lacks intermolecular hydrogen bonding.

Exam Facts

• Hydrogen bonding occurs when hydrogen is bonded to F, O, or N (highly electronegative atoms).
• Strength of hydrogen bonding: HF > H₂O > NH₃.
• Hydrogen bonding is responsible for the high boiling point of water (100°C) and HF compared to H₂S.
• Ice is less dense than liquid water due to its hydrogen-bonded open cage structure.
• H₂S contains polar covalent bonds but does not exhibit hydrogen bonding.
• Intermolecular hydrogen bonding increases boiling point, melting point, viscosity, and surface tension.
• Hydrogen bonding plays a vital role in the structure of DNA (base pairing) and proteins.

Answer: (C) 44

Explanation

The molecular weight of MCl₄ = 154.

Mass of element M = 154 − (4 × 35.5) = 154 − 142 = 12.

Since M forms MCl₄, its valency is 4, so its oxide is MO₂.

Molecular weight of MO₂ = 12 + (2 × 16) = 44.

Exam Facts

• Valency of an element is determined from the formula of its compounds.
• If the chloride is MCl₄, the valency of M = 4.
• Oxide of a tetravalent element has the formula MO₂.
• Atomic mass of oxygen = 16 u, chlorine = 35.5 u.
• Molecular mass is the sum of the atomic masses of all atoms in a molecule.
• Carbon (atomic mass = 12) forms CCl₄ and CO₂, matching the data in this question.

Answer: (B) 2n

Explanation

The molar mass of N₂ = 28 g/mol, so 14 g = 14/28 = 0.5 mole, which contains n molecules. The molar mass of NO₂ = 46 g/mol, so 46 g = 1 mole, which contains 2n molecules because 1 mole is twice 0.5 mole. Therefore, the correct answer is (B) 2n.

Exam Facts

• Avogadro’s number = 6.022 × 10²³ molecules/mol.
• Molar mass of N₂ = 28 g/mol (14 × 2).
• Molar mass of NO₂ = 46 g/mol (14 + 16 × 2).
• Number of molecules = Number of moles × Avogadro’s number.
• 0.5 mole contains half of Avogadro’s number of molecules.
• 1 mole of any substance always contains the same number of molecules, irrespective of the substance.

Answer: (D) All of the above

Explanation

Elements such as fluorine, iodine, and chlorine are not naturally radioactive elements, but each has one or more radioactive isotopes. Examples include Fluorine-18 (¹⁸F), Iodine-131 (¹³¹I), and Chlorine-36 (³⁶Cl). Therefore, all the given elements possess radioactive isotopes.

Exam Facts

• Radioisotopes are isotopes with unstable nuclei that emit α, β, or γ radiation.
• Iodine-131 (¹³¹I) is widely used in the diagnosis and treatment of thyroid disorders and thyroid cancer.
• Fluorine-18 (¹⁸F) is used in PET (Positron Emission Tomography) scans.
• Chlorine-36 (³⁶Cl) is used in groundwater dating and geological studies.
• Carbon-14 (¹⁴C) is a radioactive isotope used for radiocarbon dating of archaeological remains.
• Naturally radioactive elements generally have atomic number greater than 82 (exceptions include naturally occurring radioisotopes such as potassium-40).
• The SI unit of radioactivity is the Becquerel (Bq), while the older unit is the Curie (Ci).

Answer: (B) Aluminium metal

Explanation

In the thermit (aluminothermic) reaction, aluminium powder reduces iron(III) oxide to molten iron while itself gets oxidized to aluminium oxide. Since aluminium causes the reduction of iron oxide, it acts as the reducing agent. The molten iron produced is used for welding and repairing railway tracks.

