Mixtures and Chromatography

Mixtures are easily separated-not like compounds

• Unlike in a compound, there’s no chemical bond between the different parts of a mixture

• The parts of a a mixture can be either elements or compounds, and they can be separated out by physical methods such as filtration, crystallisation, simple distillation, fractional distillation and chromatography

• Air is a mixture of gases, mainly nitrogen, oxygen, carbon dioxide and argon

• Crude oil is a mixture of different length hydrocarbon molecules

• The properties of a mixture are just a mixture of the properties of the separate parts-the chemical properties of a substance aren’t affected by it being part of a mixture

  • For example, a mixture of iron powder and sulfur powder will show the properties of both iron and sulfur. It will contain grey magnetic bits of iron and bright yellow bits of sulfur.

You need to know how to do paper chromatography

• One method of separating substances in a mixture is through chromatography. This techniques can be used to separate different dyes in an ink. Here’s how you can do it:

• Draw a line near the bottom of a sheet of filter paper.

  • Use a pencil to do this-pencil marks are insoluble and won’t dissolve in the solvent.

• Add a spot of the ink to the line and place the sheet in a beaker of solvent, e.g. water.

• The solvent used depends on what’s being tested. Some compounds dissolve well in water, but sometimes other solvents, like ethanol, are needed.

• Make sure the ink isn’t touching the solvent-you don’t want it to dissolve into it

• Place a lid on top of the container to stop the solvent evaporating

• The solvent seeps up the paper, carrying the ink with it

• Each different dye in the ink will move up the paper at a different rate so the dyes will separate out. Each dye will form a spot in a different place-1 spot per dye in the ink

• If any of the dyes in the ink are insoluble(won’t dissolve) in the solvent you’ve used, they’ll stay on the baseline

• When the solvent has nearly reached the top of the paper, take the paper out of the beaker and leave it to dry

• The end result is a pattern of spots called chromatogram