The angle of contact, also known as the contact angle, is the angle at which a liquid/vapor interface meets the solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation:
cos(θ)=‌where:
θ is the contact angle
σSG is the interfacial tension between the solid and gas
σSL is the interfacial tension between the solid and liquid
σLG is the interfacial tension between the liquid and gas
When a soluble impurity is added to a liquid, it affects the surface tension of the liquid-gas interface (
σLG ). If the solute has a surface-active property, meaning it tends to accumulate at the surface, it typically reduces the surface tension of the liquid.
A lower
σLG (surface tension of the liquid) influences the balance of forces and consequently could decrease the contact angle if the impurity does not significantly affect the solid-liquid and solid-gas interfacial tensions. A decreased contact angle means the liquid spreads out more on the surface, leading to better wetting.
So, when considering the effects of a soluble impurity in a liquid, and assuming the impurity is surface-active and does not significantly affect the solid-liquid and solid-gas interfacial tensions, the correct answer is:
Option A: decreases
This is because the addition of the impurity decreases the surface tension of the liquid, typically resulting in a smaller contact angle and increased wetting.