Re: crystal growth and reasons why temperature and different
Postby tdaly » Sat Oct 25, 2014 12:15 pm
Thank you for this additional information. It is very helpful! It sounds like you controlled all of the right variables so that the difference must be due to using different kinds of water.
Take a look at the answer Vince Calder (of Argonne National Laboratory) gave when someone asked a similar question: http://www.newton.dep.anl.gov/askasci/g ... n99889.htm. I agree with him--other ions dissolved in tap water are probably interfering with crystal growth in your daughter's experiments. Tap water may have additional nucleation sites (which other scientists at Argonne point out in that post), but the impurities in tap water can actually stop crystals from growing larger. Those impurities aren't present in distilled water, so the crystals in distilled water can grow to larger sizes. That's the explanation for the result of your daughter's experiment.
As far as presenting her hypothesis and findings goes, your daughter's hypothesis shouldn't change after her experiment. A hypothesis is an educated guess about what will happen--you make it before doing the experiment. Her conclusion should (1) state whether her hypothesis was correct or incorrect, (2) say what the results were (crystals grew larger in tap water), and (3) briefly explain why the experiment turned out the way it did (other ions--impurities--in the tap water interfered with the growth of the crystals, keeping them from growing as large as the one in distilled water).
Last edited by tdaly on Fri Oct 31, 2014 7:29 am, edited 1 time in total.
All the best,
An Amazing Crystal Garden….and so easy to Make!
Jason Hower and Skip Rochefort, Chemical Engineering Department
Ellen Momsen, COE Women and Minorities Program
...with special thanks to Margie Haak , OSU Chemistry
Crystals have been an item of intrigue for thousands of years. Whether admiring them at a jewelers or searching for them in the ground, these shiny wonders of nature draw the attention of young and old. In nature, the amazing objects take millions of years to form in high temperatures and pressures. Now, thanks to the wonders of kitchen chemistry, you can make a veritable crystal garden in your own home. Below are two methods to make beautiful crystals and a brief explanation of how the gardens are formed.
Salt Crystals + Laundry Bluing = Crystal Garden
(school tested with many K-5 kids!)
This garden produces very nice, broccoli-like salt crystals from readily available ingredients. They tend to be fragile…but quite beautiful.
- 1 container (per child) – plastic (clear polystyrene) to-go dishes work well
- Several small pieces of porous material to grow on per container – sponges, clay pot pieces, BBQ Briquettes, etc.
- Laundry Bluing (such as Mrs. Stewart’s Liquid Bluing…careful it stains!)
- 1 small paper cup to mix ingredients (Dixie Cup)
- 1 plastic spoon
- Food Coloring (optional)
Begin by mixing 2 tablespoons of salt with 4 tablespoons of water, stirring to dissolve as much salt as possible. Next add 2 tablespoons each of ammonia and laundry bluing, again mixing as you go. The mixture will be a blue, watery sludge. Pour the sludge, liquid and extra salt, on top of the porous substrate in the plastic dish. Make sure to get all the salt out of the mixing cup and onto the porous items in the plastic dish (Note: If you use a sponge, wet it first abd squeeze dry for better sludge absorbtion). All the salt may not go into solution; this is okay as long as you pour the extra salt on top of the items in the dish. The porous materials in the dish will not immediately soak up all the liquid; this is okay. The crystals will naturally grow white, like the salt. If you want colored crystals, add a drop or two of food coloring on each item in the dish. The crystals will grow that color.
Allow the container to sit open to the atmosphere overnight. By the next day, crystals should have formed on the items in the dish and your crystal garden should be starting to bloom. You can keep the garden “in bloom” by adding 2 more tablespoons of salt on the second day, then half batches of the whole mixture from time to time. Make sure to pour the liquid into the base of the container and not on top of the already formed crystal blooms, as it will dissolve them and you will have to start all over…which is also a lot of fun!
The garden is formed by the salt after the water and ammonia evaporate away. The ammonia helps to speed the evaporation of the liquid from the mixture. The laundry bluing helps to form crystal blooms instead of crystal chunks or plates. The bluing solution is actually a colloidal suspension; it has very small particle that will not dissolve, but are held up and separated by the liquid. As the water evaporates away, the salt forms crystals using the colloidal particles as a seed, or nucleus, for growth. The liquid mixture and the salt are pulled away from the bottom of the container up to the tops of the porous material by capillary action, much the same way water spreads through a sponge. This allows you to add more mixture to the bottom and have your garden bloom and grow forever. You could experiment by leaving out the ammonia or bluing, or changing the ratios in the recipe.
Copper Sulfate Geodes….or Geode in a “half-shell”!
(adult supervision definitely required)
While the second activity makes gorgeous geodes and not a garden, it still is a fun way to explore solutions, evaporation, and crystal growth. The supplies for this activity are more difficult to obtain. Copper sulfate is not sold in grocery stores and most likely needs to be ordered or purchased from a chemical supply company. However, the directions and ingredients are very simple:
- 1 half egg shell – used as the shell for the geode
- Supersaturated Copper Sulfate solution
Begin by making a supersaturated copper sulfate solution. Heat water and add as much copper sulfate as you can when the temperature is elevated. Allow the solution to cool until you can safely handle it and pour a small amount into each half egg shell. Over night, the blue liquid will evaporate and leave translucent blue crystals that resemble the insides of a geode. The crystals are safe to touch and investigate.