Elements, Compounds and Mixtures

Five parts on the question of what counts as one substance, which sounds like vocabulary and is not. The track opens by refusing to define an element by any property you can observe, because none of them work — an element is defined by what it resists, and water passed every available test for two thousand years before electricity existed to split it. It then moves to the particle level and stays there, sorting six sealed jars that all look identical into element, compound and mixture, with six jars rather than three because the three-way answer hides the two cases that catch people out: an element whose atoms come in bonded pairs is still an element, and a mixture can be made of compounds. The middle part is the iron and sulfur demonstration, where the same two powders stirred together stay magnetic and separable, and heated until they react become a black solid no magnet will touch — the clearest evidence there is that properties belong to substances rather than to the atoms inside them. The last two parts turn that into something usable. Separation methods are not a list to memorise: every one of them exploits a property the parts do not share, so finding the difference chooses the method. And once that is clear, the rule nobody explains explains itself — a compound has one set of properties, so there is no difference left to exploit, and only a chemical reaction gets the elements back.