Chemical Demonstrations

from FAQ SCI.CHEM part 4 of 7

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Hydrophobie von Goretex mit Lösungsmitteln steuern

The ability of water-miscible solvents to mask the hydrophobic nature of Goretex can be demonstrated. Goretex is just a porous PTFE, the same material as PTFE filters - such as Millipore HF. You can easily filter liquid water through porous PTFE, provided the filter is previously wetted with a water-miscible solvent ( usually ethanol ). If a filter is set up on a vacuum flask, ensure the filter is completely wetted with ethanol, turn on the vacuum, and immediately add water - it rapidly filters through.

Once it has stopped, it only takes about 15 seconds for the air to dry the filter, then ask a student to filter more water from the same flask. No chance.

Pour off the water, surreptitiously add a few mls of ethanol, immediately followed by the same water - and watch it filter through again :-).

This is the nearest equivalent our laboratory has to the workshop practice of sending an apprentice out to purchase a spark plug for a diesel engine. It does relate slightly to the real world - indicating why "breathing" fabrics like Goretex should not be used with solvents.


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"Blue Bottle Reaction"

From: brom@yoyo.cc.monash.edu.au (David Bromage)
Date: Tue, 14 Sep 1993
Subj: Re: Need: A safe chemical display

Half fill a 1 litre flask with water and add 10g og NaOH, then add 10g of glucose and up to 1ml of 1% methylene blue. Stopper the flask and swirl gently to dissolve the contents. On standing for a few minutes the solution should turn colourless. When the flask is shaken the solution will turn blue then decolorise on standing.

Methylene blue exists in solution as a reduced colourless form and an oxidised blur form. The initially blue dye is reduced by the alkaline form of glucose and re-oxidised by dissolved oxygen. When the solution is shaken, atmospheric oxygen enters into solution at a more rapid rate than when left standing. The dye acts here as a catalyst whose colour indicates the redox state.


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Chemischer Garten

Make up (or dilute a commercial preparation) of sodium silicate to 1.1g/ml. Place this solution in a large glass container then add 'lumps' or large crystals of salts to be grown. Lumps should not be more than 0.5cm in diameter. As a salt dissolves it forms an insoluble silicate which forms a membrane around the lump of salt. The membrane is permeable to water which enters and dissolves more salt. The resulting pressure bursts the membrane releasing more salt solution to form more membrane. As the salt solution is less dense than the silicate solution, the membrane grows as a convoluted vertical tube.

        Salt                     Colour        Growth time 
  Ferric chloride                brown           1 hour
  Ferrous sulphate             grey-green        3 hours
  Cobalt chloride                purple          5 hours
  Chromium chloride            grey-green        6 hours
  Nickel sulphate            yellow-green      ~24 hours
  Cupric sulphate                blue          ~24 hours
  Potassium aluminium sulphate   white          ~1 day
To produce a "garden" which is not completely overgrown with the faster species it is necessary to take growth rates into account. Distilled water should be used as Ca and Fe in tap water can cause cloudiness.
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Oszillierende Reaktionen

A. Iodate reaction.
  Make up 3 solutions
  1) Dilute 200ml of 100 vol hydrogen peroxide to 500ml
  2) dissolve 21g of potassium iodate (KIO3) and 1.5ml of conc sulphuric
     acid in 500ml of water.
  3) Dissolve 7.8g of malonic acid and 1.4g of manganese sulphate in 400ml
     of water and add 1.5g of starch in 100ml of water.
Add equal volumes (50-100ml) of each solution to a flask in any order. Colourless-blue oscillations should start within 2 minutes. If not, try 10-20% variations in relative volumes. (try increasing 2 first). Oscillations should last up to 10 minutes but I my experience have lasted up to 3 hours.

B. The Belusov reaction

 Prepare 5 solutions.
  1) 58g of malonic acid on 500ml of water
  2) 6M sulphuric acid
  3) 21g of potassium bromate (KBrO3) in 500ml of water
  4) Dilute 5ml of solution 2 to 500ml then add 1.75g of cerous sulphate.
  5) 1.6g of 1,10-phenanthroline and 0.7g of ferrous sulphate in      100ml of water (or commercial ferroin solution to 0.025M)
Mix together 50ml of 1 to 4 and 5ml of solution 5. Blue-pink oscillations should start within a few minutes.

For either oscillating reaction the choice exists of complete mixing with uniform oscillations or waves of colour (eg in a measuring cylinder). Some interchange of reagents is possible. The Bray reaction omits malonic acid from the Iodate reaction. Malonic acid can be replaced by citric or succinic acids.


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Unbrennbares Papier

A particularly dramatic 'trick' is not to burn paper. Make up a solution containing 57% v/v ethanol and 43% v/v water with 5% w/w sodium chloride. Soak a filter circle in the solution and hold it near a flame (with tongs) just long enough to ignite. After the flames die down the filter circle will still be intact. The ethanol burns but just enough water remains in the paper to prevent ignition. NaCl is added to provide a more convincing flame. To add drama, 'burn' a banknote - but ensure that all of the note, especially the corners, is soaked.
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Silberbaum

From: lmartin@uclink.berkeley.edu (Lonnie C Martin)
Date: 17 Feb 1993
Subj: Re: Growing a Silver Tree in Beaker?

In article <...> xslkkk@oryx.com (kenneth k konvicka) writes:
>Am trying to do a demo for elementary school kids. How do you grow a tree
>of silver using copper wire(?) submerged in a solution of AgNO3? Saw one
>in high school physics class about a thousand years ago at good ol' Reagan
>HS, Austin, Tx. Was really beautiful. The silver formed nice large
>plates. Any demonstration books you could steer me toward?

What you have described is about all there is to it. I do this demonstration for the chemistry classes here at Berkeley about twice a year, or so. Just make a "tree" out of copper wire (you might clean it with sandpaper or steel wool) so that it will fit into a beaker of your choice (we use 4 litre here), and pour in the silver nitrate solution. I think we use 0.1 molar, but as long as the concentration is fairly close to that, it will work just fine.

It is not necessary to make the tree very "bushy". The silver will fill it out nicely with fuzzy thick hanging globs of crystals. The solution will change from colourless to blue, as copper nitrate is formed. A very nice experiment. You can expect this to take on the order of an hour to get fully developed.


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