Instructor Information
JCE Classroom Activity: #27
How Does Your Garden Grow? Investigating the “Magic Salt Crystal Garden”
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This activity encourages students to investigate the chemistry of the popular “Salt Crystal Garden” (1–3) and “Salt Crystal Tree” (2, 3) activities for children. Students grow salt crystals by evaporation from aqueous solutions containing various mixtures of table salt, ammonia, and laundry bluing in order to determine the purpose of each component. Crystals are composed almost entirely of NaCl; no crystals form if table salt is omitted. Bluing is a suspension of tiny particles of blue iron(III) hexacyanoferrate(II). The usual explanation is that these particles serve as seeds for formation of salt crystals, resulting in delicate flower-shaped crystals (1, 3). Published sources differ on the purpose of ammonia, stating that ammonia is added: “to increase the solubility of the salt” and may be omitted (3) or “chiefly to speed up the evaporation process” and is needed only if “all the conditions are not ideal” (1). Both sources agree that no chemical reaction takes place. In repeated tests under ambient conditions (in Madison, WI in March 2000) no flowerlike crystals were formed in the absence of ammonia. Observations suggest that a chemical reaction does take place between ammonia and bluing, and that the substance required for the formation of flowerlike crystals is a reaction product, not the blue particles. Regardless of the mechanism of crystal formation, students should be able to draw reasonable conclusions based on their results. Discussion of conflicting conclusions may lead students to design additional experiments to investigate further. WSupplemental material including color photographs is available in this issue of JCE Online.
Integrating the Activity into Your Curriculum This activity can be used to introduce: separation of solute from solution by evaporation; crystal formation or growing; preparation of solutions of specific composition; experimental design; and chromatography. The activity is easy to do and visually appealing. It could be used at the end of the school year as a review or application of many ideas introduced in class.
perforated
About the Activity
Caution: Splash-proof goggles should be worn during this activity. Ammonia can cause serious eye damage. Bluing stains equipment, clothing, and skin.
Most students should be able to complete Steps 1–7 in a single class period. To save time, stock solutions could be prepared in advance. Exact measurements are not critical to the success of the activity. The activity may be completed the following day or continued for several days. Materials for this activity can usually be obtained at grocery or discount stores. If it is not available locally, Mrs. Smith’s Bluing can be ordered from Luthor Ford & Company, P. O. Box 201405, Bloomington, MN 55431; telephone: 800/325-7785 or 612/881-1430; Internet: http://www.mrsstewart.com/mainhomepage/msbframe.htm/. The proportions of solution components used are those specified by Borgford and Summerlin (3). Solution moves up the paper by capillary action and crystals form on the paper as the solvent evaporates. Water soluble ink is not required, but if it is used chromatographic separation of the ink will occur. Ammonia may affect colors observed (4). The time required for crystal formation will vary, but some crystals should be visible within a day. If left undisturbed, crystals will continue to grow until the solution evaporates, but the paper cones may collapse under the weight of the crystal deposit. Typical observations: cone 6 is stained with blu- 1 2 3 4 6 ing; blue color does not move up the paper in 3 and 4; solution 4 loses its blue color and rusty-brown solid forms; crystals form on the outer surface of all cones except 5, forming first on 4 and 2. Crystals are: 1. cubic; 2. cubic; 3. cubic; 4. large volume of flowery or spiky crystals that fall off if the cup or cone is moved; 5. no crystals; 6. cubic. Typical con- Crystal deposits on paper cones are shown for 1, 2, 3, 4, and 6. Note clusions: crystals in 1–3 and 6 are ordinary salt crys- that 4 is substantially different from the others. Cones 1–4 included ink on the paper. tals; crystals in 4 are different; the blue substance in bluing is not soluble in water; the substance required to change the shape of the crystals is a product of the reaction of bluing and ammonia; ammonia increases the rate of evaporation. Answers to questions can be found above and on the Luthor Ford & Company WWW site http://www.mrsstewart.com/.
This Activity Sheet may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Background
Literature Cited 1. Untitled Document. http://www.mrsstewart.com/mainhomepage/msbscg.htm (accessed Mar. 2000). 2. Sarquis, M.; Sarquis, J. Fun with Chemistry, Vol. 2; Institute for Chemical Education: Madison, WI, 1993; pp 277–280. 3. Borgford, C. L.; Summerlin, L. R. Chemical Activities Teacher Edition; American Chemical Society: Washington, DC, 1988; pp 55–58. 4. The Write Stuff: Using Paper Chromatography to Separate an Ink Mixture; J. Chem. Educ. 2000, 77, 176A–176B. JCE Classroom Activities are edited and tested by Nancy S. Gettys and Erica K. Jacobsen
JChemEd.chem.wisc.edu • Vol. 77 No. 5 May 2000 • Journal of Chemical Education
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JCE Classroom Activity: #27
Student Activity
How Does Your Garden Grow? Investigating the “Magic Salt Crystal Garden” Have you ever seen or grown a “Magic Salt Crystal Garden”? In this easy and popular activity table salt is mixed with water, ammonia, and laundry bluing. The resulting solution is poured over a porous solid that sits in a bowl. Food color is added. As if by magic, a beautiful “garden” of flowerlike crystals appears on the solid. In this Activity you will grow crystals and try to figure out how the garden crystals are different from ordinary salt crystals. You should also be able to figure out what each component of the solution contributes to the process. Caution: Splashproof goggles should be worn during this activity.
