ignition
/ ig-NISH-un /
Imagine baking a damp lump of clay in a very hot kiln until it stops shrinking and becomes a stable, definite material you can finally weigh with confidence. Ignition is that fierce, final heating, applied to a chemical precipitate.
Ignition is the strong heating of a filtered precipitate, often to hundreds of degrees in a furnace, to drive off water and volatile material and convert it into a single, stable compound of exactly known composition. Only after this does the solid have a chemical formula you can trust for the calculation.
It matters because many precipitates are first collected in a damp, ill-defined form — sometimes a gel or hydrate — that no formula describes cleanly. Ignition transforms them into something definite, like an oxide, so the gravimetric factor truly applies.
Gelatinous iron(III) hydroxide is filtered and then ignited near 800 °C, where it loses all its water and collapses into the firm, weighable oxide Fe2O3.
Hard heating turns a vague hydrate into a definite oxide.
Ignition is harsher than drying. Drying merely removes moisture at low heat and keeps the same compound; ignition uses much higher temperatures and often chemically changes the solid into a new, stable form.