Elements, Compounds and Mixtures

Start with the atoms. An element is matter made of only one kind of atom, like carbon or sodium. When atoms of different elements form actual chemical bonds, sharing or transferring electrons, you get a compound, and the bonds force the elements together in a fixed ratio. Carbon dioxide is always one carbon atom for every two oxygen atoms, no exceptions, because that ratio is set by the bonds themselves. Break those bonds with a chemical reaction and you get carbon and oxygen back as pure elements, something stirring or filtering can never do.

A mixture is different because nothing bonds. Salt and sand tossed together are still salt and sand, each keeping its own properties, so you can add water, dissolve the salt, filter out the sand, and evaporate the water to get the salt back. Bronze is a mixture too, copper and tin melted together but not chemically combined, which is why you can in principle separate them again by physical means like melting points. Mixtures also don't need a fixed ratio: a mixture can be mostly copper or mostly tin and still count as bronze.

This is why air trips people up. Air is the same proportions almost everywhere you sample it, but sameness of ratio is not the test for a compound. The real test is whether a chemical bond locks that ratio in place. Nitrogen and oxygen in air sit side by side unbonded, so cooling and distilling separates them with no reaction needed, and the proportions could in theory drift without changing what the gases are. That question, bonded or just blended, is what decides compound versus mixture everywhere on this ladder, from sand and salt to sodium metal itself.