Boiling chips, sticks and stones

Someone asked why boiling stones were used for a simple distillation. This is my answer.

At the surface of a liquid, the liquid phase will be in equilibrium with a gas phase. Because energy is exchanged in changing phases, excess heat is removed by changing phases. This prevents a liquid like water from being heated beyond its boiling point.

If you heat a liquid and the heat is trapped away from a gas surface, the liquid can be superheated, it will contain more heat (energy) than needed to enter the gas phase. If all of the surrounding molecules are also in the liquid phase, there is no release for the energy. If too much heat is trapped in the liquid phase and a gas bubble forms, the conversion to the gas phase can be explosive. That is why bumping occurs.

A boiling stone or stick simply has air entrapped within. It is porous. If you heat a boiling stone, the gas will expand and be given off. If this gas meets superheated liquid, the liquid can equilibrate between the gas and liquid phase, boiling results.

You must be careful with using a boiling stone or stick. If heated too long, all of the entrapped gas will become lost and become ineffective. The other and more frequent problem is if you heat something up, and then let it cool, the gas contracts and pulls the liquid in. This can effectively block the further release of gas. Anytime you let a liquid cool from boiling to RT, add a new boiling chip as the one in the flask will be dead.

You don't need to use boiling chips. You can use an ebulator to introduce nitrogen or just stir the flask vigorously to bring the heated liquid to the gas-liquid interface. I like the latter method and when distilling, I used a magnetic stir bar and stirred it at the maximum rate I could with the stirrer. Some people use additional teflon chips with this later method, but I did not like fishing them back out.