Thermal diffusivity comparison: In general, the thermal diffusivity of gases is ________ that of liquids.

Introduction / Context:
Thermal diffusivity indicates how quickly temperature disturbances propagate through a material. It is crucial for transient heat transfer analysis in fluids such as air (gas) and water (liquid).

Given Data / Assumptions:

• Thermal diffusivity h (often denoted alpha) defined as h = k / (rho * cp).
• Typical properties at room temperature for common gases and liquids.
• No phase change; homogeneous fluids considered.

Concept / Approach:
For gases, density rho and specific heat capacity per unit volume rhocp are relatively small compared with liquids. Although thermal conductivity k of gases is lower than that of liquids, the much smaller thermal inertia (rhocp) in gases makes h larger for gases in many practical cases.

Step-by-Step Solution:
Start with h = k / (rho * cp).Compare orders of magnitude: k_gas < k_liquid but (rhocp)_gas ≪ (rhocp)_liquid.Because denominator drops faster for gases, h_gas tends to be greater than h_liquid.Therefore, temperature fields in gases adjust more rapidly for the same length scale.

Verification / Alternative check:
Typical values: h_air ~ 2.1e-5 m^2/s; h_water ~ 1.4e-7 m^2/s. The gas value is orders of magnitude higher, supporting the conclusion.

Why Other Options Are Wrong:

• Equal to / approximately half / independent of phase: contradict property data trends.
• Less than: ignores the dominant effect of much larger volumetric heat capacity of liquids.

Common Pitfalls:
Confusing conductivity with diffusivity; a higher k does not guarantee higher h if rho*cp is also much higher.

Final Answer:
greater than