How does carbon build up on the bottom of the crucible?

[forchina]

How does carbon build up on the bottom of the crucible?

[dzames]

Hey there!

So this is a question mainly about experimental technique. Generally, when using bunsen burners, we have the "safe" flame, which is yellow, and the much hotter blue-clear flame. We can use the safe flame to see where the bunsen burner is and if it is lit. But if you try to use this yellow flame to heat things, carbon will build up on the surface of whatever you are heating, because this flame is much colder and not intended for use experimentally. To stop carbon build up, use the hotter, blue flame.

The chemistry behind it is kinda complicated, but basically its about the mixture of oxygen with the bunsen burner fuel. If you notice the swivel on the bunsen burner, the flame is yellow when most of the holes are closed, and blue when they are open. This is because by closing the holes you remove access to oxygen, and so the fuel undergoes an incomplete combustion that results in this carbon residue on your crucible or beaker. When the holes are open, there's plenty of oxygen and so complete combustion of the fuel can occur.

James