Reaction:$$\mathrm{Fe_2O_3 + 2Al \rightarrow Al_2O_3 + 2Fe + Heat}$$

Exam Facts

• Reducing agent: Aluminium (Al).
• Oxidizing agent: Iron(III) oxide (Fe₂O₃).
• The thermit reaction is a highly exothermic reaction, producing temperatures of about 2500–3000°C.
• The process is also known as the Goldschmidt process or Aluminothermy.
• It is widely used for welding railway tracks, repairing heavy steel machinery, and extracting certain metals.
• Aluminium is a strong reducing agent because of its high affinity for oxygen.
• The products are molten iron (Fe) and aluminium oxide (Al₂O₃).

Answer: (D) All of the above

Explanation

Glucose, urea, and acetone are all soluble in water. Glucose and urea contain polar functional groups that form hydrogen bonds with water, while acetone is a polar organic solvent that is completely miscible with water. Hence, all the given substances dissolve in water.

Exam Facts

• Water is called the universal solvent because it dissolves a large number of ionic and polar substances.
• Glucose (C₆H₁₂O₆) is highly soluble in water due to its multiple –OH (hydroxyl) groups.
• Urea [CO(NH₂)₂] is highly soluble in water and is the most widely used nitrogenous fertilizer.
• Acetone (CH₃COCH₃) is a polar solvent and is completely miscible with water.
• “Like dissolves like”: Polar substances dissolve readily in polar solvents such as water.
• Common water-insoluble substances include benzene, kerosene, petrol, and oils, which are non-polar.
• Solubility depends on factors such as polarity, temperature, and pressure (pressure mainly affects gases).

Answer: (D) All of the above

Explanation

When magnesium burns in oxygen, it forms magnesium oxide (MgO) with the release of a large amount of heat and bright white light. It is a combination reaction because two reactants combine to form a single product, an oxidation reaction because magnesium gains oxygen, and an exothermic reaction because energy is released.

Balanced Equation:$$\mathbf{2Mg + O_2 \rightarrow 2MgO + \text{Heat}}$$

Exam Facts

• Combination reaction: Two or more substances combine to form a single product.
• Oxidation: Addition of oxygen or removal of hydrogen; in terms of electrons, it is loss of electrons.
• Reduction: Removal of oxygen or gain of hydrogen; in terms of electrons, it is gain of electrons.
• Exothermic reactions release heat to the surroundings, whereas endothermic reactions absorb heat.
• During the reaction, magnesium is oxidized and oxygen is reduced; hence it is a redox reaction.
• Magnesium burns with a dazzling white flame, producing white magnesium oxide (MgO) powder.
• Magnesium ribbon is cleaned before burning to remove the MgO layer formed due to reaction with air.

Answer: (B) Yellow precipitation of lead iodide is formed.

Explanation

When potassium iodide (KI) solution is added to lead nitrate [Pb(NO₃)₂] solution, a yellow precipitate of lead(II) iodide (PbI₂) is formed. This is a double displacement (precipitation) reaction, where insoluble PbI₂ separates from the solution.

Reaction:$$\mathbf{Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3}$$

Exam Facts

• Lead(II) iodide (PbI₂) is a bright yellow precipitate, often called “Golden Rain” crystals when recrystallized from hot water.
• This is a double displacement (metathesis) as well as a precipitation reaction.
• Potassium nitrate (KNO₃) formed in the reaction remains soluble in water.
• Lead nitrate [Pb(NO₃)₂] is one of the few soluble lead salts.
• Lead iodide (PbI₂) is insoluble in cold water but dissolves in hot water and crystallizes on cooling.
• Lead compounds are toxic and should be handled with care in laboratories.

Answer: (B) 10

Explanation

The atomic number (Z) = Mass number − Number of neutrons = 27 − 14 = 13. Therefore, the neutral atom has 13 protons and 13 electrons. Since the ion has a 3+ charge, it has lost 3 electrons, so the number of electrons in the ion is 13 − 3 = 10.

