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As an end of high school treat, my class is trying to convince our chemistry teacher to make an explosion as a demonstration. What sort of reactions are beautifully explosive but safe enough to do in a school lab environment, when performed by a teacher who knows what they're doing?

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From our friends at Periodic Videos, the following come to mind:

Gun cotton, more precisely known as nitrocellulose, which is a very simple nitration that you could show prior to the pyrotechnics.

Sodium and Potassium are also good, though you may not always get an explosion, depending upon the available hydrogen above the metal. Which suggests...

Hydrogen! This is the classic explosion in oxygen. In fact, on a small scale, the electrolysis of water into hydrogen and oxygen can be used to show that the mixture of these gases is very explosive. But keep it small!

All of these experiments avoid outright explosives, which seems prudent in a high school setting.

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    $\begingroup$ In my high school chem class we filled plastic pipette bulbs with different ratios of H2 and O2 made by H2O electrolysis, then jammed them on a partially disassembled kitchen lighter. When we pulled the trigger on the lighter, it would spark and ignite the gases, blowing the bulb down the hall. Measured the distance each ratio of gas would produce and determined ideal ratio of H2 to O2. So these demonstrations can be more than just "HEY LOOK AT THIS KICKASS FIREBALL!" $\endgroup$ – user137 Sep 10 '14 at 5:15
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    $\begingroup$ Another variant of the hydrogen plus oxygen demo is to pass the gas mixture into a soap bubble solution. The bubbles rise in the air and can be ignited with a taper or handheld propane lighter. Start out with pure hydrogen, which burns gracefully with a whoosh, the add more and more oxygen until, when you are at the stoichiometric ratio, the bubble will detonate with a loud report. $\endgroup$ – iad22agp Dec 10 '14 at 10:10
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Nitrogen triiodide is pretty spectacular and not dangerous so long as you keep the batch small. I remember my high school chem teacher had a batch left in the back of the classroom to go off randomly to scare us all. It's basically a matter of mixing aqueous ammonia and iodine crystals, leaving them to react a couple hours, then gravity filter. The result is pretty stable so long as it's damp, but explodes with a cloud of purple smoke if even slightly disturbed when dry. Even a feather or something touching is likely to set it off. As I mentioned above, keep the batch small. It doesn't take much to give a good show.

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I am giving you the link where you can find many interesting explosion experiments.but let me tell you these are little dangerous too.so be careful while performing these. http://youtu.be/sfW7KztrA8I

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Explosions usually happen too fast to be overly fascinating. Explosions with true detonation are quite dangerous.

There is quite a set of flash-powders that produce a cloud of flame and smoke. They are not true explosives as they do not detonate. The easiest one is $\ce{1Zn +1S}$ equimolar mix, it produces bright green-blue burst. I also found funny $\ce{3CuO +2Al}$ mix - it burns with a report, producing yellowish flame and black smoke, and covering nearby surfaces with yellow layer of copper, and $\ce{1Bi(OH)2NO3 +5Mg}$ - in small doses (and I mean really small) it burns with a report, bright (may be even blinding) yellow flash and mushroom cloud of bright yellow smoke. Bismuth oxynitrate is quite easy to prepare via hydrolysis of commercially available bismuth trinitrate. $\ce{2Fe +KNO3}$ mix burns without explosion, producing fountain of yellow sparks. Generally, however, mixes with metal powders must be handled with great care as the mixes may be sensitive, burn without oxygen and produce toxic fumes. Metal powder are available via specialized pyro- or chemical stores and aluminium powder is available also via painting store. However, in reciepes above the finest powder is required.

There are very few explosives that are easy to make. If you have time, you can go with synthesis of barium styphnate (lead is much more toxic, and explosion products will contain lead oxide smoke. Barium is toxic too, but to much, much lesser extent, so it is reasonable safe). It IS sensitive, and will explode if left on heating plate, but is quite easy to produce (styphnic acid is produced by nitration of resorcinol in two stage process with $\ce{NaNO2 + HCl}$ first and $\ce{HNO3}$ second, barium styphate is easily produced from barium chloride, styphic acid and a base, though don't do it without finding proper procedure), so no need to store it. I tested it and found... funny, if not very spectacular.

Here is a funny video for you http://youtu.be/7gnC0mXE0Vk

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