The Lucas reagent is a mixture of zinc chloride (ZnCl₂) and concentrated hydrochloric acid (HCl). When an alcohol is added, the mechanism at work is an SN1 substitution: the alcohol's oxygen is first activated (protonated, with ZnCl₂ acting as a Lewis acid catalyst that helps the hydroxyl leave as water), and the resulting carbocation is then captured by chloride to form an alkyl chloride.

Since this is an SN1 pathway, its rate depends entirely on how stable the intermediate carbocation is — and you already know that ranking cold: tertiary > secondary > primary. The alkyl chloride product is insoluble in the aqueous reagent mixture, so as soon as it forms, the solution turns visibly cloudy. That means the SPEED of cloudiness appearing is a direct, visible readout of how fast each alcohol class ionizes.

Before modern spectroscopic methods (like NMR) became routine and widely accessible, the Lucas test was one of the fastest ways a chemist could classify an unknown alcohol without any instrumentation at all — just a test tube, the reagent, and a stopwatch. It remains a useful teaching tool and a quick confirmatory check specifically because the underlying logic (SN1 rate tracks carbocation stability) is exactly the same mechanistic reasoning covered throughout the SN1 Characteristics lesson.

It's worth noting the test's limitation directly: it only works cleanly for alcohols with roughly six or fewer carbons, since larger alcohols become poorly soluble in the aqueous reagent mixture even before any reaction occurs, which can produce a misleading cloudy appearance unrelated to the actual substitution chemistry.