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In the experiments, which are so similar to tricks, colorless solution was reacted in one, then in another color, and it happened immediately, as if by magic wand. Indeed, the chemical reactions are very fast and usually begin immediately after mixing the reagents. However, this rule has exceptions. The reaction mixture may remain for some time, colorless, and then instantly be painted. Want - after five seconds, you want ten; you can put a "chemical clock" on the required time.

Prepare two solutions. The first: 3,9 g potassium Iodate KIO3 per liter of water. Part the second: 1 g sodium sulfite Na2SO3, 0,94 g of concentrated sulfuric acid (caution!) and a little - a few milliliters of starch paste is also per liter of water. Both solution is colorless and transparent.

Measure out 100 ml of both solutions and fast, better with stirring, hcl second to the first. The experience easier to put together - let your friend immediately starts the countdown stopwatch or clock with second hand. Six to eight seconds (the exact time depends on temperature), the liquid will immediately turn dark blue, almost black color.

Now measure again 100 ml of the second solution, and 50 ml of the first dilute with water to exactly half. With stopwatch in hand, you make sure that the time elapsed since the draining of solutions to their colouring, too, will increase in two times.

Finally, mix 100 ml of the second solution with 25 ml of the first diluted with water four times, i.e. until 100 ml. "Chemical clock" will work four times longer than in the first experiment.

This experience demonstrates one of the fundamental chemical laws - the law of the masses, according to which the reaction rate is proportional to the concentrations of the reacting substances. But here's the question: why solutions painted immediately after a pause, but not evenly and gradually, as should be expected?

Sulfuric acid in the solution displaces the Iodate - sulfite ions from their salts. When this is formed in the solution iodomethane acid HI, but she lives long and immediately reacts with adnovate acid HIO3. In the allocated free iodine. It gives a color reaction with starch.

If everything went exactly the solution and it was getting dark would gradually as iodine. However, in parallel with another process: sulphurous acid, H2SO3 reacts with the free iodine and the newly formed iodomethane acid. This reaction is faster than the previous, and iodine, not having to paint the starch, again restored to JO3--.

It turns out that the color should not appear at all? Note: during the reaction of sulfurous acid is continuously consumed, and as soon as all of it will turn into sulfur, iodine nothing will interfere to react with starch. And then the solution will instantly turn around the volume.

Diluting the solution twice and four times, you have reduced the concentration of potassium Iodate, and the reaction rate was decreased proportionally.

Explanation, it seems, took longer than the actual experience with clock...

O. Holguín. "Experiments without explosions"
M., "Chemistry", 1986

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2012-04-26 08:28:44
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