Normal filter paper burns pretty slowly.
I'm going to show you today how to make some paper that goes up in a flash.
So this is nitrocellulose or, more specifically, flash paper. By making use of
a nitration reaction, we can really accelerate the combustion process. That makes this an
ideal demonstration lower down the school, if you're doing something like the fire triangle,
or higher up in the school, if you're doing nitration reactions, obviously.
I've developed a modified version of the nitration reaction which I find really effective. It's really
good for avoiding spills, helps for the clean-up and it also makes sure that the nitration mixture
never gets too hot. For that nitration reaction, I'm using one of these – it's a polypropylene
microwavable food tray. This was advertised, I think, as some soup. You could also use ice cream tubs.
The important thing is a non-functionalised plastic – we don't want to be using something like PET ,which
will melt at a reasonably low temperature, could be hydrolysed by the acids, so look out for PP or
the recycling symbol number 5, and that will be a good choice for you. Nitration is done in a fume
cupboard while your tray is floated in a bath made in a Gratnells tray that contains about
55 grams of sodium carbonate. This is enough to neutralise the acid mixture that we're using.
With a few centimetres' depth of solution, the bath not only acts to catch any spills, quench
the nitration reaction, neutralise any excess acid, and clean-up, but also by floating the polypropylene
tray in the solution, we have an excellent heat sink to keep the nitration mixture cool.
Mix equal volumes of concentrated sulfuric acid
and concentrated nitric acid. CLEAPSS recommends 20 cm cubed of
each. This should be enough to nitrate two to three large pieces of classroom filter paper.
Using the volume suggested by CLEAPSS, you should ensure that the size of your polypropylene
tray doesn't exceed around 25 centimeters in diameter, or the liquid won't cover the
paper to nitrate it uniformly. If you can find something with handles, then that can assist
in distributing the nitration mixture as well as making it easy to hold during the clean-up.
Leave the paper nitrating for 15 minutes, and then use a pair of forceps to dump it into
the surrounding sodium carbonate bath. You'll need to prod it a few times to ensure that all of the
acid has been neutralised and, of course, you can see when this has happened as the fizzing stops.
A nitration mixture of 20 cm cubed of both acids should be sufficient to completely nitrate
two pieces of filter paper. The third one may not go quite as well. You'll be able to tell
from the texture and the color of the pieces of paper afterwards which one has actually worked,
and tear off appropriate places to to do the reaction. The paper itself once neutralised must be
absolutely bone dry. To help the process begin, squeeze it between some tissue paper to draw out
excess moisture. You will likely need to leave it overnight in order to get the best results.
Clean-up is ridiculously easy with this method, just scoop up some of the surrounding solution into the
polypropylene tray and then rinse around. Don't try to store your paper for extended periods of time.
It will decompose and it will also likely absorb a little bit of moisture from the humidity in the
atmosphere. But who's going to hang on to something this fun! in order to demonstrate you could put it
onto a gauze on a tripod at a distance like we saw before, but if you've got small pieces, say about
2 cm in size then those can safely be lit at arm's-length using a pair of forceps.
If you actually hold this out, arm's-length,
and leave a heat-resistant mat on the ground, you'll find that by lighting and letting go, a
well-nitrated piece of paper will have completely converted to gases before it hits the ground.
So, there it is: a really magical effect that's achieved in a flash!