Chemistry Paper 2

Chemical Analysis

A pure substance can be a:

  • single element
  • single compound

A pure substance is not  mixed with any other substance.

To tell if a substance is pure you can look at their melting/boiling points.

  • A pure substance melts as a specific fixed temperature.
  • A pure substance also has a specific fixed boiling point.
  • Impure substances melt and boil over a range  of  temperatures.

A formulation is a complex  mixture that has been designed as a useful product.

In a formulation, the quantity of each component is carefully  measured so that the product has the properties we need. Some formulations include: fuels, paints, cleaning products, medicines etc.

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Chemical Analysis

All separation techniques, including paper technology are physical  processes. This means that they do  not involve chemical reactions and no new  substances are made.

Paper chromatography allows us to separate substances based on their different solubilities.

  • We take a piece of chromatography  paper and draw a pencil  line near the bottom.
  • We take a dot of each of our colours  on the pencil line, making sure there is enough space on the paper.
  • Place the bottom of the paper into a solvent. A solvent is a liquid that will dissolve substances.
  • The solvent now makes its way up the  paper. It dissolves  the  ink in the coloured dots and carries them up the paper.

We call the paper the stationary  phase because it does not  move.

We call the solvent the mobile  phase because it does move.

  • If there is only one spot- the substance is pure
  • if there are multiple spots- it tells you that there is more than one substance present (impure) i.e. 2 spots, 2 substances.
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Chemical Analysis

Paper chromatography works because each chemical in the mixture will be attracted to the stationary phase (paper) to a different  extent.

Chemicals that are strongly  attracted to the stationary phase will not move very far. 

Chemicals that are only weakly  attracted will move further up the paper. 

We draw the line in pencil because it is insoluble and also if we drew it in pen, the ink would move  up the paper.

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Chemical Analysis Required Practical

  • Draw a horizontal pencil line on the chromatography paper around 2cm up in pencil. 
  • Use a capillary tube to put a small spot of each of the known food colours and the unknown colour onto the pencil spots.
  • Keep the spots relatively small, so the colours don't spread  into  each other.
  • Pour the solvent (water) into a beaker and attach the paper to a glass  rod and lower it into the beaker, where the bottom of the paper should dip into the water.
  • The solvent should not touch the ink/line, otherwise it will wash off.
  •  The sides of the paper must not touch the side walls of the beaker, because it interferes with the way that the water moves.
  • Put a lid on to the beaker to reduce evaporation of the solvent.
  • As the water moves up the paper, it carries up the colours.
  • Once the solvent has moved up a suitable amount, mark the point where the water has reached.
  • Leave the paper to dry.
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Chemical Analysis Required Practical

We can look Rf values up in a database and that will tell us the identity of the chemical.

Several different chemicals could have this Rf value. We may need to repeat this experiment using a different solvent to narrow it down further.

If this chemical has never been analysed before then there will not be an Rf value on the database. We would need to carry out further analysis to identify it. 

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Chemical Analysis

  • To test fpr hydrogen, we remove the bung of a test tube that may contain hydrogen and insert a burning splint.
  • Hydrogen gas burns rapidly and produces a pop sound.
  • To test for oxygen, we use a glowing splint and insert it into a test tube that may contain oxygen.
  • When you place the glowing splint into the test tube, the splint should relight if oxygen is present.
  • To test for carbon dioxide we use limewater (an aqueous solution of calcium hydroxide).
  • Draw some of the gas that may be CO2 into a plastic pipette and then bubble the gas through to the limewater.
  • If the limewater turns cloudy, then CO2 is present.
  • To test for chlorine, we insert damp litmus paper into the mouth of the test tube. 
  • If chlorine is present, then it bleaches the litmus paper white.
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