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Year 7 & 8 TOC

Separating mixtures


LO: Investigating and using a range of physical separation techniques such chromatography

Something we discussed earlier in this chapter is solubility. Solubility is how easily a substance dissolves in a solvent. Knowing how well different substances in a mixtures dissolve in the solution is the basis for chromatography. 

Humans have been using dies and inks for thousands of year. Many ancient cultures like the Egyptians, Greeks, Chinese and Romans all used dies and inks in their art. However,  in ancient times, obtaining die colours wasn’t as going to the shops. Ancient Greeks would make dies out of  soot and vegetable gum. Chinese would make red ink from mercury sulphate and Romans would make purple out of  extracts from glands of snails! In fact, purple was considered a royal colour because it took 12,000 snails to make 1.4 grams of dye.


Chromatography is a method of separating substances from a mixture based on how soluble they are. It works by placing a special type of paper into a mixture and allowing the water to slowly move up the paper. As the water moves up the paper, the more soluble substances move with the water. The substances which are less soluble, take longer to move up the paper. Eventually you’re left with series of substances running up the paper.  

If we place a sheet of paper in ink, the coloured dye which is most soluble will end at the top and the dye which is least soluble will stay at the bottom. 

Chromatography can be used to separate substances in drinks, air and body fluids. Science laboratories use complex machinery to detect one gram of a substance in a one thousand litre of solution. It’s used in a wide range of different industries, from Medical and Pharmaceutical Industries to Forensic Science and Testing. 

Research 3 different ways we use chromatography and explain what they are looking for. 

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