Animated Solution for Chemistry - Organic Chemistry: If dichloromethane (DCM) and water (${\text{H}}_2\text{O}$) are used for differential extraction, which one of the following statements is correct?

Differential Extraction

Imagine you are in a chemistry lab, holding a pear-shaped glass vessel called a separating funnel. You pour in a mixture of oil and water, give it a good shake, and let it rest. What happens? The two liquids separate into distinct layers. They don't mix. This simple, everyday observation is the core principle behind a powerful chemical technique known as differential extraction.

Differential extraction is used to separate an organic compound from an aqueous solution. We do this by adding an organic solvent in which the compound is highly soluble. But for this to work, the organic solvent and water must be immiscible—meaning they refuse to dissolve in one another.

In our specific problem, we are dealing with water (${\text{H}}_2\text{O}$) and dichloromethane (${\text{CH}}_2{\text{Cl}}_2$, commonly known as DCM). Water is a highly polar molecule that loves to form strong hydrogen bonds with its neighbors. DCM, on the other hand, is only slightly polar and cannot participate in this hydrogen-bonding network. Because water molecules prefer to stick to each other rather than interact with DCM, the two liquids separate into two distinct phases.

Once the liquids separate, the next logical question is: which liquid goes to the top, and which sinks to the bottom? The answer lies entirely in a fundamental physical property: density.

Gravity pulls harder on the denser liquid, forcing it to the bottom of the funnel, while the lighter liquid floats on top. Let's look at the numbers:

The density of water is approximately $1.00{\text{ g/cm}}^3$. The density of dichloromethane is approximately $1.33{\text{ g/cm}}^3$.

Because $1.33>1.00$, DCM is significantly heavier than water.

When you pour DCM and water into a separating funnel, the heavier DCM will sink and form the lower layer, while the lighter water will float and form the upper layer.

This is a crucial detail to remember in organic chemistry labs. While many common organic solvents (like diethyl ether, hexane, or ethyl acetate) are lighter than water and form the top layer, halogenated solvents (like dichloromethane and chloroform) are almost always denser than water and will form the bottom layer. Always check your densities before opening that stopcock!