matter
Matter is simply the stuff things are made of — the air you breathe, the water you drink, the chair you sit on, and you yourself. The everyday test is that matter takes up space and has weight: a balloon full of air is heavier than an empty one, even though air feels like nothing at all.
In physical terms, matter is anything that has mass and occupies volume. It is built from atoms, which combine into molecules and larger structures, and it appears in familiar forms — solid, liquid, and gas — depending on how its particles are arranged and how strongly they hold together. Physical chemistry is, at bottom, the study of how matter behaves and rearranges.
It helps to be clear about what matter is not. Light, heat, and sound carry energy but have no mass sitting still and take up no space of their own, so they are not matter. Drawing this line keeps our bookkeeping honest: when we weigh out a reaction, we are counting matter, not the energy that flows in and out alongside it.
A sealed bottle of soda weighs the same whether the gas is dissolved in the liquid or has bubbled into the space above — the matter is all still there; only its form has shifted.
Matter is conserved: it changes form without appearing or vanishing.
In everyday chemistry, matter is conserved — it is neither created nor destroyed in a reaction. The fine print from physics is that mass and energy can interconvert (E = mc²), but the effect is far too tiny to notice on a laboratory balance.