What's it all about?

Paper will burn in any direction, but if an oxidising agent is soaked into it, the paper will burn best there. Sodium nitrate solution can be painted onto absorbent paper and left to dry. A smouldering taper will set the paper on fire where the dried sodium nitrate is, and the paper will burn along the lines you have painted. Lots of fun, lots of smoke, and very smelly!
 

What is here?

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Download the Acrobat fileon this topic (119K - this identical to the one we use, including the students’ worksheet in colour), or
 
View the notes below. These are divided into:
Student notes
Teacher notes
Technician notes

Student notes

Put 10cm³ of distilled water in a beaker. Add some sodium nitrate, stir with a glass rod to dissolve it, and continue until no more will dissolve. With a small paintbrush, write a message or draw a picture on a piece of paper using the sodium nitrate solution. Make sure you use joined-up writing, and don’t complete any loops. Leave the paper to dry. Light a splint and blow it out so that the end is just glowing. Touch the glowing end to the start of the message or picture until the treated paper starts to glow and char.

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Teacher notes

Fire writing is very easy to do, but you must be careful. We find it works best on sugar paper (which is very absorbent), and the students prefer to draw an image in pencil first. The sodium nitrate solution should not be used too sparingly; otherwise it can be difficult to sustain the smouldering lines. If the fire writing seems to be burning too slowly, it can help if the paper is held almost vertically. Care must be taken, however, that it doesn’t burn too vigorously – it can burst into flames unexpectedly. Similarly, care must be taken when disposing of burnt and unburnt paper at the end of the session: we had a small fire in a waste bin two years ago, which was easily put out with a carbon dioxide fire extinguisher. This was entertaining for the students, but less so for us as the lab was brand new and, until then, never been used! Fire writing produces a lot of smelly smoke, so ensure good ventilation in the lab.

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Technician notes

Per student:

1 x bench mat
1 x 100cm³ beaker
1 x 10cm³ measuring cylinder
1 x stirring rod
1 x small paintbrush
1 x pencil

On the side bench:

paper
splints
sodium nitrate

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