Exam Facts

• Atomic Number (Z) = Number of Protons = Number of Electrons (in a neutral atom).
• Mass Number (A) = Number of Protons + Number of Neutrons.
• A cation is formed by loss of electrons, while an anion is formed by gain of electrons.
• The given atom has Z = 13, which is Aluminium (Al).
• Aluminium commonly forms the Al³⁺ ion by losing its three valence electrons.
• Protons and neutrons are present in the nucleus, while electrons revolve around the nucleus.
• A 3+ ion has three fewer electrons than protons.

Answer: (C) Sodium hydrogen carbonate and tartaric acid

Explanation

Baking soda is pure sodium hydrogen carbonate (NaHCO₃) and is not a mixture. Baking powder is a mixture of sodium hydrogen carbonate (NaHCO₃) and tartaric acid (often with starch as a drying agent). Therefore, based on the given options, Option (C) is the correct answer, but the question contains a terminology error.

Exam Facts

• Baking soda: Sodium hydrogen carbonate (NaHCO₃).
• Baking powder: Mixture of NaHCO₃ + tartaric acid (often with starch).
• On heating, baking soda decomposes: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O.
• Carbon dioxide (CO₂) released during baking makes cakes and bread soft and porous.
• Tartaric acid prevents the formation of a bitter taste by neutralizing sodium carbonate (Na₂CO₃) produced during heating.
• Washing soda: Sodium carbonate decahydrate (Na₂CO₃·10H₂O).
• Common salt: Sodium chloride (NaCl); Caustic soda: Sodium hydroxide (NaOH).

Answer: (B) 2n

Explanation

The molar mass of water (H₂O) is 18 g/mol, so 9 g = 0.5 mole, which contains n molecules. The molar mass of carbon dioxide (CO₂) is 44 g/mol, so 44 g = 1 mole, which contains 2n molecules, since 1 mole is twice 0.5 mole.

Exam Facts

• Avogadro’s number = 6.022 × 10²³ molecules per mole.
• Molar mass of H₂O = 18 g/mol (2 × 1 + 16).
• Molar mass of CO₂ = 44 g/mol (12 + 2 × 16).
• Number of molecules = Number of moles × Avogadro’s number.
• 0.5 mole contains half of Avogadro’s number of molecules.
• 1 mole of any substance always contains the same number of molecules, irrespective of its chemical nature.

Answer: (B) Magnesium

Explanation

According to Newlands’ Law of Octaves, when elements are arranged in increasing order of their atomic masses, every eighth element has properties similar to the first, just as notes repeat after every eighth note in music. In this arrangement, beryllium (Be) resembles magnesium (Mg) because both belong to the same group and exhibit similar chemical properties.

Exam Facts

• Newlands’ Law of Octaves was proposed by John Alexander Reina Newlands in 1865.
• It states that every eighth element has properties similar to the first when arranged in increasing atomic mass.
• Beryllium (Be) and Magnesium (Mg) are Group 2 (Alkaline Earth Metals).
• The law worked well only for lighter elements, approximately up to calcium.
• It failed because no space was left for undiscovered elements, and some dissimilar elements were grouped together.
• Dmitri Mendeleev’s Periodic Table (1869) overcame many limitations of Newlands’ classification by leaving gaps for undiscovered elements.
• The Modern Periodic Table is based on atomic number, proposed by Henry Moseley (1913).

Answer: (B) Carbon-14 (¹⁴₆C)

Explanation

Carbon-14 (¹⁴C) is a radioactive isotope used in radiocarbon dating to determine the age of archaeological and historical objects containing organic matter, such as wood, bones, charcoal, and cloth. It has a half-life of about 5,730 years, making it suitable for dating objects up to about 50,000 years old.

Exam Facts

• Radiocarbon dating was developed by Willard F. Libby, who received the 1960 Nobel Prize in Chemistry.
• Carbon-14 has a half-life of 5,730 years.
• It is used to date organic materials such as wood, fossils, bones, paper, and charcoal.
• Uranium-235 and Uranium-238 are used for dating very old rocks and minerals (geological dating), not archaeological artifacts.
• Tritium (³H) is used as a tracer in scientific research and in self-luminous devices.
• Oxygen-18 (¹⁸O) is a stable isotope used in climate studies, hydrology, and paleoclimatology.
• Carbon-14 is continuously formed in the upper atmosphere when cosmic rays interact with nitrogen-14.

