2 minutes reading time (387 words)

    The Thermal Decomposition of Nitrates - "Magic Writing"

    Description

    A message is written on filter paper with a solution of sodium nitrate and is then dried. Applying a glowing taper 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 only.

    Apparatus

    • Filter or blotting paper sheets - as large as possible.
    • Wooden taper.
    • Bunsen burner or hair-drier.
    • Small paint brush.

    Chemicals

    • Sodium nitrate (V), 10g

    Method
    Before the demonstration
    Make a saturated solution of sodium nitrate by adding about 10g of solid to 10mL of water and stirring. Using a small paintbrush (or a length of wooden taper), write a message on the filter paper. Use joined up writing! Dry the message using a hair-drier 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.

    The demonstration
    Pin up the filter paper in the sight of the audience. Apply a glowing taper to the start of the message until the treated paper starts to glow and char. Remove the taper and watch as the glow and charring works its way along the message, leaving the untreated paper untouched.

    Teaching tips

    This demonstration could be used to introduce the fire triangle: fuel, heat and oxygen. With older students it could be used to revise the equations for the decomposition of nitrates.

    Theory

    The reaction that occurs is:
    2NaNO3(s) → 2NaNO2(s) + O2(g)
    The oxygen produced is sufficient to keep the treated paper smouldering while the untreated paper does not burn.

    Extensions

    Try other metal nitrates (see below). What effect does the nitrate concentration have?

    Further details

    Potassium nitrate works in the same way as sodium nitrate.
    Lithium nitrate also works although it decomposes slightly differently due to the higher charge density on the lithium ion.

    4LiNO3(s) → 2Li2O(s) + 4NO2(g) + O2(g)

    Lead nitrate will also work:

    2Pb(NO3)2(s) → 2Pb(s) + 4NO2(g) + O2(g)

    Ammonium nitrate does not work because it does not give off oxygen as it decomposes:

    NH4NO3(s) → N2O(g) + 2H2O(l)

    Although nitrogen(I) oxide (N2O) will itself decompose to give oxygen, there is presumably either insufficient N2O to keep the paper smouldering or the temperature is too low to bring about decomposition.

    Safety

    Wear eye protection. It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.

    Light Scattering by a Colloid (The Tyndall Effect)...
    Red, White and Blue
     

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    Thursday, 18 October 2018

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