Question

Why does the viscosity of a liquid decrease with increasing temperature?

Short answer

Viscosity decreases with temperature because molecules move faster, reducing intermolecular forces.

Step-by-step solution

Understanding Viscosity

Viscosity is a measure of a liquid's resistance to flow. It indicates how 'thick' or 'sticky' a liquid is. A highly viscous liquid, like honey, flows slowly, while a less viscous liquid, like water, flows more easily.

Effect of Temperature on Molecules

When a liquid is heated, the temperature increase gives more energy to the molecules. This extra energy causes the molecules to move faster and with more vigor.

Molecular Interactions

In a liquid, viscosity arises from the intermolecular forces that hold molecules together. As the molecules move faster due to increased temperature, the intermolecular forces become less effective at holding them together.

Decreased Interaction Leads to Lower Viscosity

The faster movement of molecules disrupts their ability to 'stick' to each other, effectively reducing the liquid's resistance to flow, or its viscosity.

Key concepts

Molecular Interactions

Molecular interactions are the forces that occur between molecules, keeping them cohesive and determining how they behave. In liquids, these interactions are key to understanding properties like viscosity. Different types of molecular interactions include:

• Van der Waals Forces: These are weak attractions or repulsions between molecules or parts of molecules that come from temporary dipoles or induced polarity.
• Hydrogen Bonds: A stronger type of interaction occurring specifically when hydrogen is bonded to electronegative atoms like oxygen or nitrogen, common in water and alcohols.
• Dipole-Dipole Interactions: These occur in polar molecules where the positive end of one molecule is attracted to the negative end of another.

These interactions determine the viscosity of a liquid. The stronger the intermolecular forces, the more the molecules 'stick' together, increasing viscosity. As we explore the effects of temperature, you'll see how these interactions play a significant role.

Temperature Effect on Liquids

Temperature has a profound effect on the behavior of liquids, primarily affecting molecular speed and energy. When a liquid is heated, its molecules receive added energy. This energy translates into movement, making molecules vibrate and collide more frequently.

• Increased Energy: As temperature rises, molecules move more rapidly and with greater force.
• Impact on Viscosity: Faster molecular movement weakens intermolecular forces, allowing molecules to slide past each other more easily.
• Temperature-Viscosity Relationship: Commonly, a higher temperature results in lower viscosity. This is why liquids like honey become easier to pour after being warmed.

Understanding these effects helps explain why heating a liquid decreases its viscosity: the molecular interactions weaken, resulting in less resistance to flow.

Intermolecular Forces

Intermolecular forces are central to understanding the viscosity of liquids. They include all the forces of attraction and repulsion between molecules, excluding those due to covalent bonds.

• Types of Forces: Similar to molecular interactions, these include van der Waals forces, hydrogen bonds, and dipole-dipole interactions.
• Role in Viscosity: These forces govern how tightly molecules cling to each other, directly influencing the liquid's viscosity.
• Affect By Temperature: With an increase in temperature, these forces are less effective as added energetic movement overcomes the attractions.

As the intermolecular forces become weaker with heat, the resistance experienced by consecutive layers of liquid reduces, leading to decreased viscosity. Thus, understanding intermolecular forces is crucial for explaining changes in the flow characteristics of liquids with temperature.