Q74.Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A :
Water is an excellent solvent.
Reason R :
Water has high dielectric constant.
In the light of the above statements, choose the most appropriate answer from the options given below :
(1) Both A and R are correct and R is the correct explanation of A
(2) Both A and R are correct but R is not the correct explanation of A
(3) A is correct but R is not correct
(4) A is not correct but R is correct
The correct answer is option (1): Both A and R are correct and R is the correct explanation of A.
Statement Analysis
Assertion A is true—water dissolves more substances than any other liquid (“universal solvent”) due to its polarity and hydrogen bonding. Reason R is true—water’s dielectric constant (ε ≈ 78-80 at 25°C) is exceptionally high among common liquids. R directly explains A: high dielectric constant reduces electrostatic attraction between ions (F ∝ 1/ε per Coulomb’s law), enabling dissociation of ionic compounds like NaCl into solvated Na⁺ and Cl⁻.
Option Breakdown
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Option (1): Correct—high ε weakens ion pairing, making solvation thermodynamically favorable.
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Option (2): Wrong—R causally explains solvent power via charge screening.
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Option (3): Wrong—R accurately states water’s dielectric property (~80 vs. ethanol ~25).
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Option (4): Wrong—A universally accepted; polarity + high ε define solvent excellence.
Introduction: Water Is an Excellent Solvent High Dielectric Constant
NEET physical chemistry tests solvent properties via assertions like “water is an excellent solvent” explained by “high dielectric constant.” Option (1) links Coulombic force reduction (F ∝ 1/ε) to ionic dissolution, core to solution chemistry. This solves A+R causality with molecular mechanism.
Why High Dielectric Constant Makes Water Universal Solvent
Water’s εr ≈ 78 screens charges:
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Ionic compounds: NaCl → Na⁺(aq) + Cl⁻(aq); hydration shell stabilizes separated ions.
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Polar molecules: Dipole-dipole + H-bonding dissolve sugars, alcohols.
Coulomb’s law: F = kq₁q₂/(εr²) → high εr weakens lattice energy.
Dielectric Constant Comparison
Option (1) Causal Link Confirmed
Trap: Thinking polarity alone suffices—ε quantifies screening efficiency. High ε + H-bonding = synergistic solvent power.
NEET Prep Tip: Mnemonic—”ε Shields, Water Yields Ions.” Link to Born solvation energy (ΔG ∝ 1/εr). Practice: Compare NH4Cl solubility in H2O vs. C6H6.
