|MadSci Network: Earth Sciences|
Salt crystals can grow in a matter of minutes or it can take weeks. The time depends on the method you use and the environment where you are growing the crystals (is the room hot or cold, humid or dry?) To grow crystals really quickly, you need prepare a hot saturated solution, cool it, and provide a place for crystals to grow. A saturated solution is one in which as much salt as possible has dissolved. You can look up how much salt you need ion books like the one I list below, or if you know a chemist, ask them to check in their "CRC Tables." You might also find the CRC Tables in the library. It's a huge book, and lists many things about many compounds. You want to look up the solubility of sodium chloride. You may need a librarian to help you. Instead of looking it up, however, you can just keep adding salt until there is always salt left on the bottom, no matter how long you stir it. You may be surprised at how much salt it takes, so don't start with more than a cup (~200 mililiters) of water. Table salt will dissolve faster than rock salt, but has some powdery material added to it that will make it harder to see if it has dissolved. This powder absorbs humidity and is the reason why "When it rains, it pours..." as the Morton salt box says. The powder won't really affect your experiment, so you don't need to worry. If you saturate a solution at room temperature, it will not grow crystals until some of the water evaporates. Since there is less water, some of the dissolved salt that was in that part of the water now has no place else to go and it comes out of solution to grow crystals. Putting a small hole in the cover over a very clean dish of just saturated salt water and adding one little crystal to give the salt a place to grow is an excellent way to grow really big, really nice crystals, but it takes weeks and weeks. I've grown salt crystals a centimeter across, but it took two months. You sounded like faster was better, so back to how to do it fast. To make crystals grow fast, you need to have an oversupply of salt in the solution. Some people call this a supersaturated solution (more than saturated). If you heat water up, a lot more salt can dissolve than could at room temperature. The higher the temp, the more will dissolve. Before you start, get a very clean heat-safe bowl (Pyrex is good, or a canning jar) ande have it ready. Then get an adult to supervise (and maybe help) and heat your water to not quite boiling (little bubbles rising, but no big bubbles; if you let it boil, salt crystals will form on the surface of the liquid as you watch, but they will be tiny little needle shapes) and start stirring in salt. Again, you want to do this until some salt is left on the bottom no matter how much you stir. Stop stirring and let the water settle until it is very clear. You may need to turn the heat down to keep it from boiling, but don't let it cool much at all, you should still see the little bubbles. Once it has cleared, remove it from the heat a carefully pour only the liquid into your bowl or jar. Be very careful not to pour off any crystals, because they will lead to the formation of a mass of tiny crystals on the bottom of the jar as it cools. If your bowl or jar is very clean (no dust) and you didn't spill any salt grains in with the water, you should be able to cool the jar down without having any crystals form. Now you have a jar with a whole bunch of salt just dying for an excuse to come out of solution. Any rough surface (such as a piece of string) will do, but you can tie a little salt crystal into the string to provide an extra good place for new crystals to grow. You'll need to tie a small washer or other weight onto the string to keep it down in the solution. You should have visible crystals in a half day to a day and certainly within two days. This method drives faster growth, so the crystals will often be unevenly formed and cloudy and it is very hard to get them bigger than a few milimeters across, so you will want a magnifying glass to get a really good look. For more directions and tips for successful crystal growing, there is one excellent source you need. Crystals and Crystal Growing by Alan Holden and Phylis Morrison. MIT Press, Cambridge, Massachusetts, 1982. Library of Congress Catalog Number: QD921.H58 ISBN for paperback edition: 0-262-58050-0 This book has recently been reissued and should be available at a library or bookstore near you. I looked for websites, but didn't find any that I thought would be very helpful. Once the rush is over, you should try the slow evaporation method to make some really nice crystals. Also, some other materials grow better than salt. An easy one is alum, which you can sometimes find with the spices in the supermarket (it's used sometimes to make pickles) or get from a pharmacist. Whenever you grow anything other than salt, you need to be careful to wash your hands well after handling the material or the crystals and to make sure that your crystals can't get into the hands of younger kids who could mistake them for candy. Good luck, David Smith Geology and Environmental Science Department La Salle University, Philadelphia
Try the links in the MadSci Library for more information on Earth Sciences.