Try This
You will need table salt, magnifying glass, white coffee filters, ruler, scissors, 5 small paper clips, 7 plastic or paper 8–10-oz. cups (if taller than 7 cm, trim to a height of 5–7 cm), water, measuring spoons or small graduated cylinders, household ammonia solution, laundry bluing, stirring rod or spoon, paper towels, and forceps. Water-soluble marker(s) are optional. __1. Sprinkle a few crystals (grains) of table salt on a clean surface. Examine them with a magnifying glass. Describe the crystals. How are they alike or different? __2. From a coffee filter, cut a pie-shaped wedge, 8–10 cm long and 12 cm at the wide end (1/4 of a 4-cup filter). Roll the wedge into a cone so that at the base the edges overlap by no more than 0.5 cm. Fasten the bottom edges with a small paper clip. The cone does not have to stay closed at the top, but it should stand upright when placed in one of the cups (pointed end up) and should not touch the walls. (If necessary, unroll the cone and cut off some paper to make it fit inside the cup.) Repeat to make a total of six paper cones. Optional: Use a water-soluble marker to draw a line around the circumference of five of the cones about 2 cm from the base, using different colors for segments of each line, if you wish. __3. Remove cones from cups and set them aside. Pour water into one of the empty cups until it is about 1–1.5 cm deep. Measure and record the volume of water in the cup by pouring it into a graduated cylinder or measuring spoon(s), then discard the water. Dry the cup with a paper towel. __4. Number the cups 1–7. In cups 1–5 and 7, mix approximately the volume of solution recorded in step 3 with the composition (by volume) listed below. Leave cup 6 empty. You may assume the volumes are additive. (For example, if the volume in step 3 was 6 teaspoons (t.), use 2 t. of salt and 4 t. of water in cup 1.) Stir each solution well. The salt may not dissolve completely. Observe and describe each solution. Cup 1. 1 part salt, 2 parts water Cup 2. 1 part salt, 2 parts water, 1 part household ammonia solution Cups 3 and 7. 1 part salt, 2 parts water, 1 part laundry bluing Cup 4. 1 part salt, 2 parts water, 1 part household ammonia solution, 1 part laundry bluing Cup 5. 2 parts water, 1 part household ammonia solution, 1 part laundry bluing __5. Find a well-ventilated shelf, window sill, table top, or other stable surface, where the cups can be left for a few days without being disturbed. Cover the surface with paper towels to protect it from solutions or crystals that may climb out of the cups. Place cups 1–5 on the surface. Carefully place one of the marked paper cones in each cup, pointed end up, so that the cone stands on the bottom of the cup without touching the walls. What happens? __6. Stand the remaining unmarked cone in empty cup 6. Very slowly and carefully, pour the contents of cup 7 over the cone in cup 6. Wet the entire surface of the cone. If the cone falls over, use forceps to return it to an upright position. Carefully place cup 6 on the paper towels with cups 1–5. Cup 7 may be reused or placed in the trash. __7. Without moving the cups, make observations about 15 minutes later. Has there been any change? Record observations hourly for the rest of the day (or for as long as possible). Check the cones the following day, and daily thereafter until crystals form on some or all of them. __8. Use a magnifying glass to examine the crystals deposited on the cones. Compare these with the salt crystals in step 1. If time permits, leave the cups undisturbed for 2–3 days and continue to observe them daily. When your observations are complete, use forceps (not bare hands!) to remove the paper cones from the cups. Any remaining solutions can be poured down the sink. If you like, you can stand the cones upright on labeled paper towels to dry, and keep them for display. Otherwise, cups, cones, and crystals are safe to place in the trash.
Questions __1. Based on your observations, what do you think is the purpose of each ingredient in the salt crystal garden solution? Comment on the suggestion in some crystal garden “recipes” that use of ammonia is optional. __2. Chemically, what is laundry bluing? How is it used in laundering clothing? Does it have any other household uses?
Information from the World Wide Web (accessed March 2000) Welcome to Bradley Products/Mrs. Stewart’s Bluing. http://www.mrsstewart.com/ National Chemistry Week – Experiments – Crystal Garden. http://www.chem-inst-can.org/ncw/experiments/egarden.html National Chemistry Week – Experiments – Growing Crystals. http://www.chem-inst-can.org/ncw/experiments/egrowingcrystals.html This Activity Sheet may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 77 No. 5 May 2000 • JChemEd.chem.wisc.edu
