If we increase the temperature of the liquid, its surface tension will __________.

CONCEPT:​

• Surface tension: The property of liquid surfaces to shrink into the minimum surface area possible is known as surface tension.
• The surface tension of a liquid depends on temperature, impurity in the liquid, nature of the liquid, etc.

EXPLANATION:

• The surface tension of liquids decreases with an increase in temperature.
  • The surface tension depends on the intermolecular forces of liquid.
  • As temperature increases, molecules of liquid move more rapidly.
  • So, the intermolecular forces between the liquid molecules become less stable.
  • Therefore, Surface tension decreases.
• So correct answer is option 4 i.e. surface tension decreases with an increase in temperature.

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• Other factors on which surface tension depends:
• Nature of liquid: On changing the liquid the surface tension will change. So it depends on the nature of the liquid.
• Presence of impurity: The surface tension of water increases when soluble impurities are added to it. 
  • Insoluble impurities like detergent decrease the surface tension
  • It has both effects depending on the impurity.
• Area of the free surface: Surface tension does not depend on the area of the free surface.
  • When the surface area of a liquid is increased, molecules are allowed to move from the bulk phase to the interface.
  • As a result, the number of molecules per unit area remains the same or the distance between the molecules at the interface.
  • Therefore, the surface tension does not change.