Describe, in terms of particles, why thermal conduction is poor in gases and most liquids.
Key Concepts
Thermal conduction is the transfer of kinetic energy from high‑temperature particles to neighbouring lower‑temperature particles.
The rate of conduction depends on how easily particles can collide and exchange energy.
Particle spacing and the strength of intermolecular forces are the main factors that differentiate solids, liquids and gases.
Particle‑level Explanation
In a solid the particles are packed closely together in a regular lattice and are held by strong intermolecular forces. When one particle gains kinetic energy (heats up), it vibrates more vigorously and quickly transfers part of this energy to its immediate neighbours through direct collisions. This chain of collisions propagates the heat efficiently through the material.
In gases and most liquids the situation is very different:
Large average separation of particles
In a gas the average distance between molecules is many times their own diameter.
In a liquid the particles are closer than in a gas but still have appreciable free space compared with a solid.
Because of this separation, the frequency of collisions is much lower, so energy transfer by direct impact is slow.
Weak intermolecular forces (especially in gases)
Gases have negligible attractive forces; molecules move independently.
Most liquids have moderate forces (e.g., hydrogen bonding in water) but these are still much weaker than the rigid bonds in a solid lattice.
Weak forces mean that when a molecule collides, only a small fraction of its kinetic energy is transferred before it moves away.
Random, high‑speed motion
Gas molecules travel in straight lines until they collide, leading to a random walk that spreads energy slowly.
In liquids, molecules also move in a random, “slipping” manner, which disrupts the orderly transfer of kinetic energy.
Mathematical Formulation (for reference)
The macroscopic rate of heat transfer by conduction is given by Fourier’s law:
\$ Q = -k A \frac{dT}{dx} \$
where:
\$Q\$ = heat transferred per unit time (W)
\$k\$ = thermal conductivity of the material (W m⁻¹ K⁻¹)
\$A\$ = cross‑sectional area through which heat flows (m²)
\$\frac{dT}{dx}\$ = temperature gradient (K m⁻¹)
Because \$k\$ is much smaller for gases and most liquids than for solids, the same temperature gradient produces far less heat flow.
Comparison of Thermal Conductivity
State of Matter
Typical \$k\$ (W m⁻¹ K⁻¹)
Particle Arrangement
Reason for Low Conduction
Solid (e.g., copper)
≈ 400
Close‑packed lattice, strong bonds
Frequent collisions, efficient energy transfer
Liquid (e.g., water)
≈ 0.6
Particles close but with free space, moderate forces
Less frequent collisions, weaker energy transfer
Gas (e.g., air)
≈ 0.025
Widely spaced molecules, negligible forces
Very few collisions, random motion limits transfer
Implications for Everyday Situations
Thermos flasks use a vacuum (essentially no gas) to minimise conduction.
Insulating materials (e.g., foam) trap air in tiny pockets, exploiting the poor conductivity of gases.
Water’s relatively low conductivity compared with metals means it is a good coolant but not as efficient as metal heat sinks.
Suggested diagram: Schematic showing particle spacing and collision frequency in a solid, liquid and gas, with arrows indicating heat flow direction.