Effect of temperature on thermal conductivity of liquids and gases

In this Article we learn about why thermal conductivity of liquid decreases whereas that of gas increases by increasing temperature.

As we all know Thermal conductivity of fluids depends upon two  main factor i.e.

1. Molecular  Collision 
2. Molecular Diffusion

Lets first know what is molecular Diffusion & Molecular Diffusion means.

In case of molecular collision, the atoms, molecules & ions of an object/substance colliding with the neighbouring molecules. when by any means thermal energy of the molecules increases, the Kinetic increases also gets increased as a result the movement of the molecules becomes more faster and starts collision among the nearby molecules.

Whereas in molecular diffusion case, when the thermal energy get increased random motion of the molecule increases in different directions.

One can also termed molecular diffusion as, random irregular motion.

Now onwards Lets see,

Why thermal conductivity of liquid decreases whereas that of gas increases by increasing temperature ? 

The molecules of the liquids are relatively closely packed in compare of gases molecules. The distance between the liquid molecules is small and the covalent bond is stronger than that of gases.

As the temperature increases the liquid starts expanding and the molecules starts moving apart. But here in case of liquid,  cohesive force dominants over the molecular diffusion. As a result thermal conductivity decreases while increasing temperature.

Note : Except for water, aqueous solutions, and multihydroxy molecules, the thermal conductivity of most liquids decreases with temperature.

In case of gas, molecular diffusion dominants over molecular collision. Here the covalent bond is not so strong as that of liquid molecules bonding in-between. As the temperature increases, Kinetic energy of the molecules increases and starts colliding with higher rate and more faster. This as a result increases the thermal conductivity of gas by increasing temperature.

Chart of few materials and their respective thermal conductivity at room temperature.

ArticleSource : ScienceDirect, Wikipedia

ImageSource : ResearchGate; Electronics Cooling