Cambridge O Level Biology · Syllabus 5090 · Biological Molecules
Ethanol Emulsion Test
What is Ethanol Emulsion Test?
A food test for lipids. The sample is mixed or shaken with ethanol and the ethanol extract is then added to water. If lipid is present the clear mixture turns into a milky-white emulsion, best seen against a suitable background. If lipid is not detected the mixture stays clear. The positive result is an emulsion of tiny droplets suspended in the water, not a precipitate, and no heating is used because ethanol is flammable.
This definition is part of the Biological Molecules chapter in Cambridge O Level Biology.
Ethanol Emulsion Test in context
A food test is a laboratory procedure in which a reagent is added to a food sample and a change in colour or appearance shows whether a particular biological molecule is present. Iodine solution turns from orange-brown to blue-black with starch. Benedict's solution, heated in a hot-water bath, turns from blue through green, yellow and orange to brick-red with glucose or maltose, and a coloured precipitate forms. Biuret reagents turn from blue to lilac or purple with protein. In the ethanol emulsion test a clear mixture turns into a milky-white emulsion with lipid. Every test is qualitative: it shows presence at a detectable level, not an amount.
Questions students ask about Ethanol Emulsion Test
Is the positive result of the ethanol emulsion test a precipitate?
No. It is a milky-white emulsion: fine droplets of lipid suspended through the water. A precipitate is an insoluble solid that can settle out, and the mark scheme for this test is worded for "emulsion". Mix the sample with ethanol, add the ethanol extract to water and look at it against a dark background; a clear mixture means lipid was not detected. No heating is used at any stage, because ethanol is flammable and must be kept away from flames.