Why should the distilling flask be filled to no less than one thirds of its capacity?
If the neck of your flask is short, you don't want to fill the
flask too high (more than about 1/2), or else liquid from the flask
will enter your distilling tube and get distilled out when you heat
it. Remember the idea of a distillation is to heat up your
substance to the vapor phase and let the vapor diffuse toward the
distillation tube so that only purified product drips into the
collecting flask.