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

Surface tension and viscosity both decrease with increasing temperature because higher temperatures weaken the intermolecular forces within a fluid. Weaker intermolecular forces result in lower surface tension (reduced surface particle attraction) and in lower viscosity (reduced internal friction). Substances with high surface tensions tend to have high viscosities because both properties are related to the strength of the intermolecular interactions within the fluid. Strong intermolecular forces result in greater surface elasticity (surface tension) and resistance to flow (viscosity).

Step-by-step solution

Define Surface Tension and Viscosity

Surface tension can be thought of as an elastic property of the surface of a liquid that causes it to behave like a stretched elastic membrane. It is due to the unbalanced attractive forces between the molecules at the surface of a liquid. In other words, surface tension is the force that acts upon a liquid's surface to prevent liquid particles from spreading. Viscosity, on the other hand, is a measure of a fluid's resistance to flow. It is essentially the internal friction of a fluid, and it describes how thick or sticky it is. Viscosity is influenced by the forces between fluid particles and how easily they can move past each other.

Temperature Effect on Surface Tension

As temperature increases, the kinetic energy of molecules also increases, and they move more vigorously. This increased movement weakens the attractive forces between the molecules at the surface of a liquid, resulting in a decrease in surface tension. The decreased attractive forces between liquid particles at higher temperatures make it easier for particles to break away from the surface, lowering the surface tension. In other words, surface tension decreases with increasing temperature because the intermolecular forces become weaker as temperature rises.

Temperature Effect on Viscosity

Viscosity also tends to decrease when a liquid's temperature is increased. As mentioned earlier, viscosity is a measure of a fluid's resistance to flow, and it depends on the internal forces and interactions between its particles. When temperature increases, the kinetic energy of a fluid's particles also increases. This leads to weaker molecular cohesion and, therefore, less fluid resistance to flow. As a result, viscosity decreases with increasing temperature as the internal friction between a fluid's particles becomes weaker with increasing molecular energy.

Relationship between Surface Tension and Viscosity

Both surface tension and viscosity are influenced by the intermolecular forces within a fluid. Substances with strong intermolecular forces will tend to have higher surface tensions, as these forces work to hold the surface particles together more tightly. Such substances will also tend to have higher viscosities, as strong intermolecular forces make it more difficult for particles to flow past each other. Therefore, substances with high surface tensions often have high viscosities because both properties are related to the strength of the intermolecular interactions within the fluid. These strong interactions contribute to more resistance to flow (viscosity) and greater surface elasticity (surface tension).

Key concepts

Viscosity

Viscosity is a crucial property of fluids that measures their resistance to flow. Imagine honey being poured – it flows slowly because it has a higher viscosity compared to water. Viscosity is essentially the internal friction within a fluid, arising from the cohesive forces among its molecules.

These internal forces can include attraction between the molecules themselves, including van der Waals forces and hydrogen bonding. These attractions make the fluid "stickier" or more "thick," resisting motion across its layers.

• High Viscosity: Fluids like honey have strong intermolecular forces, making them more resistant to flow.
• Low Viscosity: Water is more fluid and flows easily due to weak intermolecular forces.

Because viscosity is so tightly linked to the molecular interactions within a liquid, understanding this concept helps us understand why some substances pour smoothly while others resist being moved.

Intermolecular Forces

Intermolecular forces are the glue that holds molecules together, influencing how liquids behave. When these forces are strong, molecules adhere tightly, resulting in higher surface tension and viscosity.

Types of Intermolecular Forces:

• Van der Waals forces: These are weak attractions between molecules that contribute to a fluid's properties.
• Hydrogen bonds: A special type of attraction seen in molecules like water that results in significant tension and cohesion.

When a substance has strong intermolecular forces, its surface molecules are "pulled" inward more effectively, leading to high surface tension. Likewise, these forces prevent the molecules in the consumer fluid from sliding past each other easily, resulting in high viscosity.

Thus, understanding intermolecular forces provides insight into why fluids behave the way they do, including their resistance to flowing and their ability to stretch across a surface without breaking.

Temperature Effect

Temperature affects both surface tension and viscosity significantly by altering the kinetic energy of molecules. As temperature rises, molecules gain more energy and move with increased speed and vigor.

Effect on Surface Tension:

• Increased molecular movement weakens the attractive forces at the surface, reducing surface tension.

Effect on Viscosity:

• Higher temperatures decrease viscosity because particles can freely move past one another without strong cohesion.

Temperature acts as a trigger that reduces the strength of intermolecular forces, allowing a fluid to flow more easily and its surface to stretch further. This highlights why on a hot day, motor oils become more fluid, and cold liquids become sluggish in cooler environments.