AnnabelleBuda
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6 years ago Teachers Information Sheet
This practical seeks to address aspects of the Working Scientifically and Material World sections of the syllabus. It is also linked to the English syllabus. In this experiment, students will cover the following syllabus dot points:
- observe some irreversible changes that common materials undergo to identify that the changes may result in new materials or products, eg rusting iron, burning paper, cooking a cake and making toffee.
- classify some observable changes that materials undergo as reversible or irreversible.
- explore how materials are used in innovative ways for specific purposes, eg the use of soft-fall materials in playgrounds and geotextiles to retain water in landscaping
- identify the properties of materials used in a familiar product and relate them to its use
- create literary texts that experiment with structures, ideas and stylistic features of selected authors
Equipment List:
Invisible Inks:
- Lemon juice/ vinegar
- Laundry detergent (whitening)
- White crayons
- Highlighter
Developing agents and other equipment:
- Black light
- Heat lamp, iron, candle etc
- White paper
- Water colours
- Plastic cups
- Thin paint brushes/ q tips
- Iodine solution
Method
Create a station for each ink and divide students between the stations.
- Read and answer the Inquiry worksheet.
- Use the paint brushes and one of the invisible inks to compose a short spy letter on paper. If you want to use a code you can!
- Put the letters on a rack to dry and move on to step 3.
- On the grid paper provided, test the different methods of making the invisible ink visible again! The solutions are iodine solution, black light, watercolour and heat. They should keep a record of how each ink is made visible.
- Take a spy letter that is not your own from the rack and attempt to make the ink visible using the methods available.
- Write the message from the letter in pen/ pencil and record which invisible ink you think was used.
- Answer the Reflection worksheet.
Background Information
Invisible inks have been used throughout history in the world of espionage, remaining relevant even until the 1990’s! While quite low tech in comparison to the encryption and cyber security used today, the chemistry behind these inks is still functional. The student enquiry sheet will contain some history about the types and development of invisible inks throughout history.
There are two main categories of invisible inks, organic (which are the type examined here and can be developed through the use of household materials, heat or UV light) and sympathetic (which is developed through a reaction with a specific reagent depending on the type of ink).
The reactions for developing the invisible inks in this example are all irreversible (chemical) reactions and so rely on the specific properties of the inks to work.
Lemon juice invisible ink:
Heat: The lemon juice is acidic as it contains citric acid. Citric acid is a weak acid which is in a lot of food we consume, it is what gives lemons, oranges etc. a sour taste. The acid weakens the paper fibres where it is applied, which means these parts of the paper are more susceptible to heat. When heat from a hot light bulb or hair dryer is applied, the unweakened paper is unaffected, but the weaker areas undergo a chemical reaction called “oxidation” when exposed to heat. Oxidation has the effect of turning those areas darker, revealing the ink. It is the same process that turns fruit such as apples darker over time when you cut them and expose the insides to air.
Iodine: The iodine solution bought in stores (Lugol’s solution) is potassium iodide and water. It is a dark brown colour due to the presence of the iodine molecule (I2) but reacts with starch to form a dark blue complex. Paper, potatoes and many plants contains starch and so when iodine solution is painted over the paper, the paper becomes a dark blue colour. However, aside from citric acid, lemon juice also contains vitamin C, which prevents this colour change by reacting the the I2 instead. This reaction is known as a “reduction” and is the opposite to the “oxidation” mentioned above. Because the vitamin C reacts with the iodine solution, the places where the lemon juice is placed should be revealed in white.
Laundry Detergent invisible ink:
Laundry detergent, teeth whitening agents, highlighters, tonic water and many other everyday substances will glow under a blacklight (UV light). These things all contain phosphors, which are substances which absorb the higher energy UV light and then re-emit the light as a lower energy visible light. Depending on the concentration, this may work with the lemon juice as well. If you do not have laundry detergent, another UV-light developed invisible ink can be made with diluted highlighter ink.
White crayons:
While not exactly an ink, white or plain wax crayons on white paper will also be invisible. To “develop” this will not require a reaction at all, but relies on the physical properties of the wax. By painting watercolours over the paper and wax, the message will be revealed. This is because the adhesion of the water to the wax is weaker than the water to itself, so little droplets are formed and can be wiped off the wax. On the paper, however, the watercolour soaks into the fibres. Thus the message is revealed in white.
Troubleshooting
- Use dilute solutions of lemon juice and laundry detergent so they aren’t immediately visible.
- Avoid skin and clothing contact with the iodine solution as it will stain.
- Avoid eye contact with all reagents.
Student Inquiry Sheet
- What kinds of areas do you think invisible inks are/were useful in?
- What do you think a spy would consider good properties for an invisible ink?
- Compose your secret spy letter here (you could include a password, a question, a riddle etc.):
- Have a guess about which of the solutions will develop the invisible inks before you test.
Student Reflection Sheet
- Which of the solutions developed which of the invisible inks? Put a tick in the box for the ones that worked and a cross for those that didn’t.
- Which invisible inks do you think would be terrible for spies to use? Give a reason for each one.
- When developing (making visible) the invisible inks, which processes do you think were reversible and which do you think were irreversible? Why?
- Do some research and find out about an actual invisible ink that was used by spies and how it works.
- What do you think would happen if you left the inks on the paper and didn't develop them? Would they eventually appear on their own?
MSDS
Potassium Iodide solution: Manufacturer_SDS (3).pdf
Lemon juice: Manufacturer_SDS (1).pdf
Laundry Detergent: Manufacturer_SDS (4).pdf
Vinegar: MSDS
Testing Practical No. 3 (Invisible Inks)
1) Use assigned ink to write your message. 2) Test each ink against the developing reagents. 3) Identify which ink was used to create which message.
Making the invisible inks and developing reagents
The white vinegar was applied to the printed grid using a cotton bud (q-tip). For the white crayon section of the grid, a white candle was used. Omo active clean front and top loader washing powder was mixed in water (1/2 teaspoon powder, 100 mL water) and stirred lightly to dissolve. It was then applied to the grid with a cotton bud. The lemon juice was diluted in water (50:50, water:lemonjuice) and applied to the printed grid with a toothpick for accuracy. The grid sheet was then allowed to dry.
To test the developing reagents, they were each applied to there own column on the grid.
The students will then analyse the grid to see which worked and how well.
The lemon juice, wax and laundry detergent were each revealed by the iodine, however the vinegar was not. The laundry detergent likely contains some additive which prevented the starch in the paper from reacting with the iodine. The vinegar does not contain the vitamin C which reacts with the iodine to prevent the reaction of iodine and starch. Thus the vinegar grid square was not revealed by iodine.
The water colour only revealed the wax as expected. The water was unable to adhere to the wax, so the white colour is revealed.
A candle was used for the heat test. It was waved half a centimetre under the paper until the paper began to brown. A hot iron was tested first, but did not reveal anything. The heat test revealed the vinegar and lemon due to the presence of acid in both which weakened the carbon bonds in the paper, so it was more quickly affected by heat. The laundry detergent was not revealed by the heat test. The wax was revealed, though this was simply because it was melted into the paper when exposed to heat.
Problems
The use of iodine: The iodine used here was 10% w/v. Lower concentrations are available which are deemed safe for use in schools. Candle: The teacher should handle the candle for this activity.
Grid Invisible ink.docx