Modes of heat transfer by molecular collisions In which phase of matter is conductive heat transfer by molecular collision the smallest, all else being equal?

Introduction / Context:
Conduction arises from molecular interactions (collisions and energy exchange). The efficiency of this mechanism depends strongly on intermolecular spacing and bonding, which vary across solids, liquids, and gases.

Given Data / Assumptions:

• No bulk motion (pure conduction comparison).
• Similar temperature gradients and characteristic length.
• Typical engineering materials at standard conditions.

Concept / Approach:
Thermal conductivity k reflects how readily energy is transferred microscopically. In solids, lattice vibrations and free electrons (in metals) enable high k. In liquids, closer molecular proximity yields moderate k. In gases, large mean free paths and low density give the smallest k under comparable conditions.

Step-by-Step Solution:
Compare typical k values: metals >> nonmetal solids ≈ liquids > gases.Gases have fewer collisions per unit time due to low density.Therefore, the smallest conductive transfer by molecular collision occurs in gases.

Verification / Alternative check:
Practical evidence: still air is a good insulator; adding convection (air motion) dramatically increases heat transfer, confirming conduction alone is weak.

Why Other Options Are Wrong:

• Solids/liquids exhibit higher conductivities than gases in most cases.
• “None of these” contradicts empirical data.

Common Pitfalls:
Confusing conduction with convection; assuming water (a liquid) always conducts poorly—while not high, it is superior to air.

Final Answer:
Gases