Why should a distilling flask be filled to not more than two-thirds of its capacity at the beginning of a distillation procedure?

Answer 1

If the flask is more than half full it is possible if it starts to boil and bump that some of the mixture could contaminate your distilled/re-condensed liquid by it bumping into the liquid that has been seperated.

Answer 2

An empty or dried flask will cause the temperature to rapidly increase and may explode.

By leaving a small residue of liquid in the boiling flask, you will not overheat the flask and break it, nor will you char the last drops of residue, which causes cleaning difficulty. Moreover, some compounds, such as ethers, secondary alcohols, and alkenes, form peroxides by air oxidation. If a distillation involving one of these compounds is carried to dryness, the peroxides could explode.

Answer 3

If a distilling flask is filled more than two-thirds full, there is a chance of the liquid boiling (or bumping) over into the condenser or receiving flask without vaporizing. This would result in an impure distillate. Also, if the flask were too full, there would be too little surface area for rapid evaporation and therefore the distillation would be very slow.

Answer 4

If a distilling flask is filled more than two-thirds full, there is a chance of the liquid boiling (or bumping) over into the condenser or receiving flask without vaporizing. This would result in an impure distillate. Also, if the flask were too full, there would be too little surface area for rapid evaporation and therefore the distillation would be very slow.

You should also consider that a liquid will increase in volume when heated and leaving a flask no more than 2/3 full allows room for expansion. This is common good practice in all chemistry.

Answer 5

When the liquid being distilled is heated, it will expand slightly, possibly foam or bubble. Leaving adequate space for the reaction prevents spills or burns.