Question

SEP Construct an Explanation Look at Figure 1. Why does surface tension increase with increasing salinit

Answer 1

Why surface tension rises with salinity (as in Figure 1)

1. Observation: Figure 1 shows that adding salt raises the water’s surface tension.
2. Molecular cause: Dissolved ions (e.g., Na+, Cl–) become strongly hydrated and are energetically favored to stay in the bulk liquid rather than at the air–water interface. That means the interface becomes depleted of solute (negative surface excess).
3. Cohesion effect: Because ions bind water molecules by ion–dipole forces, the net attractive forces among molecules in the bulk increase. The difference in bonding strength between bulk and the interface therefore grows, so more energy is required to create additional surface area. That increased energy per unit area is an increased surface tension.
4. Thermodynamic statement: The Gibbs adsorption relation links changes in surface tension γ to the surface excess Γ of solute; for simple electrolytes Γ < 0, so raising concentration increases γ (dγ = −RT Γ d ln a).
5. Note: This behavior applies to salts whose ions are excluded from the surface; some larger, polarizable anions (e.g., I−, SCN−) are surface-active and instead lower surface tension.

So, in short: salts increase water’s bulk cohesion (and are excluded from the interface), which raises the energy cost of forming surface area and thus increases surface tension.