Answer: (D) Butane

Explanation

Liquefied Petroleum Gas (LPG) mainly consists of butane (C₄H₁₀) along with propane (C₃H₈). In domestic LPG used in India, butane is generally the chief component, while the proportion of propane is adjusted according to climatic conditions. Therefore, butane is the correct answer.

Exam Facts

• LPG stands for Liquefied Petroleum Gas.
• It mainly contains butane (C₄H₁₀) and propane (C₃H₈).
• Butane is usually the major component of domestic LPG; propane is added in higher proportions in colder regions because it vaporizes more easily.
• Methane (CH₄) is the chief component of Natural Gas and CNG (Compressed Natural Gas), not LPG.
• Ethyl mercaptan (Ethanethiol) is added to LPG to give it a distinct smell, helping detect leaks.
• LPG is a clean-burning fuel used for domestic cooking, heating, and industrial purposes.
• LPG is stored as a liquefied gas under moderate pressure in cylinders.

Answer: (C) Copper and Tin

Explanation

Bronze is an alloy primarily composed of copper and tin. It is harder, stronger, and more corrosion-resistant than pure copper, making it suitable for statues, coins, medals, bearings, and machine parts.

Exam Facts

• Bronze = Copper (Cu) + Tin (Sn).
• Brass = Copper (Cu) + Zinc (Zn) — a commonly confused alloy in competitive exams.
• Solder = Lead (Pb) + Tin (Sn) (traditionally; modern lead-free solders are also common).
• Steel = Iron (Fe) + Carbon (C).
• Stainless steel contains iron, chromium, nickel, and carbon, making it resistant to rust.
• The Bronze Age was characterized by the widespread use of bronze tools and weapons before the Iron Age.
• Bronze is widely used for statues, bells, medals, bearings, ship propellers, and coins due to its strength and corrosion resistance.

Answer: (A) F⁻

Explanation

An oxide ion (O²⁻) is formed when an oxygen atom (atomic number = 8) gains 2 electrons, so it has 10 electrons. A fluoride ion (F⁻) is formed when a fluorine atom (atomic number = 9) gains 1 electron, giving it 10 electrons as well. Therefore, O²⁻ and F⁻ are isoelectronic.

Exam Facts

• Isoelectronic species are atoms or ions having the same number of electrons but different nuclear charges.
• O²⁻ = 10 electrons (8 + 2).
• F⁻ = 10 electrons (9 + 1).
• Other common 10-electron isoelectronic species are Ne, Na⁺, Mg²⁺, Al³⁺, N³⁻.
• Cl⁻ has 18 electrons, while K⁺ also has 18 electrons; hence they are isoelectronic with Ar, not with O²⁻.
• Li⁺ has only 2 electrons, making it isoelectronic with He.
• In an isoelectronic series, ionic radius decreases as nuclear charge (atomic number) increases.

Answer: (D) Solid CO₂

Explanation

Dry ice is the solid form of carbon dioxide (CO₂). It is called “dry” because it does not melt into a liquid; instead, it changes directly from solid to gas by sublimation at −78.5°C. This property makes it useful as a cooling agent without leaving any liquid residue.

Exam Facts

• Dry ice = Solid carbon dioxide (CO₂).
• Dry ice sublimes directly from solid to gas at −78.5°C under atmospheric pressure.
• Sublimation is the process in which a solid changes directly into a gas without passing through the liquid state.
• Dry ice is widely used for preserving food, transporting vaccines and medicines, and creating artificial fog in stage shows.
• Ordinary ice is solid water (H₂O) and melts at 0°C, whereas dry ice does not melt under normal atmospheric conditions.
• Carbon dioxide is colorless, odorless, non-flammable, and denser than air.
• Dry ice should be handled with insulated gloves because it can cause severe cold burns (frostbite).

