A chemist has to determine the identity of an unknown substance. It could be poisonous lead(II)fluoride (PbF2) or harmless dinitrogen tetraoxide (N2O4). Conduct an experiment to prove that the unknown substance is poisonous.
We are looking to distinguish between lead (II) fluoride (PbF2) and dinitrogen tetraoxide (N2O4). We really need to look no further than basic observation. Lead fluoride is a white, odorless, powdery solid. Dinitrogen tetraoxide can be either a gas or a liquid. N2O4 is in equilibrium with nitrogen dioxide, which has a brownish look to it. So no chemical reaction or experiment is required to distinguish between the two. If the unknown substance is a white solid, it's lead fluoride. If it's a colorless liquid or a gas, it's dinitrogen tetraoxide. Incidentally, you should be aware that both chemicals are quite toxic. Dinitrogen tetraoxide is a strong oxidizer and can be used as rocket fuel. The fumes are toxic to breathe.
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