Which is the best explanation of his results? The heating changed some of the sample to gas, causing the mass to decrease without breaking down the sample. Therefore, the original sample is a compound. The appearance stayed the same, showing that the sample was not broken down by heating. Therefore, the original substance is an element. The mass decreased during heating and some of the impurities escaped. Therefore, the original substance is an element. The chemical reaction with acid changed, showing that the sample was broken down by heating. Therefore, the original substance is a compound.