Answer: (D) Sn and Pb

Explanation

Solder is a fusible alloy traditionally made of tin (Sn) and lead (Pb). It has a low melting point, allowing it to melt easily and join metal parts without melting the metals being joined. Hence, it is widely used in electrical and plumbing work.

Exam Facts

• Solder = Tin (Sn) + Lead (Pb) (traditional soft solder).
• The common composition is 60% tin and 40% lead (60/40 solder), though other ratios are also used.
• Lead-free solder (mainly tin with silver and copper) is increasingly used due to environmental and health concerns.
• Brass = Copper + Zinc (Cu + Zn).
• Bronze = Copper + Tin (Cu + Sn).
• Steel = Iron + Carbon (Fe + C).
• Solder is used for joining electrical wires, electronic components, and metal sheets because of its low melting point.

Answer: (A) Sulphur

Explanation

When an onion is cut, its cells are damaged, releasing enzymes that convert sulfur-containing compounds into a volatile gas called syn-Propanethial-S-oxide. This gas reaches the eyes and reacts with moisture to form a mild irritant, stimulating the tear glands and causing tears.

Exam Facts

• The tear-producing compound in onions is syn-Propanethial-S-oxide (lachrymatory factor).
• It is formed from sulfur-containing amino acid derivatives present in onion cells.
• The enzyme responsible for its formation is lachrymatory factor synthase (LFS).
• Onions belong to the Allium genus, which also includes garlic, leek, and shallot.
• Garlic also contains sulfur compounds such as allicin, responsible for its characteristic odor and medicinal properties.
• Chilling onions before cutting or cutting them under running water reduces the release of the irritant gas.
• Sulfur compounds are responsible for the pungent smell and taste of onions and garlic.

Answer: (D) Allotropes

Explanation

Diamond and graphite are allotropes of carbon. Allotropes are different structural forms of the same element existing in the same physical state but having different arrangements of atoms, resulting in different physical properties. Hence, the correct answer is allotropes.

Exam Facts

• Allotropy is the property of an element to exist in two or more different structural forms in the same physical state.
• Major allotropes of carbon include diamond, graphite, fullerene (C₆₀), graphene, and carbon nanotubes.
• Diamond is the hardest naturally occurring substance and is used in cutting and drilling tools.
• Graphite is soft, slippery, and a good conductor of electricity due to the presence of free electrons.
• Graphene is a single layer of graphite and is one of the strongest known materials.
• Isotopes are atoms of the same element with the same atomic number but different mass numbers (e.g., ¹²C, ¹³C, ¹⁴C).
• Isotones are atoms of different elements having the same number of neutrons.

Answer: (B) Oxidizing

Explanation

Potassium permanganate (KMnO₄) is a strong oxidizing agent. It purifies water by oxidizing organic impurities, iron, manganese, hydrogen sulfide, and certain microorganisms, thereby improving water quality. Hence, its purification action is mainly due to its oxidizing property.

Exam Facts

• Chemical formula: KMnO₄ (Potassium permanganate).
• It is commonly known as Condy’s crystals.
• KMnO₄ is a strong oxidizing agent, especially in acidic medium.
• It is used in water treatment, disinfection, wound cleaning (dilute solution), and qualitative redox titrations.
• The characteristic color of potassium permanganate is deep purple (violet).
• In acidic medium, permanganate ion (MnO₄⁻) is reduced to Mn²⁺ during oxidation reactions.
• Chlorination using chlorine or bleaching powder is the most common large-scale method of drinking water disinfection, while KMnO₄ is used for specific oxidation purposes.

Answer: (D) Fossils

Explanation

Radiocarbon (Carbon-14) dating is used to determine the age of once-living organic materials, such as fossils, bones, wood, charcoal, and plant remains. It measures the decay of Carbon-14 (¹⁴C), a radioactive isotope, to estimate the time since the organism died. Hence, among the given options, fossils is the correct answer.

