Some pairs of liquids, like ethanol and water, mix completely into one uniform liquid — we say they are miscible, and the mixture is homogeneous (the same all the way through). Other pairs, like oil and water, refuse to mix. They are immiscible: the mixture is heterogeneous, with a visible boundary between two separate layers.
Immiscible liquids arrange themselves by density. The densest liquid sinks to the bottom; the least dense floats on top. Water is denser than oil, so water forms the bottom layer and oil sits above it. A separating funnel can even hold three or more layers at once — they simply stack in order of density, densest at the bottom.
While liquid runs out at the bottom, air must get in at the top to take its place. With the stopper still in, no air can enter, the pressure above the liquid drops, and the flow quickly stops. Always remove the stopper before opening the tap.
Left a bowl of curry to stand? The oil floats up and forms a shiny layer on the watery gravy — the same density story. At home you can skim the oil off with a spoon, blot it with paper, or chill the dish so the oil turns solid and lifts off. A separating funnel is simply the neat, precise lab version of that skimming.
If the two liquids are miscible (like ethanol and water), there is no boundary to drain to — the funnel is useless. Miscible liquids must be separated by fractional distillation, which uses their different boiling points, or in some cases analysed by chromatography.
| Type of liquid mixture | Separation technique |
|---|---|
| Immiscible liquids (e.g. oil + water) | Separating funnel |
| Miscible liquids (e.g. ethanol + water) | Fractional distillation |