Art and Science of Crystals
Inquiry Lab

Purpose

In this home-inquiry lab you will explore the growth of crystals. Mineral substances, when dissolved in water, will often form crystals as the water evaporates or cools. These crystals can take a variety of forms depending on the substances they are made of and on the conditions under which they grow. Although crystal formation may appear to be organic there is nothing living. Instead, atoms and molecules naturally stack together in regular repeating patterns, which when grown large enough, become obvious without any magnification.

Materials

Background

Solubility

Water is a substance that is easy to take for granted. We require it to live but it usually plays the role of a theatrical prop, easily ignored in the excitement of events going on around it. The chemical truth of the matter, though, is that water is a miraculous material. It is safe and non-toxic but still it is one of the most powerful solvents known to science. So many substances easily dissolve in water that it is fairly called the universal solvent. For example, salt (NaCl) and sugar (C₁₂H₂₂O₁₁) both dissolve in water. These are very different kinds of chemical substances, for all that they look alike. Salt is an ionic compound of a metal and a non-metal and the bonds between its atoms are among the strongest known. Salt has a very high melting point as proof of this strength: 801°C (1,474°F). When salt melts the bonds between the atoms must be broken—the high melting point shows that a large amount of energy is necessary to break these bonds. Sugar is a compound of non-metal atoms bound by covalent bonds. The bonds holding the atoms together are reasonably strong but they are not broken when sugar melts. The bonds that do break are based on the weak forces of attraction holding one molecule to the other in a solid or liquid form. As proof of these weaker bonding forces, sugar melts (if you haven't burned it first) at 186°C (367°F). Despite their differences, salt and sugar are both very soluble in water.

Although they both dissolve in water, there is a difference between sugar and salt in how well they dissolve. Salt will dissolve in water only until it makes up about 26% of the solution by mass. That is, when 26 g out of 100 g of the solution are salt. The other 74 g are water. Sugar, on the other hand, will dissolve until it makes up 40% by mass of the solution. Sugar is much more soluble than salt. Different substances all have their own unique amounts that represent the maximum mass that will dissolve in a given amount of water. When the maximum amount of any substance is dissolved in water we call the solution saturated. There is an activity available in which students can explore solubility as a function of temperature in a pencil-and-paper exercise in which they put data on a graph and answer questions about it. Find it on my site here. Saturated solutions will not allow any more of the dissolved material to dissolve. Additional solid that is added appears to remain at the bottom of the container, unchanged. Chemists measure the solubility of a substance based on its concentration when the solution is saturated. Substances with higher solubility make saturated solutions with larger dissolved masses.

Solubility is also subject to changes in temperature. Gases, such as oxygen (O₂), dissolve better in cold water than in hot water. Solids, on the other hand, usually have a higher solubility in hot water than in cold water. For example, at a temperature of 90°C a saturated solution of sugar will be 45% sugar by mass instead of 40%. By making a solution at a high temperature it is possible to dissolve more solid than at low temperature. Compared to the amount that dissolves at room temperature, a solution made at high temperature is supersaturated. A supersaturated solution is one in which more material is dissolved than the usual maximum amount at a given temperature. When the temperature falls as the solution cools off the extra dissolved material will form crystals.

Crystal Formation

Image source: Aaron Keller
Salt at the Atomic Level

Image source: Aaron Keller
Salt Crystals Showing Typical Square Shape

A crystal is a special solid form of a pure substance that has a regular three-dimensional geometrical shape. Each substance in the world has its own unique crystalline shape.

Image source: Aaron Keller
A water molecule

Image source: Aaron Keller
A borax snowflake

Crystals will form from a saturated solution left out on a table simply because the water evaporates away. When there is less water, less of the solid can remain dissolved. As a result, the solid begins to form crystals. These crystals may be so small that they appear to be like a fine powder. Visible crystals form when crystallization is slow. If evaporation takes a long time then bigger crystals have time to form. In the picture of salt crystals at right it is possible to see that they all have the typical square shape of salt crystals. Their different sizes are a result of different amounts of time spent growing as the water evaporated. This type of crystal formation is one that the in-class portion of this activity addresses. The ‘magic trees’ that you grow are made of crystals of salt that form as a result of the rapid evaporation from a piece of paper soaked in the solution.