Exam Facts

• Carbon-14 (¹⁴C) is the radioactive isotope used in radiocarbon dating.
• Half-life of Carbon-14 = 5,730 years.
• Radiocarbon dating is effective for samples up to about 50,000 years old.
• It is used to date wood, charcoal, bones, fossils, cloth, paper, and other organic remains.
• Willard F. Libby developed the radiocarbon dating technique and received the 1960 Nobel Prize in Chemistry.
• Uranium-Lead and Potassium-Argon dating methods are used for very old rocks and minerals, not recent organic remains.
• Carbon-14 is continuously produced in the upper atmosphere by the interaction of cosmic rays with nitrogen-14.

Answer: (C) Henri Becquerel

Explanation

Henri Becquerel discovered natural radioactivity in 1896 while studying uranium salts. He observed that uranium emitted invisible rays capable of exposing photographic plates even without sunlight. This discovery marked the beginning of the study of radioactivity.

Exam Facts

• Henri Becquerel discovered natural radioactivity in 1896.
• Marie Curie and Pierre Curie discovered the radioactive elements Polonium (1898) and Radium (1898).
• Ernest Rutherford classified radioactive emissions into alpha (α) and beta (β) rays and is known as the Father of Nuclear Physics.
• Paul Villard discovered gamma (γ) rays in 1900.
• Enrico Fermi discovered artificial (induced) radioactivity using neutron bombardment and made major contributions to nuclear reactor development.
• The SI unit of radioactivity is the Becquerel (Bq), named in honor of Henri Becquerel. The older unit is the Curie (Ci).
• Natural radioactivity occurs spontaneously in unstable nuclei such as uranium, thorium, and radium, whereas artificial radioactivity is induced by nuclear reactions.

Answer: (D) NaNO₃

Explanation

Sodium nitrate (NaNO₃), commonly known as Chile saltpetre, is a nitrogenous fertilizer. It supplies nitrate (NO₃⁻) ions, which are readily absorbed by plants for growth. The other options are elemental substances and are not used directly as fertilizers.

Exam Facts

• Sodium nitrate (NaNO₃) is known as Chile saltpetre because it occurs naturally in Chile.
• It is a nitrogenous fertilizer that provides readily available nitrate nitrogen to plants.
• Urea [CO(NH₂)₂] is the most widely used nitrogenous fertilizer in India (46% nitrogen).
• Ammonium sulfate [(NH₄)₂SO₄] and ammonium nitrate (NH₄NO₃) are also important nitrogenous fertilizers.
• Fertilizers are classified into nitrogenous, phosphatic, and potassic fertilizers.
• NPK fertilizers supply the three major plant nutrients: Nitrogen (N), Phosphorus (P), and Potassium (K).
• Nitrogen (N₂) constitutes about 78% of Earth’s atmosphere, but plants cannot use atmospheric nitrogen directly.

Answer: (B) Propane and butane

Explanation

Liquefied Petroleum Gas (LPG) is primarily a mixture of propane (C₃H₈) and butane (C₄H₁₀). The proportion of these gases varies depending on climate and intended use. LPG is widely used as a domestic cooking fuel because it burns efficiently with minimal smoke.

Exam Facts

• LPG = Propane (C₃H₈) + Butane (C₄H₁₀).
• Methane (CH₄) is the chief component of Natural Gas and CNG (Compressed Natural Gas), not LPG.
• Ethyl mercaptan (Ethanethiol) is added to LPG to impart a distinctive smell, helping detect gas leaks.
• LPG is stored in cylinders as a liquefied gas under pressure.
• CNG is mainly methane and is considered a cleaner automotive fuel than petrol or diesel.
• PNG (Piped Natural Gas) supplied to homes also consists mainly of methane.
• LPG has a high calorific value, making it an efficient fuel for domestic and industrial applications.