Thermal decomposition of nitrates: ‘writing with fire’

A close-up photograph of a lighted wooden splint against a black background

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Make an invisible message ‘glow’ by applying a lighted splint to filter paper treated with sodium nitrate in this demonstration. Includes kit list and safety instructions.

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.

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