In adventure novels, tells the story of old times, sometimes referred to letters written colorless ink; cunning enemies do not know the secret of a secret code, and only noble heroes can turn the invisible into the visible...
And the secret here and there, it has long been known. Some colorless substances as would occur under the action of heat, forming a colored compound. Such substances include, for example, lemon juice or onion. Dip them in the pen and make an inscription on a piece of paper - nothing visible. And now hold a piece of paper over the closed cooker or over the flame, but far enough away so that the paper is not broke, and the label will be clearly visible. The same experience good with milk and diluted vinegar.
Several similar experiences - but not with natural substances, and chemical reagents. Pour into a small test tube quite a bit on the tip of a knife, ammonium chloride and add about a teaspoon of water. In a clear solution dip pen, write or draw something on paper and allow to dry. After strong heating inscription or drawing will be clearly visible.
Even more striking, this experience is obtained with a highly diluted solution of cobalt chloride CoCl2. After drying lines on a white background is almost invisible, because it CoCl2* 6H2O (and it is formed after drying) - pale pink. But when the sheet is heated, part of the water of crystallization is cleaved, and the salt becomes blue. If re-wetted her, breathing on paper or, even better, holding it over the steam, the image disappears, because again formed shestibalny hydrated.
Perhaps this experience you have already met. Here's a version of it, much less known. We are not going to heat the sheet of paper, and to show how you can take part of water without heating, will deliver a preliminary experience.
Pour into test tube a little concentrated solution of chloride of cobalt pink. Add an equal volume of acetone and mix: the color of the solution will be blue! Dilute the solution with water, and he will again become pink.
What happened? Acetone is a good solvent of water and can take it away from other substances. But if so, it is written with a solution of cobalt chloride can be with acetone and without any heat? So. The sheet of paper, wipe with a cotton swab dipped in acetone, and the result will be the same as when heated.
Another experience with writing without a pen without ink. Straighten a piece of foil from chocolate and pin it buttons to the plate. One of the buttons attach to the negative pole of the battery. To the positive pole attach the nail, cleaned emery cloth. A sheet of writing paper moisten almost colorless solution of sodium chloride with the addition of red blood salt To3[Fe(CP)6], put on top of the foil and tap the highlight of the paper: it appears as a blue trace. During electrolysis ions Fe2+ , interacting with red blood salt, give Turnbull blue Fe3[Fe(CP)6]2. It penetrates into the paper and is fixed in the fibers. If instead of blood salt to take potassium thiocyanate KSCN or ammonium NH4SCN , then we'll not get the blue trail, and the red, because thiocyanate is formed of iron red.
Such experiments can be set not only with writing a paper, but with patches of pure white cloth.
Turning the invisible into the visible often has a tremendous help to those who disclose the crime, Saying that the crime scene always remain traces, only they are not always immediately noticeable. And criminologists are looking for in the first place fingerprints, because everyone is unique - as unique human person. Of course, the experts have subtle ways and suitable substance to prevent, detect and faint prints; we will use a rather crude but simple way.
Prepare a mixture of equal quantities of talc and carbon black (talc sold in pharmacies, and getting soot is described in the article about how to obtain pigments). Breathe on my finger to lightly moisten it, and push it on to a clean sheet of paper. Mark on the sheet unnoticeable, but if you sprinkle it with the prepared mixture and carefully spread with a soft brush (or just shake the leaf) and pour over the mixture, then the paper will remain a clear fingerprint. On paper were invisible fat traces, and they were adsorbiroval chastity black mixture.
The same experience can be supplied with different objects and surfaces - take an old newspaper, cardboard box, plastic or glass. In the latter case, better adhesion it is necessary to take more talc; after the excess mixture is removed from the glass, slightly pre-heat the glass is then prints on transparent surface will become more pronounced.