Another way in which crystals can form is when a solution which is saturated at a high temperature is allowed to cool down. As it cools the solution becomes super-saturated and the excess solid crystallizes out. If the solution cools quickly then the crystals that form will be small. On the other hand, if the solution is allowed to cool more slowly then this will allow larger crystals to form. In the at-home portion of this activity borax will be dissolved in water at high temperature. Next it will be allowed to cool so that crystals may form. It is usually possible to create very attractive decorative ornaments by allowing the crystals to form on pipe cleaners which have been bent into pleasing shapes. For example, see the pictures at left.

Crystallization and the ‘Magic Tree’

The paper trees you will make in the in-class portion of the lab may form a strange and wonderfully different kind of crystal. Salt crystals naturally form in cubic shapes but when the mixture of water, salt, ammonia, and Prussian Blue pigment is just right then they form a different type of crystal called a dendritic crystal. Dendritic crystals have a branched form which looks like tiny trees. Solids crystallizing out of a solution as the water evaporates away may form dendritic crystals just because they form very quickly. The dendritic crystal is in actually a collection of extremely tiny crystals, each growing separately from the others and never growing very large. If evaporation is very fast, crystals may form branching, tree-like structures. Another reason a solid may form dendritic, rather than typical crystals, is when an anti-caking agent is added. For example, commerical salt sold in grocery stores usually include very small amounts of chemicals that interefere with normal crystal growth. These are safe to eat and keep the salt free-flowing. One of these, Yellow Prussiate of Soda, is a close relative of one of the ingredients you will use to make your magic trees, the Prussian Blue pigment. (Canning and Pickling Salt does not include an anti-caking agent).

In this lab the salt crystals form dendritically because of some complex interactions between the ingredients. The Prussian Blue pigment is a chemical called ferric hexacyanoferrate: Fe₄[Fe(CN)₆]₃ (also called iron(III) hexacyanoferrate(II)). When in basic solution, as in ammonia, some of the iron(III) ions (Fe³⁺) precipitate as iron(III) hydroxide (Fe(OH)₃). The iron(III) hydroxide is brown and looks like mud and when it forms the solution loses its blue color. Once this happens the hexacyanoferrate ions are free in solution and when they attach to the surface of a growing crystal of salt they stick very tightly. Because they do not come off easily, and because they prevent the crystal from growing on surfaces where they are stuck, the hexacyanoferrate ions prevent large crystals from forming and also prevent crystals from being able to stick together. In this way their presence prevents salt from growing its normal cubes and instead brings about a growth of crystals that appears almost organic, as if a living creature were developing before our eyes. In fact, the crystals are called dendritic precisely because they resemble incredibly tiny trees (the word is based on the Greek word for tree).

In-class Lab Procedure

Safety and Disposal

The following list does not cover all possible hazards, just the ones that can be anticipated. Move slowly and carefully in the lab: haste and impatience have caused more than one accident.

• If you choose not to wear safety glasses you are choosing to sit out the lab.
• Prussian Blue pigment is non-toxic but should not be consumed.
• Ammonia is a household cleaner with a high pH (it is alkaline). As such it is corrosive to skin and eyes and all bodily contact should be avoided. Wash hands thoroughly after accidental contact. Remove contact lenses and rinse eyes for fifteen minutes in case of contact with eyes.
• All materials are safe to go into the trash, once dry. To clean crystals out of dishes simply rinse with plenty of water. Small amounts of vinegar will remove brown deposits.

The Challenge

In this in-class portion of the lab you will investigate which combination of ingredients is required to create beautiful crystal trees covered in delicate dendritic salt crystals. Also, you will attempt to determine what the role of each ingredient is in the creation of the unusual crystals.

Part I: The Paper Tree

In this part you will read and follow instructions on how to make a simple paper cone which will stand up well even when wet.

