General Science question from Sub Inspector exam, 2022 by JKSSB
Which of the following when added to pure water usually increases its surface tension?
Last updated Jun 24, 2026
Correct Answer:
Option B —
Sodium chloride
Sodium chloride (NaCl): When a highly soluble inorganic salt like sodium chloride is added to pure water, it breaks apart into ions (Na⁺ and Cl⁻). The interactions between these ions and the water molecules (ion-dipole interactions) are actually stronger than the hydrogen bonds between the water molecules themselves. This pulls the molecules closer together, which slightly increases the surface tension.
Detergents, Kerosene oil, and Phenyl: These substances are either surfactants (surface-active agents) or strongly hydrophobic. They tend to gather at the surface of the water and disrupt the cohesive hydrogen bonds between the water molecules, which decreases the surface tension. This is exactly why detergents are used for cleaning—they lower water's surface tension so it can more easily penetrate fabrics and wash away dirt.
Answer verified by Quintessence Classes faculty — Karan Nagar, Srinagar.
