Every gas problem in this chapter turns the same four dials: pressure $P$ in kilopascals (the gas drumming on its container), volume $V$ in litres (the room it fills), temperature $T$ (how hard the particles jiggle), and amount $n$ in moles (the headcount). Behind them all sits one law you will meet properly soon — $PV = nRT$, read aloud: P V equals n R T. Learn the cast now; the units name each player on sight.

One doctrine before anything else: gas laws speak kelvin, never Celsius. Celsius hangs its zero on melting ice — a fine place for weather, a false floor for physics. Kelvin starts at absolute zero, where the jiggling stops, so DOUBLE the kelvin really is double the jiggle. The bridge: $T_K = T_{^\circ C} + 273$ (273.15 in full — this course rounds, and so do we). Taught once, here. Trapped everywhere after.