Thermal decomposition of nitrates: ‘writing with fire’

In this experiment, students observe as a message is written on filter paper with a solution of sodium nitrate, before being dried to render it invisible. As the teacher applies a lighted splint to the start of the message, the treated paper begins to glow and smolder, revealing the hidden message.

The demonstration takes about 10–15 minutes. It could be a student activity, but with a large class it will need a well-ventilated laboratory .

The message drawn on the paper should be such that when the treated areas burn through, the letters, and the sheet of paper as a whole, remain intact.

Equipment

Apparatus

• Eye protection
• Filter or blotting paper sheets – as large as possible
• Wooden splints
• Hot-air blower, eg Hair dryer (see note 4 below)
• Small paint brush
• Beaker, 100 cm³
• Stirring rod

Chemicals

• Sodium nitrate(V) (OXIDISING, HARMFUL), about 10 g

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout. 
• Sodium nitrate(V), NaNO₃(s), (WARNING: OXIDISING; HARMFUL) – see CLEAPSS Hazcard HC082.
• Make sure the hot-air blower is electrical safety tested. If a hot-air blower is not available, judicious use of a Bunsen flame or an oven provides an alternative method for drying the paper.

Procedure

Before the demonstration 

Make a saturated solution of sodium nitrate by adding about 10 g of solid to 10 cm³ of water and stirring.

For the demonstration

1. Using a small paintbrush (or a length of wooden splint), write a message on the absorbent paper. Use joined up writing! Design the message so that the sheet of paper will remain in one piece after the message has burnt through.
2. Thoroughly dry the message using a hot-air blower, or by holding the paper well above a Bunsen flame. The message will be virtually invisible, so mark the start of it with a light pencil mark.
3. Fix the paper where the audience can see it easily, and away from combustible material.
4. Apply a glowing splint to the start of the message until the treated paper starts to glow and char.
5. Remove the splint and watch as the glow and charring work their way along the message, leaving the untreated paper untouched.

Teaching notes

If lesson time is limited, the writing of the message and the drying process could be carried out before the demonstration begins.

This experiment could be used to introduce the fire triangle: fuel, oxygen and energy.

With older students, the demonstration could be used to revise the equations for the decomposition of nitrates. In this particular case, sodium nitrate decomposes to give sodium nitrite (sodium nitrate(III)) and oxygen, and it is the oxygen released which helps to promote the burning process and produce the glow and charring:

2NaNO₃(s) → 2NaNO₂(s) + O₂(g)

Most other nitrates will also produce a similar effect, but potassium nitrate is less effective because it is less soluble and some other nitrates may give off very toxic nitrogen dioxide when they decompose.