i am doing a chemistry lab and this was one of the question on my review,and it says it in the packet that you should never weigh a hot object but it doesn"t tell why.
first of all because of safety. handling hot objects just isn"t as safe as handling room temperature objects.
also, hot objects will radiate heat in the form of convection currents, making slight changes (instabilities) in your readings. this is very noticeable in highly accurate digitial scales used in most modern laboratories.
As air is warmed, the energy from the heat causes the molecules of air to move faster and ƒᴀʀтher apart. This means the mass is less dense than if the item was cold (where the molecules are close and compact).
Might also come from the fact you don"t want to put any hot item on a scale as it would ruin the scales (especially if using a digital scale)...