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CONCEPT:

Interconversion of States of Matter and Types of Changes

• Physical Change: A change in the state or appearance of a substance without altering its chemical composition. Example: Melting of ice.
• Chemical Change: A change that leads to the formation of a new substance with a different chemical composition. Example: Rusting of iron.
• The interconversion of states of matter (solid, liquid, gas) is a physical change because the chemical composition of the substance remains the same.

EXPLANATION:

• "The interconversion of states of matter is a physical change" – This is correct because the chemical composition does not change during state changes (e.g., melting, boiling).
• "During burning of a candle, both chemical and physical changes take place" – This is correct because melting of wax is a physical change, while combustion of wax is a chemical change.
• "During a physical change, the chemical composition of molecules changes" – This is NOT correct because, by definition, a physical change does not alter the chemical composition of a substance.
• "Rusting of iron is a chemical change" – This is correct because rusting involves the chemical reaction of iron with oxygen and water to form iron oxide.

Therefore, the correct answer is "During a physical change, the chemical composition of molecules changes".

CONCEPT:

Homogeneous and Heterogeneous Mixtures

• A homogeneous mixture is a mixture in which the components are uniformly distributed, and there is no visible boundary of separation between them. Examples include solutions like salt water or sugar water.
• A heterogeneous mixture is a mixture in which the components are not uniformly distributed, and the different components can often be seen as distinct phases. Examples include sandy water or oil and water.

EXPLANATION:

• Sulphur in carbon disulphide: This is a homogeneous mixture because sulphur dissolves completely in carbon disulphide, forming a single phase.
• Mixture of sugar and salt crystals: This is a heterogeneous mixture because the sugar and salt crystals remain as distinct solids and do not form a single uniform phase.
• Sandy water: This is a heterogeneous mixture because sand does not dissolve in water and forms a separate layer or particles in the water.
• Sulphur in water: This is a heterogeneous mixture because sulphur does not dissolve in water and appears as a separate solid phase.
• Among the options, only Option 1 (Sulphur in carbon disulphide) is a homogeneous mixture, while the rest are heterogeneous mixtures.

Therefore, the correct answer is Sulphur in carbon disulphide, as it is NOT a heterogeneous mixture.

EXPLANATION:

Atomicity

• Atomicity is defined as the number of atoms present in a single molecule of an element or compound.
• For elements:
• Phosphorus exists as P₄, so its atomicity is 4.
• Nitrogen exists as N₂, so its atomicity is 2.
• Neon is a noble gas and exists as monoatomic (Ne), so its atomicity is 1.
• Sulphur exists as S₈, so its atomicity is 8.
• Matching List I (Element) with List II (Atomicity):
• A. Phosphorus → Atomicity = 4 (Option 4)
• B. Nitrogen → Atomicity = 2 (Option 1)
• C. Neon → Atomicity = 1 (Option 2)
• D. Sulphur → Atomicity = 8 (Option 3)

Therefore, the correct answer is Option 2: A - 4, B - 1, C - 2, D - 3.

CONCEPT:

Redox Reaction and Reducing Agent

• In a redox reaction, one compound is oxidized (loses electrons) and another is reduced (gains electrons).
• The reducing agent is the compound that donates electrons (is oxidized) during the reaction.

EXPLANATION:

• In the given reaction:

N₂H₄ + 2H₂O₂ → N₂ + 4H₂O

• Hydrazine (N₂H₄) is oxidized to nitrogen gas (N₂). This means it loses electrons.
• Hydrogen peroxide (H₂O₂) is reduced to water (H₂O). This means it gains electrons.
• Since N₂H₄ is oxidized, it is the reducing agent, as it donates electrons to H₂O₂.

Therefore, the compound acting as the reducing agent is N₂H₄.

CONCEPT:

Purest Source of Water

• Water sources such as rivers, seas, wells, and rain have varying levels of purity based on their exposure to contaminants and natural filtration.
• The purity of water is determined by the absence of dissolved salts, impurities, and pollutants.
• Rainwater is considered the purest form of natural water because:
• It is formed by the process of evaporation and condensation, where water vapor rises from the earth's surface and condenses to form droplets.
• This process eliminates most impurities, resulting in water that is free from dissolved salts and major contaminants.

EXPLANATION:

• Each water source has distinct characteristics:
• River Water: Contains dissolved salts, organic matter, and pollutants from the surrounding environment.
• Sea Water: Highly saline due to dissolved salts, making it unsuitable for direct consumption.
• Well Water: May contain dissolved minerals and contaminants depending on the soil and rock layers it passes through.
• Rain Water: It is naturally distilled through evaporation and condensation, making it the purest form of water.
• Hence, rainwater is considered the purest source of water.

Therefore, the correct answer is Rain water.

