Demonstration

A message is written on filter paper with a solution of sodium nitrate and is then dried, rendering it invisible. Applying a glowing splint to the start of the message makes the treated paper smolder and the message is revealed as the glow spreads its way through the treated paper.

Lesson organisation

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.

Apparatus Chemicals

Eye protection

Filter or blotting paper sheets – as large as possible

Wooden splints

Hot-air blower, e.g. Hair dryer (Note 1)

Small paint brush

Beaker (100 cm3)

Stirring rod

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

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection. 

Sodium nitrate(V), NaNO3(s), (OXIDISING, HARMFUL) - see CLEAPSS Hazcard.

1 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.

The demonstration
a 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.

b 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.

c Fix the paper where the audience can see it easily, and away from combustible material.

d Apply a glowing splint to the start of the message until the treated paper starts to glow and char.

e 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.

Health & Safety checked, August 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015