Question

Why is very important to wait until the crucible is fully coleed to room temperature before weighting

Answer 1

If you do not wait until the crucible is at room temperature, its density will vary constantly until it reaches this temperature, since density is a property that varies with temperature. For the above reason, weighing a hot crucible will not allow the measurement of a constant weight on the balance, since the weight of the object will be constantly changing, which will not allow a constant reading.