And pure substances, because they don't have concentrations, aren't included in equilibrium calculations.
这就是为什么水既可以显酸性也可以显碱性。
可汗学院:生物化学
And so just to get our terminology, we call these different substances-- these pure substances that have these specific properties at certain temperatures and react in certain ways-- we call them elements.
A chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e., without breaking chemical bonds. Chemical substances can be chemical elements, chemical compounds, ions or alloys.