1. Use a glue stick to glue a piece of construction paper to a piece of manila folder. Press firmly to make a strong bond.
2. Cut out a circle with a 12 cm diameter from the construction paper reinforced with manila folder. Cut the circle in half on a diameter. Fold the half-circle into quarters and tape the cut sides together. Open this shape out into a cone, shaping it with your fingers until it is self-supporting.
3. If desired, cut out the shapes of tiny trees or other things using the same manila/construction paper material. The more edges and points these shapes have, the more crystal growth you will see on them. The shape should be able to lean against the cone without falling down, even when wet.

Part II: The Basic ‘Magic Tree’

Published recipes for growing a ‘magic tree’ provide the information that the following ingredients are needed: salt, water, household ammonia, and Prussian Blue pigment solution. Most of these sources say nothing about the purpose of each of the ingredients. By setting up experiments in which you leave out one or more of these ingredients you should be able to learn how they work together and explain the role of each component.

What follows is a generic recipe meant to produce a total volume that will not overflow a typical petri dish. You will be asked to plan a series of experiments in which you choose how to vary this basic recipe, by leaving out different ingredients, in order to figure out what is going on. In order for you to be familiar with the procedure and so that you will be able to create the basic ‘magic tree’ first follow the recipe below to create your ‘magic tree’. Afterward you will discuss your plans with your lab partner and your teacher about how to set up the experimental portion.

1. Obtain a 100-mL beaker.
2. Using a graduated cylinder, measure out 20 mL of tap water. Add it to your beaker.
3. Using a graduated cylinder, measure out 5 mL of household ammonia solution. Be careful handling this liquid as it is corrosive. To measure it, pour a little into a clean small beaker (50-mL size). Then measure the solution by pouring from the beaker into the graduated cylinder. Do not pour directly from the bottle into the graduated cylinder! Pour any excess solution back in the bottle and put the cap back on the bottle.
4. Add the ammonia solution to the water.
5. Obtain a weighing dish and a small lab scoop. Use a digital lab balance to measure out 0.10 grams of powdered Prussian Blue pigment. Remember to put the cap back on the bottle.
6. Add the powder to the beaker. Use a wash bottle and a very small amount of water to wash all of the powder into the beaker. Your teacher can show you how to do this effectively.
7. Stir the ingredients with a glass stirring rod.
8. Note: After you add the powdered Prussian Blue pigment to the water and ammonia you may notice that something strange has happened after a few minutes. You are witnessing a chemical reaction between the iron ions in pigment and the hydroxide ions from the ammonia solution. Tiny, solid, brown crystals will form and the solution will lose its blue color. Pause to observe this.
9. Obtain a petri dish (just the top or bottom only) and set the empty petri dish in a safe place, specified by your teacher. Do this before you add liquid to the dish. Create a slip of paper to place partly under your dish that has your name, class period, and today’s date on it. This is so you can know which one is yours later.
10. Slowly pour the solution you made into the petri dish.
11. Measure out 12 grams of salt (sodium chloride, NaCl) using a weighing dish and a 3-beam balance. Do not forget to add the mass of the weighing dish! Add it to your petri dish and stir. You should notice that not all of the salt will dissolve. This is on purpose because it ensures that the solution you make is absolutely saturated with salt.
12. Place your cone in the middle of the dish. If you add a little tree or other shape be careful to lean it against the cone it so it won’t tip over. Watch for a few minutes as the water begins to creep up through the construction paper. Within 24 - 48 hours this recipe will cause the cone to be completely covered with beautiful white crystals! The speed depends on the humidity in the room.
13. You could also use a little food coloring to decorate it your cone or tree further. Simply dab a drop of food coloring on the tip of any branch of your tree to add some color to the crystals that grow on that branch.
14. Once the crystals have finished forming, take some photos for your report.
15. Next, you will do an experiment to see what happens when you leave out one or more of the ingredients.

