The quality and quantity of the crystals formed unfortunately depends on many factors: the evaporat

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We continue our journey into the salt with an experiment. The schools have started recently and my soul "educational" re-emerges in this period. The salt is a raw material that costs little, is not dangerous, and can be used for various experiments to do with kids. When a child I played with the "small chemical" experiments which I liked were the ones which used to produce the crystals. I had produced some beautiful blue crystals of copper sulphate (a classic), large white crystals of alum, sugar crystals electric warmers and other substances. The salt, however, was never mentioned in my books chemistry experiments. "I wonder why?", I wondered. Only later I learned that to produce beautiful crystals of salt needed a long time, because of the particular solubility of sodium chloride. Let's start here then. How much salt is dissolved in water?
Take a container with a little 'tap water and start adding salt (coarse or until it does not matter) while stirring. Initially this will melt rapidly, then gradually slow the rate of dissolution until you can no longer add additional salt in solution: it is said that the solution is saturated. This happens because the water is able to maintain electric warmers in solution only a certain amount of salt. The excess remains undissolved on the bottom. The solubility of a substance electric warmers are the grams that can dissolve in 100 g of pure water. However, the solubility depends on temperature: if we increase the water temperature increase both the speed of dissolution is the amount of sodium chloride that can be dissolved. The solubility of almost all the substances increases with temperature. That of sodium chloride, however, does not vary too much (that of sugar varies much more for example). At 20 C one hundred grams of water dissolve slightly more than 35 grams of sodium chloride, while at 80 C if they dissolve about 38.
The precise determination of the solubility curve is difficult from the experimental electric warmers point of view, and in fact in the literature values are a little 'different. I refer to the article "On the solubility of sodium electric warmers chloride in water" H. Langer and H. Offermann, Journal of Crystal Growth 60, 389 (1982) which gives the following formula for the solubility of sodium chloride as a function of temperature expressed in degrees Celsius:
One of the simplest methods used to rapidly produce crystals of a substance soluble in water is to take advantage of the different solubility at different temperatures. It heats the water and add the maximum amount of the substance soluble at that temperature. At this point the solution is left to cool. Decreasing the temperature decreases the amount electric warmers of substance that can remain in solution, and then the excess goes to form crystals. This is the preferred method to get the beautiful crystals of copper electric warmers sulphate (found in the agricultural consortium if you want). This method, however, is not ideal for the salt because the difference electric warmers in solubility is not very high between hot water and water at ambient temperature.
For more convenience for all my experiments, rather than dissolve all times of the salt in the water, I prefer to keep on hand a bottle of saturated solution obtained by adding to a liter of water 400 grams of salt. Prepare electric warmers the solution a few days before helping with a funnel, shaking vigorously each time the content. You can see it in the photo: the undissolved salt remains on the bottom. Above the solution is saturated.
the way then in a plastic container to keep those food and leave to cool. The hot water evaporates a bit 'and electric warmers cooling of small crystals are formed on the surface which then sink and begin to grow. In the half-day tour of the beautiful crystalline sizes here at the bottom of the container.
The quality and quantity of the crystals formed unfortunately depends on many factors: the evaporation rate