How the Separating Funnel Works

A separating funnel relies on a plain physical fact: two liquids that do not dissolve into each other, such as oil and water, will settle into distinct layers when left still, with the denser liquid sinking below the lighter one. The funnel's pear shape and its bottom stopcock are what turn that settling into a usable separation: the mixture of two immiscible liquids is poured in through the wide top opening, the funnel is stoppered, and it is gently agitated to bring the two liquids into close contact, which is essential when the goal is extraction, letting a dissolved substance move from one liquid into the other across their shared boundary.

Once agitation stops, the funnel is left to stand until the two layers separate cleanly again, the denser liquid pooling in the narrow lower section just above the stopcock. Opening the stopcock lets that lower, denser layer drain out in a controlled stream, and the operator closes the stopcock again the instant the boundary between the two liquids reaches the tap, leaving the lighter liquid behind in the funnel to be poured out separately through the top. The stopcock is the whole innovation over older decanting methods: it lets the denser liquid be drawn off precisely, without disturbing or mixing the layer left above it.