A compound is a substance that can be separated into elements by electrolysis or thermal decomposition. It has all the features of the substance class plus this one further feature: it comes apart, by these specific methods, into simpler substances.
There is no entity whose name is "compound." Like the other class names in chemistry, the word names a group. Its members are particular substances (water, table salt, baking soda, the rust on iron) whose feature in common is that, given the right method, they yield up simpler substances and leave nothing of themselves behind. The methods are specific. Filtering and settling are not the test; the test is electrolysis or thermal decomposition. A substance that can be coaxed apart by these methods is a compound. A substance that cannot is something else.
A compound has all the features of the substance class. Its property values are uniform throughout. Its phase_of_matter value is one. It is a single component material. By every macroscopic measure it behaves as a substance behaves; you would not, by looking, know to call it a compound. What distinguishes it is what happens under the decomposition test, and what comes out the other side.
This is how the hierarchy of materials reaches its lower bound. Materials are composed of substances and mixtures; substances are sorted into compounds and elements; compounds, run through the right decomposition, give back elements. The chain has a stopping point because elements do not give back anything simpler under these methods. Compound and element together carve up the substance class, and they do so by a clean criterion: does it come apart, and how?
Question
A learner is told that water can be broken down by passing electric current through it, and what is recovered are the substances hydrogen and oxygen. How does that piece of evidence place water in the compound class?
Reveal Merrill's answer →Hide answer
Water has all the features of the substance class. Property values are uniform throughout, the phase_of_matter value is one, it is a single component material. The further test, the one that sorts substances into compound and element, is whether the substance can be decomposed by electrolysis or by thermal decomposition. Water can. So water is placed in the compound class. The substances that come out of the decomposition (hydrogen, oxygen) are placed, by the same kind of test, in the element class.