How to make the invisible visible. Fun chemistry experience at home

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How to make the invisible visible. Chemical experiments

Entertaining experiments in chemistry

Entertaining experiences at home / Chemistry experiments for children

Adventure novels that tell of ancient times sometimes mention letters written in colorless ink; cunning enemies do not know the secret of cryptography, and only noble heroes can turn the invisible into the visible...

And there is no special secret here, it has long been known. Some colorless substances seem to appear under the action of heat, forming colored compounds. Such substances include, for example, lemon juice or onion juice. Dip a pen in them and make an inscription on a piece of paper - nothing is visible. Now hold a piece of paper over a closed electric stove or over a flame, but far enough away that the paper does not flare up, and the inscription will become clearly visible. The same experiment works well with milk and diluted vinegar.

A few more similar experiments - but not with natural substances, but with chemical reagents. Pour into a small test tube quite a bit, on the tip of a knife, of ammonium chloride and add about a teaspoon of water. Dip the pen in a clear solution, write or draw something on paper and let dry. After strong heating, the inscription or drawing will become clearly visible.

This experiment is even more effective with a highly dilute solution of cobalt chloride CoCl₂. After drying, the lines on a white background are almost invisible, because the crystalline CoCl₂* 6H₂O (namely, it is formed after drying) - pale pink. But when the leaf is heated, part of the water of crystallization is split off, and the salt becomes blue. If you moisten it again by breathing on the paper or, even better, by holding it over steam, the image disappears, because the hexahydrate crystalline hydrate is again formed.

You may have already experienced this experience. Here is his version, much less known. We will not heat the sheet with the inscription at all, but in order to show how it is possible to take away part of the water without heating, we will set up a preliminary experiment.

Pour some concentrated pink cobalt chloride solution into a test tube. Add an equal amount of acetone and stir: the color of the solution will turn blue! Dilute the solution with water and it will turn pink again.

What happened? Acetone dissolves water well and can take it away from other substances. But if so, can something written with a solution of cobalt chloride be developed with acetone and without any heat? Exactly. Wipe the sheet with the inscription with a cotton swab dipped in acetone, and the result will be the same as when heated.

Another experience with writing - no pen and no ink. Straighten a sheet of foil from chocolate and pin it with buttons to the board. Connect one of the buttons to the negative pole of the battery. Attach a nail cleaned with sandpaper to the positive pole. Moisten a sheet of writing paper with an almost colorless solution of table salt with the addition of red blood salt K₃[Fe(CN)₆], put it on top of the foil and touch the paper with a nail: a blue mark will appear on it. During electrolysis, Fe ions²⁺, interacting with red blood salt, give turnbull blue Fe₃[Fe(CN)₆]₂. It penetrates the paper and is fixed in its fibers. If instead of blood salt we take potassium thiocyanate KSCN or ammonium NH₄SCN, then it will turn out not a blue trace, but red, because red iron thiocyanate is formed.

Such experiments can be done not only with writing paper, but also with scraps of clean white cloth.

Turning the invisible into the visible is often of great help to those who solve crimes. They say that there are always traces at the scene of a crime, but they are not always immediately noticeable. And criminologists look first of all for fingerprints, because they are unique for each person - just like human faces are unique. Of course, experts have subtle ways and suitable substances to detect very faint prints; we will use a rather crude, but simple way.

Prepare a mixture of equal amounts of talc and carbon black (talc is sold in pharmacies, and the preparation of carbon black is described in the chapter on pigments). Breathe on your finger to slightly moisten it, and press it against a clean piece of paper. The trace on the sheet is invisible, but if you sprinkle it with the prepared mixture, carefully spread it with a soft brush (or just shake the sheet) and pour off the excess mixture, then a clear fingerprint will remain on the paper. There were invisible grease marks on the paper, and particles of the black mixture were adsorbed on them.

The same experience can be made with different objects and surfaces - take an old newspaper, a cardboard box, a plastic or glass cup. In the latter case, for better adhesion, you need to take more talc; after the excess mixture is removed from the glass, warm the glass slightly - then the prints on the transparent surface will become more distinct.

Author: Olgin O.M.

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