Page 25 - Curriculum Visions Dynamic Book

Boiling tube cannon
In the demonstration on this page some crystals of orange ammonium dichromate have been strongly heated in a boiling tube. This adds heat energy to the crystals. Notice that the boiling tube is heated at the open end. This means that the reaction first occurs in the crystals near the open end, and only later to those near the closed end.
The first change is that the crystals glow red hot. At this point, the external heating is removed. The chemical decomposition gives out heat (as nitrogen molecules form), which causes the reaction to spread along the mixture. The crystals decompose to give nitrogen gas, water (as steam) and a bulky green solid called chromium oxide.
As the reaction continues, the chromium oxide blocks the open end of the tube, so as the heat energy is used to create nitrogen gas, pressure builds up towards the closed end of the tube until eventually the gas pressure is enough to shoot the chromium oxide out as fragments. This explosive violence is similar to the way ash is shot out from a volcano, or cannonballs, grapeshot
or shells fly out of a piece of artillery. In every case it is the rapid creation of nitrogen gas that builds up pressure and eventually breaks apart any nearby solids.
decompose: to break down a substance (for example by heat or with the aid of a catalyst) into simpler components.
In such a chemical reaction only one substance is involved.
explosive: a substance which, when a shock is applied to
it, decomposes very rapidly, releasing a very large amount of heat and creating a
large volume of gases as a shock wave.
 Fine particles of chromium oxide are ejected from the boiling tube cannon.
EQUATION: Heating ammonium dichromate
Ammonium dichromate ➪ nitrogen + chromium oxide + water (NH4)2Cr2O7(s) ➪ N2(g) + Cr2O3(s) + 4H2O(l)
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