Question

Water is a liquid over the range of temperatures most compatible with life. The energy required to convert one mole of liquid water to one mole of water vapor is called

• A. Heat of vaporization
• B. Dielectric constant
• C. Heat of fusion
• D. Hydration shell

Hint:

Water's high "Heat of Vaporization" is the thermodynamic foundation of transpiration-mediated cooling in plants. It allows leaves to dissipate large amounts of solar heat through the evaporation of relatively small volumes of water.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Water possesses unique physical and thermodynamic properties owing to the extensive hydrogen-bonding network between its highly polar molecules.
This network requires substantial energy inputs to undergo phase transitions.

Step 2: Detailed Explanation:
Let us evaluate each physical and thermodynamic property of water:
Heat of Vaporization (A): This is the amount of energy required to convert one mole of liquid water into one mole of gaseous water vapor at a constant temperature.
For water, this value is exceptionally high (\(\approx 44\text{ kJ/mol}\) at \(25^\circ\text{C}\)) because many hydrogen bonds must be broken to release water molecules into the gaseous phase.
This high heat of vaporization is physiologically vital for plants, as it enables efficient evaporative cooling (transpiration) to protect leaves from overheating under direct sunlight.
Dielectric Constant (B): This is a measure of a solvent's ability to reduce the electrostatic attraction between oppositely charged ions.
Water's high dielectric constant makes it an excellent universal solvent for ionic salts.
Heat of Fusion (C): This is the energy required to convert one mole of solid ice into liquid water at its melting point.
Hydration Shell (D): This is the organized layer of water molecules that surrounds and stabilizes solute ions or polar molecules in a solution.

Step 3: Final Answer:
The energy required to convert liquid water to water vapor is called the Heat of vaporization, corresponding to option (A).