How to Separate Materials by Density

Liquids separate naturally by density.

Density is a measure of how much mass is contained within a given volume of space. Density can be used to separate mixtures. For example, oil floats on top of water because oil is less dense than water.

Standard Liquid Mixtures

Liquid mixtures separate naturally by density. To isolate one layer, use a turkey baster to suck up the mixture and then let it stand for a few minutes. When the layers are clearly visible, squeeze the turkey baster to remove the bottom layer.

Homogenized Liquids

Homogenized mixtures, such as whole milk, can be frozen to force separation by density. When milk is frozen, the water molecules form a tight lattice structure, forcing out other molecules that don't fit into the lattice. The fat thus becomes separated from the rest of the milk and floats to the top. To remove the fat, simply scrape it off of the frozen skim milk.

Small Solids

Small solids can be separated by using a centrifuge. The sample is placed at the top of vials of salt solution, which are then capped and positioned in the centrifuge. As the centrifuge spins, the particles migrate downward and outward through the liquid. The more dense particles migrate further.

References

• US Department Of Energy: Oil and Water
• US Department Of Energy: Centrifuge Separation
• Terence A. Brown: Genome Replication -- Genomes -- NCBI Bookshelf