Question

When common salt is dissolved in water, the surface tension of saltwater is

• A. Increased
• B. Decreased
• C. Not changed
• D. First increases then decrease

Hint:

Remember: Highly soluble inorganic salts (like \(\text{NaCl}\)) increase the surface tension of water, whereas organic impurities and surfactants decrease it.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Surface tension is caused by cohesive forces between liquid molecules at the surface interface.
Adding a solute to a solvent alters these intermolecular forces, changing the surface tension of the solution.

Step 2: Detailed Explanation:
When an inorganic, highly soluble electrolyte like common salt (sodium chloride, \(\text{NaCl}\)) is dissolved in water:
- The salt dissociates into sodium (\(\text{Na}^+\)) and chloride (\(\text{Cl}^-\)) ions.
- Strong electrostatic interactions (ion-dipole forces) develop between these ions and the polar water molecules.
- This ion-solvent attraction is stronger than the hydrogen bonds between water molecules alone.
- This strong attraction pulls the surface water molecules more tightly inward toward the bulk of the solution.
- Consequently, additional work is required to bring water molecules to the surface, which increases the surface tension of the liquid.
In contrast, organic solutes (like surfactants, alcohol, or soap) accumulate at the surface and decrease surface tension.

Step 3: Final Answer:
The surface tension of the saltwater is increased.