Question

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 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.

The distilling flask will be filled not more than two-thirds of its capacity at the beginning of a distillation procedure, becuase to avoid the risk of the contents, product and impurities, bubbling up and over into the condenser which is intended to only take vapour of the product.