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Dissolving sodium chloride in water is a physical change because the chemical composition of the salt (NaCl) and water (H₂O) does not change. The salt ions simply disperse within the water molecules. The original substances can be recovered by physical means, such as evaporating the water, which leaves the solid salt behind. Hence, no new substance is formed, and it is easily reversible. A. addition of water to calcium oxide is a chemical change. Calcium oxide (quicklime) reacts vigorously with water in an exothermic reaction to form a new substance, calcium hydroxide. B. corrosion of iron is a chemical change. Iron reacts with oxygen and moisture in the air to form iron oxide (rust), which is a new substance with different properties from the original iron. D. burning of charcoal is a chemical change. Burning (combustion) involves the reaction of charcoal (primarily carbon) with oxygen to produce new substances, such as carbon dioxide and possibly carbon monoxide, which cannot be easily reversed to get the original charcoal back. The correct option is C. dissolving sodium chloride in water.

The main components of air are nitrogen (~78%), oxygen (~21%), noble gases (~0.9%), carbon dioxide (~0.04%), and variable amounts of water vapour. After these steps, all oxygen, carbon dioxide, and water vapor are removed, leaving nitrogen and the noble gases (predominantly argon) as the remaining unabsorbed components. Recall that Air is a mixture of gases. These gases comprises mainly nitrogen, oxygen, carbon(IV) oxide, noble gases, and water vapour. Alkaline pyrogallol is used to absorb oxygen from the air. Potash (potassium carbonate, though potassium hydroxide is more common in this context) absorbs carbon(IV)oxide (carbon dioxide). Fused calcium chloride is a drying agent used to absorb water vapor (moisture). After passing the air sample through all three substances, the components of air that are left unabsorbed are the chemically inert noble gases (like argon, neon, etc.) and the relatively unreactive nitrogen, which together make up the bulk of the remaining gas. Correct answer is noble gases and nitrogen - Option C

The electron configuration provided is 1s² 2s² 2p⁶ 3s² 3p⁵. The outermost energy level (principal quantum number n = 3, or the third shell) contains the valence electrons. To find the number of valence electrons, the electrons in the 3s and 3p sub-shells are summed: 2 + 5 = 7 valence electrons. Elements with 7 valence electrons in their outermost shell belong to Group 7 (or Group 17 in modern IUPAC notation) of the periodic table, known as the halogens. The element with this configuration is chlorine (atomic number 17). The correct option is A - Group 7

A polar covalent bond occurs between atoms that have a significant difference in electronegativity. Electronegativity is an atom's ability to attract shared electrons towards itself. Due to this difference, the bonding electron pair (the electrons involved in the bond) spends more time closer to the more electronegative atom. This results in an unequal sharing of the electron pair, creating a partial negative charge (?−) on the more electronegative atom and a partial positive charge (?+) on the less electronegative atom. B. equal sharing of electron pair: This describes a nonpolar covalent bond, which typically occurs between atoms of the same element (e.g., in O\(_2\) or H\(_2\)) where the electronegativity difference is zero or negligible. C. equal sharing of electron lone pair: A lone pair (or unshared pair) is a pair of valence electrons that is not involved in bonding. The concept of "sharing" applies to bonding pairs. D. unequal sharing of electron lone pair: Lone pairs are associated with a single atom and are not shared between atoms in the context of a bond, so this option is chemically inaccurate in describing the bond itself. The correct option is A. unequal sharing of electron pair.

The cell notation convention places the anode on the left and the cathode on the right, separated by a double vertical line representing the salt bridge. A single vertical line represents a phase boundary (solid|aqueous). The reaction involves: Oxidation (anode, left side): Mg is oxidized to Mg²⁺ Mg → Mg\(^{2+}\) + 2e\(^-\). This is represented as Mg l Mg\(^{2+}\) Reduction (cathode, right side): Pb²⁺ is reduced to Pb, Pb\(^{2+}\) + 2e\(^-\) → Pb . This is represented as Pb\(^{2+}\) l Pb The complete notation is Mg l Mg\(^{2+}\) ll Pb\(^{2+}\) l Pb

According to Graham's Law of Diffusion, the rate at which a gas diffuses is inversely proportional to the square root of its molar mass. Therefore, gases with lower molar masses diffuse faster than those with higher molar masses. For the rate of diffusion to increase, the molar masses of the gases must decrease in the sequence listed. The molar masses for the gases in option B are calculated using the provided atomic weights (H=1, O=16, N=14, S=32): SO\(_2\)= 32 + (16 x 2) = 64 g/mol O\(_2\)= 16 x 2 =32 g/mol NH\(_3\) = 14 + (1 x 3) = 17 g/mol H\(_2\) = 1 x 2 = 2 g/mol The order of molar masses is 64 g/mol > 32 g/mol > 17 g/mol > 2 g/mol. Since the molar masses decrease in this sequence, the rate of diffusion increases. The correct arrangement of gases in the order of increasing rate of diffusion is B.

The solution with a pH of 8.0 is likely to turn red moist litmus paper blue, which corresponds to option B. Litmus paper is an indicator used to test for acidity or basicity: Acids (pH below 7) turn blue litmus paper red. The solutions with pH 2.5 and 3.5 are acidic. Bases (pH above 7, or alkaline) turn red litmus paper blue. The solution with pH 8.0 is basic. Neutral solutions (pH exactly 7.0) cause no color change in either red or blue litmus paper. Bases turn red litmus paper blue.Therefore, only the basic solution (pH 8.0) will turn red litmus paper blue.

