When impurities are mixed in liquid, surface tension of liquid decreases. The soluble substances when dissolved in water, decreases the surface tension of water.

Highly soluble impurity salt dissolved in water , surface tension of water increases . Insoluble impurities like detergent decreases the surface tension of water.

Introduction:
When impurity is added to a liquid, it affects the properties of the liquid. One such property is the surface tension of the liquid. Surface tension is the force acting on the surface of a liquid that tends to minimize its surface area. It is caused by the cohesive forces between the liquid molecules. The presence of impurities can disrupt these cohesive forces and ultimately affect the surface tension of the liquid.

Explanation:
1. Surface Tension and Impurities:
When an impurity is added to a liquid, it gets adsorbed at the liquid-air interface. The impurity molecules have different chemical properties compared to the liquid molecules, which can disrupt the cohesive forces and interactions between the liquid molecules. This disruption weakens the attractive forces between the liquid molecules, leading to a decrease in surface tension.

2. Effect on Cohesion:
Cohesion is the force of attraction between like molecules. In the absence of impurities, the cohesive forces between the liquid molecules are strong, resulting in a high surface tension. However, when impurities are introduced, they interfere with the cohesive forces by either reducing the strength of the attractive forces or by creating a barrier between the liquid molecules. This disruption weakens the cohesion and reduces the surface tension.

3. Adsorption of Impurities:
Impurities tend to get adsorbed at the liquid-air interface. When an impurity molecule adsorbs at the interface, it disrupts the arrangement of the liquid molecules, leading to a decrease in surface tension. The adsorbed impurity molecules create a barrier between the liquid molecules, preventing them from forming cohesive forces effectively.

4. Nature of Impurities:
The effect of impurities on surface tension depends on the nature of the impurity. Some impurities may have a greater disruptive effect on the cohesive forces, leading to a more significant decrease in surface tension. On the other hand, certain impurities may have a minimal effect on surface tension. The nature and concentration of the impurities play a crucial role in determining the extent of the effect on surface tension.

Conclusion:
In summary, when impurity is added to a liquid, the surface tension of the liquid decreases. This is due to the disruption of cohesive forces between the liquid molecules caused by the presence of impurities. The adsorption of impurity molecules at the liquid-air interface weakens the cohesive forces and reduces the surface tension. The nature and concentration of impurities determine the extent of the effect on surface tension.