Magic writing with sodium nitrate solution

Student Sheet

In this practical I will be:

• Writing messages using an invisible ink.
• Changing the conditions of the invisible ink to see if messages become visible.
• Using my knowledge of metal salts to explain how changing the conditions the ink is under affects its visibility. 

Introduction:

You have been working for King Agis of Sparta, developing invisible inks for writing secret messages. You know some of his armies are running low on supplies, including the chemicals used to make some inks visible. It can also be difficult to find enough firewood to heat the entire paper in order to read the message. You therefore need to develop an invisible ink that reveals its image with only a small amount of heat. Like all good science-artists, you decide to investigate further…

Wear eye protection.
Wear disposable gloves.
Tie back long hair.

Equipment:

• Filter or blotting paper sheets – as large as possible
• Wooden taper (spill)
• Matches
• Bunsen burner or hairdryer
• Small paint brush
• Tongs

Or

• 1 large piece of filter paper or blotting paper 
• 1 wooden spill
• 1 small paint brush
• 1 pencil
• 1 paperclip
• Access to a hair dryer or warm radiator
• Access to a washing line
• Sodium nitrate saturated solution

Method:

Using a small paintbrush (or wooden taper or spill), write a message on the filter paper with the saturated sodium nitrate solution. It is important to use joined up writing.Mark the start of your message with a light pencil mark.

Dry the message using a hair dryer or by holding the paper on a warm radiator. When dry the message will be almost invisible.

Use a paper clip to hang the filter paper from a washing line above the bench or hold the paper with tongs over a sink. Turn on the tap to run some water gently. 

Light the spill and then blow it out leaving the end glowing hot. Apply the glowing taper/spill against the start of the message until the treated paper starts to glow and char.

Remove the taper/spill and watch as the glow and charring works its way along the message, leaving the untreated paper untouched. 
If the paper begins to burn too much it can be dropped into the sink with running water to put it out safely.

Going further:

Try other metal nitrates such as potassium nitrate or lithium nitrate. 

• What type of chemical is a metal nitrate?
• Why does the solution have to be saturated?

Theory:

The reaction is thermal decomposition, a chemical decomposition caused by heat. 

For a chemical to decompose the temperature must be just right. Metal salts decompose at different temperatures. When the metal is near the bottom of the reactivity series the salts generally decompose at low temperatures, and when the metal is near the top of the reactivity series the salts generally decompose at high temperatures. This is because stronger bonds form between the ions of salts of metals towards the top of the reactivity series. Strong bonds break less easily so the temperatures need to be high.

The temperature at which a substance chemically decomposes is known as the decomposition temperature.

When the reaction takes place it is usually endothermic. Heat is required to break the chemical bonds in the compound undergoing decomposition. If decomposition is exothermic, the reaction will occur very fast and possibly result in an explosion.

The reaction for the thermal decomposition of sodium nitrate 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.

Potassium nitrate works in the same way as sodium nitrate.

Lithium nitrate also works in a similar way but decomposes slightly differently due to the higher charge density on the lithium ion:

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

Lead nitrate will work and the reaction is:

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

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

NH₄NO₃(s) → N₂O(g) + 2H₂O(l)

Nitrogen(I) oxide (N₂O) does not work. It will decompose to give oxygen, but there is presumably either insufficient N₂O to keep the paper smouldering or the temperature is too low to bring about decomposition.

Forensic scientists have used information about the thermal decomposition of paints by fire to identify the original paint.

Teacher and Technician Sheet

In this practical students will:

• Write messages using an invisible ink.
• Change the conditions of the invisible ink to see if messages become visible.
• Use their knowledge of metal salts to explain how changing the conditions the ink is under affects its visibility. 

Introduction for teachers:

(This topic could start with a group discussion about what chemical change is, what burning means, when things burn, what they need to burn, etc. During this discussion the teacher introduces the following ideas, especially the words in bold.)

In this student activity a message is written on to filter paper with colourless, dilute, aqueous solutions of metal nitrates. 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.

Over time some compounds decompose either due to heat or light, and this change can result in, for example, a change in colour due to the formation of a new compound. 

The activity follows on from the investigation on invisible inks. It can be used as an introduction or finale to a topic on burning. It could be used to introduce the fire triangle: the nitrate is the fuel, heat from the burning spill, and oxygen from the air and the thermal decomposition of the nitrate salt. With older students it could be used to revise equations for the decomposition of nitrates. It is also a nice end to a term, giving a little bit of fun but still staying on subject.

(This practical can be completed by students working in pairs). 

Curriculum range:

All ages can take part in this activity since the aim is to gain some understanding of the thinking of the chemist and chemical change.  It links with:

• setting up simple practical enquiries, comparative and fair tests;
• reporting on findings from enquiries and observations, including oral and written explanations, displays or presentations of results and conclusions;
• using straightforward scientific evidence to answer questions or to support their findings;
• building a more systematic understanding of chemical change;
• asking questions and developing a line of enquiry based on observations of the real world, alongside prior knowledge and experience;
• using appropriate techniques, apparatus, and materials during laboratory work, paying attention to health and safety;
• making and recording observations using a range of methods for different investigations; and evaluate the reliability of methods and suggest possible improvements;
• presenting observations using appropriate methods;
• interpreting observations and identify patterns using those observations to draw conclusions;
• presenting reasoned explanations, including explaining data in relation to predictions and hypotheses; and
• learning about the concept of thermal decomposition.

Hazard warnings:

Wear eye protection. 
Wear disposable gloves.
Tie back long hair.
Ensure that the room is well ventilated.
It is the responsibility of teachers to carry out an appropriate risk assessment on the chemicals.

Equipment:

Per pair of students

• Large sheets of white filter paper, chromatography paper or blotting paper.
• Hair dryer (optional).
• Wooden taper (spill).
• Small paint brush.
• Saturated solution of sodium nitrate (sodium nitrate (V), NaNO₃). OXIDISING & HARMFUL

Or 

• 1 large piece of filter paper, chromatography paper or blotting paper 
• 1 wooden spill
• 1 small paint brush 
• 1 pencil
• 1 paperclip
• Access to a hair dryer or warm radiator
• Access to a washing line
• 10 cm³ saturated solution of sodium nitrate (sodium nitrate (V), NaNO₃) in water 
• 100cm³ beaker

Technical notes:

The small paint brushes should be about the size used for painting models. Wooden spills will do if these are not available.
You can make up a set of stock saturated solutions of sodium nitrate by adding about 10 g of solid to 10 cm³ of water in a 100 cm³ beakers and stirring. Then leave to settle.

Results:

When touched with a glowing taper or spill the nitrate begins to decompose and burns with the release of oxygen.

If the solution is weaker than a saturated one it will not burn the ‘writing’ but may set the paper on fire.

Going further:

Try other metal nitrates such as potassium nitrate and lithium nitrate. 

• What type of chemical is a metal nitrate?
• Why does the solution have to be saturated?

You could use a more dilute solution to demonstrate the difference between the saturated solution and a weaker solution.)