Temperature dependence — Which statement correctly describes how viscosity changes with increasing temperature for liquids and gases?

Introduction:
Understanding how viscosity varies with temperature is essential for pump sizing, mixing scale-up, and heat- and mass-transfer correlations. Liquids and gases exhibit opposing trends because their molecular transport mechanisms differ fundamentally.

Given Data / Assumptions:

• Moderate pressure range where idealized trends apply.
• No phase change over the temperature span considered.
• Comparisons are qualitative (increase vs decrease).

Concept / Approach:
In liquids, viscosity is dominated by cohesive intermolecular forces; heating reduces these cohesive constraints, allowing molecules to slide past one another more easily, thus lowering viscosity. In gases, viscosity reflects momentum transfer by molecular motion; increasing temperature raises molecular speed and mean kinetic energy, increasing momentum exchange and, therefore, viscosity.

Step-by-Step Solution:
Identify liquid mechanism: stronger intermolecular attractions at low T impede flow; higher T weakens these restraints.Identify gas mechanism: higher T implies faster molecules and more frequent, energetic collisions that enhance momentum transport.Conclude opposite trends: liquids thin with heat; gases thicken modestly with heat.

Verification / Alternative check:
Empirical correlations (e.g., Andrade for liquids, Sutherland or Chapman–Enskog for gases) quantitatively capture these trends and are standard in transport property databases used by engineers.

Why Other Options Are Wrong:

• (a) Both decreasing contradicts gas behavior.
• (b) Both increasing contradicts liquid behavior.
• (d) Claims gases decrease, which is opposite typical gas trends.
• (e) Temperature effects are often decisive in design and cannot be neglected.

Common Pitfalls:
Extrapolating gas trends to very high pressures where non-ideal effects intervene; conflating viscosity with density changes, which may move in different directions.

Final Answer:
Viscosity of gases increases with temperature, but viscosity of liquids decreases with temperature.