MadSci Network: Earth Sciences

Re: How long does it take crystals to grow?(after mixing rock salt & water)

Date: Wed Nov 29 21:43:46 2000
Posted By: David Smith, Faculty Geology, Environmental Science
Area of science: Earth Sciences
ID: 975454503.Es

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

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