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.

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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

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