CONCEPT:

Temporary Hardness of Water

• Temporary hardness of water is caused by the presence of dissolved bicarbonates of calcium and magnesium.
• These bicarbonates form when water interacts with carbon dioxide and minerals in the soil, resulting in soluble compounds.
• Temporary hardness can be removed by boiling the water, which decomposes the bicarbonates into insoluble carbonates.

EXPLANATION:

• Bicarbonates of magnesium are responsible for temporary hardness because:
• Bicarbonates dissolve in water and cause hardness.
• Boiling the water decomposes bicarbonates into insoluble magnesium carbonate, removing the hardness.
• Other salts (e.g., sulphates and chlorides) cause permanent hardness, which cannot be removed by boiling.

Therefore, the correct answer is Bicarbonates of magnesium.

CONCEPT:

Types of Chemical Reactions

• Combination Reaction: Two or more substances combine to form a single product.
• Decomposition Reaction: A single compound breaks down into two or more simpler substances.
• Single Displacement Reaction: One element replaces another element in a compound.
• Double Displacement Reaction: Exchange of ions occurs between two compounds forming new compounds.

EXPLANATION:

• A. CaO(s) + H₂O(l) → Ca(OH)₂(aq)
• This is a Combination Reaction because two reactants (CaO and H₂O) combine to form a single product (Ca(OH)₂).
• Type: Combination (3)
• B. CaCO₃(s) → CaO(s) + CO₂(g)
• This is a Decomposition Reaction because a single compound (CaCO₃) breaks down into two simpler substances (CaO and CO₂).
• Type: Decomposition (4)
• C. Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)
• This is a Single Displacement Reaction because Pb replaces Cu in CuCl₂ to form PbCl₂ and Cu.
• Type: Single Displacement (1)
• D. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
• This is a Double Displacement Reaction because ions are exchanged between Na₂SO₄ and BaCl₂ to form BaSO₄ and NaCl.
• Type: Double Displacement (2)

CONCEPT:

Endothermic and Exothermic Reactions

• An endothermic reaction is a chemical reaction that absorbs heat from its surroundings. The energy required to break the bonds of the reactants is greater than the energy released when the products are formed.
• An exothermic reaction, in contrast, releases heat to the surroundings as the energy released during the bond formation of products is greater than the energy required to break the bonds of the reactants.
• In endothermic reactions, the change in enthalpy (ΔH) is positive, while in exothermic reactions, ΔH is negative.

EXPLANATION:

• CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
• This is a combustion reaction of methane. Combustion reactions are typically exothermic as they release a large amount of heat.
• C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l)
• This is the process of cellular respiration, which is an exothermic reaction as energy is released in the form of ATP.
• N₂(g) + 3H₂(g) → 2NH₃(g)
• This is the Haber process for ammonia synthesis, which is exothermic as heat is released during the reaction.
• 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)
• This is the thermal decomposition of lead nitrate. Thermal decomposition reactions are endothermic as they require heat to break the bonds in the reactants.

Therefore, the correct answer is 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g).

CONCEPT:

Water Gas

• Water gas is a mixture of carbon monoxide (CO) and hydrogen gas (H₂).
• It is produced by passing steam over heated carbon (usually coke) in a controlled environment.
• The chemical reaction involved is:

C + H₂O → CO + H₂

• Water gas is widely used as a fuel and as a source of hydrogen for industrial purposes.

EXPLANATION:

• Water gas is primarily composed of carbon monoxide (CO) and hydrogen (H₂).
• It is different from other mixtures like carbon dioxide (CO₂), nitrogen-based compounds (e.g., NH₃ or H₂ + N₂).

​Therefore, the correct answer is CO + H₂.

CONCEPT:

Properties of Allotropes of Carbon

• Diamond, graphite, and fullerenes are allotropes of carbon, meaning they are different structural forms of the same element.
• The properties of these allotropes depend on their structure and bonding:
• Diamond: Has a three-dimensional network of covalent bonds, making it extremely hard. It is an electrical insulator as all its valence electrons are involved in bonding.
• Graphite: Has a layered structure with delocalized electrons between layers, making it a good conductor of electricity and an effective lubricant.
• Fullerenes: Consist of spherical or tubular molecules composed of carbon atoms arranged in five- and six-membered rings.

EXPLANATION:

• Diamond is hard due to extended covalent bonding in it.
• This is a correct statement. Diamond's hardness is due to its strong three-dimensional network of covalent bonds.
• Fullerene is composed of five- and six-membered rings.
• This is a correct statement. Fullerenes are made up of carbon atoms arranged in five- and six-membered rings, forming spherical or tubular shapes.
• Diamond is a good conductor of electricity.
• This is an incorrect statement. Diamond is an electrical insulator because all its valence electrons are used in covalent bonding, leaving no free electrons for conduction.
• Graphite is used as a lubricant.
• This is a correct statement. Graphite's layered structure allows layers to slide over each other, making it an excellent lubricant.

Therefore, the correct answer is Diamond is a good conductor of electricity.