Why heat and salt diffuse at different rates.

Why do heat and salt diffuse at different rates? This seems to always be puzzling students when talking about double diffusion.

Well, why should they diffuse at the same rate? The processes of molecular diffusion of heat and salt are very different.

In the case of heat, a transfer of heat only means that particles hit and transfer energy. The warmer the substance, the faster the particles’ movements. So faster particles hitting slower particles will transfer momentum, and by slowing down one of the particles and speeding up the other one, this is de facto a heat exchange.

A transfer of salt on the other hand means that ions have to be transported from a region of higher concentration to a region of lower concentration. In order for this to happen, they have to travel over physical distances larger than just the wiggling connected to molecular movement, and they have to exchange place with water molecules and clusters.

Clearly, this second process is a lot more time-consuming than the first one?