Question

(a) Explain why surface tension and viscosity decrease with increasing temperature. (b) Why do substances with high surface tensions also tend to have high viscosities?

Short answer

(a) Surface tension and viscosity decrease with increasing temperature because higher temperatures increase the average kinetic energy of the molecules, causing them to move more vigorously and weakening the intermolecular forces. This results in a decrease in surface tension and a reduction of internal friction within the fluid, leading to lower viscosity. (b) Substances with high surface tensions also tend to have high viscosities because both properties are a result of strong intermolecular forces. Stronger intermolecular forces lead to higher surface tension due to greater cohesive forces at the surface and higher viscosity because of greater internal friction within the fluid.

Step-by-step solution

Part (a): Surface tension and temperature

Surface tension is the force per unit length acting on the surface of a liquid, which tries to minimize the surface area. This is due to the imbalance of the cohesive forces between the liquid molecules at the surface. At higher temperatures, the average kinetic energy of the molecules increases, causing them to move and vibrate more vigorously. This leads to a weakening of the cohesive forces between the molecules. As a result, surface tension decreases with increasing temperature.

Part (a): Viscosity and temperature

Viscosity is a measure of a fluid's resistance to flow or deformation due to an applied force. It depends on the internal friction within the fluid, which is related to the intermolecular forces between the fluid molecules. As the temperature increases, the kinetic energy of the molecules also increases, causing them to move more rapidly and thus weakening the intermolecular forces. This reduces the fluid's internal friction, leading to a decrease in viscosity with increasing temperature.

Part (b): Relationship between surface tension and viscosity

Both surface tension and viscosity are influenced by the intermolecular forces between the molecules of a substance. Stronger intermolecular forces lead to higher surface tension, as the cohesive forces at the surface are greater. Similarly, stronger intermolecular forces lead to higher viscosity, as the internal friction within the fluid is greater. Therefore, substances with high surface tensions tend to have high viscosities because both properties are a result of strong intermolecular forces.