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.
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.
Filter or blotting paper sheets – as large as possible
Hot-air blower, e.g. Hair dryer (Note 1)
Small paint brush
Beaker (100 cm3)
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
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.
Before the demonstration
Make a saturated solution of sodium nitrate by adding about 10 g of solid to 10 cm3 of water and stirring.
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.
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
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
This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.
The experiment is also part of the Royal Society of Chemistry’s Continuing Professional Development course: Chemistry for non-specialists