Question

The dielectric constant of water is............

• A. 0
• B. 20
• C. 40
• D. 80

Hint:

Remember: Water's high dielectric constant (\(\approx 80\)) is the primary reason why it acts as a universal solvent.
It decreases the electrostatic forces between ions in ionic crystals, facilitating their dissolution.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The dielectric constant (or relative permittivity, \(\epsilon_r\)) is a measure of a substance's ability to insulate charges and reduce the electrostatic force of attraction between dissolved ions.
Water's high polarity and molecular structure give it one of the highest dielectric constants among common liquids.

Step 2: Detailed Explanation:
At room temperature (\(20^\circ\text{C}\) to \(25^\circ\text{C}\)), the dielectric constant of pure water is approximately \(80\) (specifically around \(78.4\) to \(80.1\)).
This extraordinarily high value is due to the strong dipole moment of the water molecule and its ability to form a dynamic network of hydrogen bonds.
According to Coulomb's Law, the electrostatic force (\(F\)) between two charges \(q_1\) and \(q_2\) separated by a distance \(r\) in a medium is:
\[ F = \frac{1}{4\pi\epsilon_0\epsilon_r} \frac{q_1 q_2}{r^2} \] Because water has a dielectric constant (\(\epsilon_r\)) of approximately \(80\), it reduces the attractive force between positive and negative ions by a factor of \(80\) compared to a vacuum.
This allows the ionic components of salts (like \(\text{Na}^+\) and \(\text{Cl}^-\)) to easily dissociate and become surrounded by hydrating shells of water molecules.
This makes water an exceptionally good "universal solvent" for polar and ionic substances.

Step 3: Final Answer:
The dielectric constant of water is approximately \(80\).