A sealed beaker contains liquid Bromide, which has a brown color. We heat the beaker and after a while, the top of the beaker becomes brown. Then, the whole beaker becomes brown, until it stops. Why does the beaker become brown?

Why does the beaker become brown?

2 Answers
Mar 24, 2018

The experiment as described is neither practical NOR safe....


You don't heat a volatile liquid in a sealed container. Why not? You risk creating a bomb. And here, you use liquid bromine, the which is one of the most reactive and corrosive liquids you can find in a laboratory, and it is treated with a great deal of respect.

Mar 24, 2018

The beaker becomes brown because bromine molecules are moving into the vapour phase.


The liquid bromine sits at the bottom of the beaker.

As you heat the beaker, more bromine molecules have enough kinetic energy to escape the attractions of their neighbours.

They move into the vapour phase.


However, bromine vapours are dense, so I would expect them to move gradually up the beaker rather than going directly to the top.

If the sides of the beaker are cold, the bromine will condense to a liquid and run back down the walls.

On continued heating, you will eventually reach the critical point (315 °C and
100 bar).

You won't be able to distinguish the liquid from the vapour because they will have the same density.