Rusting is the specific term for the corrosion of iron (and its alloys like steel) that occurs when it reacts with oxygen and moisture in the air to form a reddish-brown, flaky substance called rust (hydrated iron(III) oxide). This is an electrochemical and oxidation process that deteriorates the metal. B. Burning is a combustion process involving rapid reaction with oxygen, usually producing heat and light, and forming different products (ashes, smoke, etc.), not the slow oxidation that results in rust. C. Galvanizing is a method of rust prevention, where a protective coating of zinc is applied to the iron, not the process of corrosion itself. D. Alloying is the process of mixing iron with other metals (like chromium and nickel to make stainless steel) to create a material that is more resistant to corrosion, not the corrosion process itself. The correct answer is rusting - option A

Solubility is expressed as the mass of solute per 100g of solvent. It can be expressed mathematically below as: Solubility = \(\frac{mass of solute}{mass of solvent}\) x \(\frac{100}{1}\) = \(\frac{mass of salt}{mass of water}\) x \(\frac{100}{1}\) Mass of salt = 2g Mass of water = Mass of solution - mass of salt = 12.5 - 2 = 10.5g Solubility = \(\frac{2}{10.5}\) x \(\frac{100}{1}\) = 19.0476 = 19.05 g

Propane, C\(_3\)H\(_8\) reacting with chlorine ((Cl\(_2\)) in sunlight is a free radical substitution reaction, also known as photochemical chlorination, where sunlight (UV light) provides energy to break the \(Cl-Cl\) bond, allowing chlorine atoms to replace hydrogen atoms on the propane molecule, forming chloropropanes and hydrogen chloride (HCl).

During the electrolysis of brine (sodium chloride solution), hydrogen gas (\(H_{2}\)) is produced at the cathode (negative electrode) because water molecules are more easily reduced than sodium ions (\(Na^{+}\)), forming hydrogen gas and hydroxide ions (\(OH^{-}\)). This process also generates sodium hydroxide (caustic soda) and chlorine gas at the anode, making it a crucial industrial process.

In the Orsat apparatus for flue gas analysis, ammoniacal cuprous chloride, CuCl\(_2\) is specifically used to absorb CO. The CO acts as a Lewis base and forms a soluble coordination complex or adduct with the copper(I) ions. A. \(\text{H}_{2}\) (Hydrogen) and C. \(\text{N}_{2}\) (Nitrogen) are generally unreactive with the solution and remain after the absorption process. These are typically determined by difference in a gas analysis experiment. D. \(\text{CO}_{2}\) (Carbon dioxide) is absorbed by a different solution, typically an alkaline solution like potassium hydroxide (\(\text{KOH}\)), in the Orsat apparatus.

Water exists as a liquid at room temperature primarily due to the presence of strong hydrogen bonds between adjacent water molecules. Hydrogen bonds are a strong type of intermolecular force (force between molecules) that cause water molecules to attract each other and resist separation into a gas phase at typical room temperatures. A. electrovalent bond (ionic bond) is a strong force that exists between ions, like in table salt (NaCl), not within or between water molecules. Water is a polar covalent compound. B. covalent bond is the strong intramolecular force that holds the hydrogen and oxygen atoms together within a single water molecule (\(\text{H}_{2}\text{O}\)). The existence of the molecule itself depends on the covalent bonds, but the liquid state (how molecules interact with each other) is due to the intermolecular hydrogen bonds. C. van der waals forces are weaker intermolecular forces present in all molecules. While present, they are much weaker than hydrogen bonds. If only van der Waals forces were present, water would be a gas at room temperature, similar to other hydrides like hydrogen sulfide (\(\text{H}_{2}\text{S}\)).

Reforming, specifically catalytic reforming, is a petroleum refining process used to convert low-octane, straight-chain or cyclic hydrocarbons into high-octane aromatic hydrocarbons. This process improves the quality of gasoline (petrol) and involves dehydrocyclization (cyclization and dehydrogenation). Cyclohexane (\(\text{C}_{6}\text{H}_{12}\)) is a cycloalkane that is converted into the aromatic compound benzene (\(\text{C}_{6}\text{H}_{6}\)) during this process, with the release of hydrogen gas (\(\text{H}_{2}\)). This is a key example of a reforming reaction.The general reaction is: \(\text{C}_{6}\text{H}_{12}\rightarrow \text{C}_{6}\text{H}_{6}+3\text{H}_{2}\). A. hexene to hexane is a hydrogenation reaction (addition of hydrogen to an alkene to form an alkane), not reforming. Reforming typically involves dehydrogenation (removal of hydrogen) and cyclization. C. hexane to carbon(IV)oxide and water vapour is the complete combustion of hexane, which produces energy but is not a petroleum refining process for product reformation. D. hexane to butane and ethane is an example of cracking, a process that breaks down larger hydrocarbons into smaller, more valuable ones, which is distinct from reforming. The correct answer is option B

Carbon(II)oxide, (CO), is primarily a product of the incomplete combustion of carbon-containing fuels such as natural gas, coal, wood, and oil. When there is an inadequate supply of oxygen during burning, CO is produced instead of the less harmful carbon dioxide (CO₂). A. photochemical smog is a type of air pollution resulting from the reaction of sunlight with nitrogen oxides and volatile organic compounds; carbon monoxide is involved in the complex cycles of reactions, but it is a primary pollutant produced by combustion, not the source of the smog itself. The primary components of photochemical smog are secondary pollutants like ozone and peroxyacetyl nitrate (PAN). B. respiration is a natural process in living organisms that produces carbon dioxide (CO₂) as a byproduct, not carbon monoxide (CO). CO₂ is an essential component of the carbon cycle and not typically considered an air pollutant in the same toxicological sense as CO (though it is a major greenhouse gas). D. decomposition of sewage involves biological processes that can produce various gases, such as methane (CH₄) or hydrogen sulfide, but not significant amounts of carbon monoxide as an air pollutant. The correct option is C. combustion of fuel.