Temperature Effect – Dynamic Viscosity of Liquids For most common liquids, the dynamic viscosity decreases as temperature rises, because increased molecular activity reduces resistance to shear.

Introduction:
Designers must account for temperature-dependent viscosity when sizing pumps, estimating losses, and selecting lubricants. Liquids and gases behave oppositely with temperature: liquids thin out, gases thicken slightly.

Given Data / Assumptions:

• Newtonian liquids (water, oils) over normal engineering temperature ranges.
• No phase change or chemical reaction.
• Pressure effects modest compared to temperature effects.

Concept / Approach:

Viscosity reflects internal friction between fluid layers. As temperature increases, liquid molecules gain energy, weakening intermolecular cohesion that impedes sliding, so dynamic viscosity mu drops. Empirical correlations (e.g., Andrade equation) capture this strong inverse relation.

Step-by-Step Solution:

Verification / Alternative check:

Viscosity-temperature charts for oils show order-of-magnitude drops from ambient to elevated temperatures; water viscosity roughly halves between 20°C and 50°C.

Why Other Options Are Wrong:

Unaffected: Contradicts data. Increases: Applies to gases, not liquids. Other options mix unrelated effects or oversimplify complex dependencies.

Common Pitfalls:

Applying gas trends to liquids; neglecting that kinematic viscosity also changes because density varies mildly with temperature.

Final Answer:

decreases