Let's analyze each statement to determine which one is incorrect.

Option A: Viscosity of liquid always decreases with increase in temperature.

This statement is generally true for most liquids. As temperature increases, the kinetic energy of molecules also increases, leading to a reduction in the intermolecular forces that contribute to viscosity, hence reducing the viscosity.

Option B: Surface tension of liquid always decreases with increase in temperature.

This statement is also generally true. As temperature increases, the cohesive forces between molecules at the surface decrease, which lowers surface tension.

Option C: Viscosity of liquid always increases in presence of impurity.

This statement is incorrect. The effect of impurities on viscosity can vary depending on the nature of the impurity and the liquid. Some impurities may increase viscosity, while others may decrease it. For example, adding sugar to water increases its viscosity, whereas adding alcohol decreases it.

Option D: Surface tension of liquid always increases in presence of impurity.

This statement is also incorrect. The effect of impurities on surface tension depends on the nature of the impurity. For instance, detergents added to water reduce its surface tension, while some other impurities may increase it.

Therefore, the incorrect statement is:

Option C: Viscosity of liquid always increases in presence of impurity.

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MCQ (Single Correct Answer)