Clever Chromatography

What if the substances we’re interested in are colourless? There aren’t any spots to see!

There are two simple ways of getting around this problem.

Using fluorescence

If the stationary phase ( the solvent part) has a substance added to it which fluoresces or glows when exposed to UV light, the glow is masked at the position where the spots are on the final chromatogram – even if those spots are invisible to the eye.


That means that if you shine UV light on the paper, it will all glow apart from where the spots are. The spots show up as darker patches, like this.

While the UV is still shining on the plate, you obviously have to mark the positions of the spots by drawing a pencil circle around them. As soon as you switch off the UV source, the spots will disappear again.

Showing the spots up chemically

In some cases, it’s possible to make the spots visible by reacting them with something which makes a coloured product. A good example of this is in chromatograms produced from amino acid mixtures. Amino acids are the building blocks of proteins.

The chromatogram is allowed to dry and is then sprayed with a solution of ninhydrin. chromo2Ninhydrin reacts with amino acids to give coloured compounds, mainly brown or purple. Have a look.

In another method, the chromatogram is again allowed to dry and then placed in an enclosed container (such as another beaker covered with a watch glass) along with a few iodine crystals.

The iodine vapour in the container may either react with the spots on the chromatogram, or simply stick more to the spots than to the rest of the plate. Either way, the substances we’re interested in may show up as brownish spots.

Separation Techniques

A little bit of revision first of all. A mixture contains two or more substances which have not reacted chemically with each other. A mixture is made of little bits of each substance mixed together.

A mixture can be separated by physical methods, a compound can not.

For example:
A mixture of iron filings and sulphur can be separated by using a magnet to attract the iron.
Iron is a magnetic material but sulphur is not.

If a mixture of iron filings and sulphur is heated the iron reacts with the sulphur and the compound iron sulphide [FeS] is formed which is not a magnetic material and cannot be separated by using a magnet.

Separation Techniques – Physical Methods.





We’ve already covered paper chromatography and Rf values. You should be able to work these outThese compounds are coloured. What if they're colourless? We calculate a number between 0 and 1 – the smaller distance travelled by the pigment divided by the larger distance travelled by the solvent. This is the Rf value for this pigment. The yellow pigment has a value of 14.5/18.5 = 0.78.

Here’s a thought. Sometimes the ‘pigments’ are colourless.  What can we do with these?  Here’s the answer.

Kinetic Theory Summary for Y9

kinetic theory.jpgEasy stuff for year 9 Chemists and Physicists

Useful stuff to start with…

  • Atoms = tiny particles 1 x 10-10m diameter (100 billion across a small fingernail)
  • Molecules = groups of atoms bonded (chemically stuck) together
  • Elements = unique number of nucleons[1], electrons, 118 in all (smallest H)
  • Compounds = 2 or more elements bound together chemically (Fe2O3, NaCl, H2SO4)
  • Mixtures = substances not chemically bound occupying the same space (Fe filings, sand)

Kinetic Theory

Three states of matter – solids, liquids and gases. (there are 5 actually, but we only need 3 of them)

Matter made up of very small particles in constant random motion. Accounts for diffusion and Brownian Motion. [Clicking on either of these links takes you to another post on this website. Scroll down to see them.]

This little applet shows what happens when you take the lid off a perfume bottle. There’s no way of stopping it, so just watch for a bit, then go back.

In solids the particles are packed very closely together. They vibrate (as if connected by tiny springs) about fixed positions and have strong forces of attraction between them.

Solids : have a high density

can not be compressed

do not flow

have a fixed shape

have a fixed size

In liquids the particles are close together but not as close as they are in solids. They can move around in any direction and are not fixed in position. The forces of attraction between them are still quite strong but, again, not as strong as in solids.

Liquids : have a medium density

can not be compressed

can flow

have the shape of their container

have a fixed size

In gases the particles are very far apart with large distances between them. They move around very quickly in all directions and the forces of attraction between them are very, very weak.

Gases : have a very low density

can be compressed

can flow

have the shape of their container

have the size of their container

You can change the state of a substance by heating or cooling it.

When the change is from a liquid to a solid it is called freezing.

When the change is from a gas to a liquid it is called condensing.

When the change is from a solid to a liquid it is called melting.

When the change is from a liquid to a gas it is called boiling or forced evaporation.

Here’s a summary…

ht diag

[1] Nucleons – the number of protons plus neutrons in the nucleus – the very tiny bit in the middle

Elements for Year 8’s

This little game is quite fun to play.  If you get to the end AND you can figure out how to get a printout of the ‘Well done…’ like the one below, hand it in to me….Merits available, of course.ptcross

If I can figure it out, so can you…

Also, what about a link between History and Chemistry? Try this crossword puzzle. It’s called ‘Known to the Ancients’. Three merits if you solve it correctly and work out how to hand it in…

Just for information – here’s a new element you might not know much about.

Vanadium. This is about 10cm long
Vanadium. This is about 10cm long

Vanadium is a bluish, grey metal used as an alloy to strengthen steel to make spanners and other tools, also huge structures like bridges and buildings. It’s named after the Norse goddess of beauty, Vanadia or Freya. How many other uses can you find for this element? Merits for the best answers…

Frictionless Motion – the Linear Air Track

When you have studied the 4.3 Further Mechanics 73-75 handout given out in class, try this…

This is a simulation of a linear air track, which you might have seen briefly before in the IG course.  The trollies or carts float on a cushion of air,  so can move horizontally with a gentle push at more or less constant speed. latIf the carts have small magnets attached to them, they can either stick together and collide inelastically or repel each other without touching, simulating elastic interactions. You can vary the speeds along a straight line, also the masses of the carts, hence calculate momenta and kinetic energies.

Also read 4.3 Further Mechanics 76

For a momentum calculator, click on this link. There’s lots that Wolfram Alpha can help you with. It’s worth having a look around, especially at the Maths parts.