The thermal decomposition of nitrates – ‘writing with fire’
The thermal decomposition of nitrates – ‘writing with fire’
31
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 smoulder 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 and chemicals
Eye protection
Filter or blotting paper sheets – as large as possible
Wooden splints,
Hot-air blower, e.g. Hair dryer (see note 1)
Small paint brush
Beaker (100 cm3)
Stirring rod
•
Sodium nitrate(V) (Oxidising, Harmful), about 10 g
Technical notes
Sodium nitrate (Oxidising, Harmful) Refer to CLEAPSS® Hazcard 82
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. 95
Procedure
•
HEALTH & SAFETY: wear eye protection
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 the paper where the audience can see it easily, and away from combustible material.
Fix
d
31
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 smoulder 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 and chemicals
Eye protection
Filter or blotting paper sheets – as large as possible
Wooden splints,
Hot-air blower, e.g. Hair dryer (see note 1)
Small paint brush
Beaker (100 cm3)
Stirring rod
•
Sodium nitrate(V) (Oxidising, Harmful), about 10 g
Technical notes
Sodium nitrate (Oxidising, Harmful) Refer to CLEAPSS® Hazcard 82
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. 95
Procedure
•
HEALTH & SAFETY: wear eye protection
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 the paper where the audience can see it easily, and away from combustible material.
Fix
d