Question

(a) What is the relationship between surface tension and temperature? (b) What is the relationship between viscosity and temperature? (c) Why do substances with high surface tension also tend to have high viscosities?

Short answer

The relationship between surface tension and temperature is that as temperature increases, surface tension of a liquid generally decreases due to increased molecular vibrations and kinetic energy. The relationship between viscosity and temperature is that for most liquids, viscosity decreases with increasing temperature, as molecules slide past each other more easily. However, in gases, viscosity generally increases with temperature due to more frequent molecular collisions. Substances with high surface tension usually have high viscosities because both properties are related to the cohesive forces between molecules within the substance, originating from intermolecular interactions like van der Waals forces, hydrogen bonding, or dipole-dipole interactions.

Step-by-step solution

Part (a): Relationship between surface tension and temperature

Surface tension is a property of liquids that arises from the unbalanced molecular cohesive forces at the surface or interface between a liquid and another medium. In general, as temperature increases, the surface tension of a liquid decreases. The reason behind this is that with the increase in temperature, the molecular vibrations and kinetic energy of the molecules increase, leading to weaker cohesive forces between them. As a result, the surface tension decreases.

Part (b): Relationship between viscosity and temperature

Viscosity is a measure of a fluid's resistance to flow or the internal friction within a fluid. It is directly related to the cohesive forces between the molecules and their tendency to stick together, as well as the thermal energy within the fluid. In most liquids, viscosity decreases with increasing temperature. This is because the increased thermal energy allows the molecules to slide past each other more easily, thus reducing the internal friction and lowering the viscosity. However, this relationship is different for gases. In gases, viscosity generally increases with temperature, as the increased kinetic energy of the molecules leads to more frequent collisions between them and a greater resistance to flow.

Part (c): Connection between surface tension and viscosity

Substances with high surface tension usually have high viscosities as well. The underlying reason for this is that both surface tension and viscosity are related to the cohesive forces between the molecules within the substance. These forces result from various intermolecular interactions, such as van der Waals forces, hydrogen bonding, or dipole-dipole interactions. In general, if a substance has strong cohesive forces, it will have both high surface tension and viscosity. This is because the molecules have a strong tendency to stick together, resulting in a high resistance to flow (viscosity) and a high force required to break or deform the surface/interface (surface tension).