Part III: The Experiment

This is the most important part of the in-class lab. Once you know the recipe for how to make the ‘magic tree’ you can vary that recipe in order to figure out the purpose of each ingredient. Work together with your lab partner and your teacher to make a table recording the variations you will make to the recipe. There are three useful ways to vary the recipe to isolate the purpose of each ingredient by leaving out one or more of them. Make a plan, write it in your lab notebook, share it with your teacher to get some feedback, then carry out your plan. Remember to keep the total volume of liquid a constant for all trials that you set up. For any trials including powdered Prussian Blue pigment you will need no more than 0.10 grams. Clearly label each trial with a description of the ingredients, your name, your lab partner’s name, and the date and time. Over the next 1 to 3 days take time to look at and write down observations about your experiments in your lab notebook.

To help you get ideas for what to do, here are some things you will need to take time to observe:

• Find out what crystals of salt look like when they are unmodified by additional ingredients. You will need only salt and water to find out.
• Find out what happens if you leave out the Prussian Blue pigment but include everything else.
• Find out what happens if you use the Prussian Blue pigment but leave out the ammonia.

At Home Lab Procedure

Safety and Disposal

• The borax should not be consumed. Keep away from pets and small children, who may put the ornaments in their mouth.
• Take pains to be neat and tidy with the borax. It is safe to use in the kitchen in your usual cooking pots and with your usual utensils provided you rinse them off very well. Borax is a cleansing agent and can be used in the kitchen to clean dishes. Like any cleaner, it should not be consumed. Clean all surfaces thoroughly to prevent accidentally consuming borax.
• Because this craft involves the use of boiling water any small children who are helping should be assisted by someone old enough to use the stove safely.
• There are no concerns about the disposal of the materials in this activity. All un-used materials may be thrown in the trash and excess borax solution may go down the drain.

Borax Crystal Ornaments

For this at-home experiment you will actually engage in an arts and crafts activity that takes advantage of scientific knowledge. You will be able to express yourself creatively while also learning about how solubility varies with temperature and how crystallization can occur when a hot, concentrated solution cools.

Part I: Making Ornaments

1. Use your imagination to create seasonal ornaments by shaping pipe cleaners into whatever shape you like. Make a snowflake or a bow. If you would like to help decorate the chemis-tree in your teacher’s room then make an ornament in the shape of a piece of chemical glassware or a molecule and bring it to school.
2. After shaping your ornaments tie a string to the point from which you’d like to hang it. Or use floral wire or ornament hangers to make a hook. Attach the hook to the top of your ornaments. Alternatively, just bend one end of the pipe cleaner into a hook shape.
3. Obtain as many heat-proof containers as the number of ornaments you would like to make. They should be large enough that when you hang the ornament by its string or hook the ornament can hang without touching the sides or bottom. Be sure that they can safely be completely covered with boiling-hot water.
4. Hang your ornaments in your containers by suspending them from a popsicle stick or a pencil.
5. Most mugs are about a cup and a half in volume. Pint mason jars are two cups (1 pint = 2 cups). Total up the volume of borax solution you will need to make the ornaments you have planned.

Part II: Growing Crystals

1. To make the super-saturated borax solution you will need boiling water. For every cup of boiling water you use, add 6 tablespoons of borax powder. Mix up the solution by boiling the total amount of water in a pot on your home stove. Measure out the water based on the total you calculated previously.
2. Measure the borax powder into a cup while you wait for the water to boil.
3. Once the water is boiling remove it from the heat. Wait a moment or two for the bubbles to stop coming up to the surface. Slowly add the borax powder to the hot water. Sometimes adding a finely divided solid to very hot water will cause it to come to a boil very quickly. Be careful!
4. Stir the solution with a metal spoon until all of the borax dissolves. If any borax remains at the bottom without dissolving then pour the liquid into another container and leave the solid behind. You do not want any solid material left at the bottom of your crystallization containers.
5. Remove an ornament from its container. Add the hot borax solution. (Be careful pouring boiling-hot water!) Replace the ornament in the solution, being careful not to let it touch the sides or bottom.
6. By the next day your ornaments should be covered with beautiful clear crystals. Take them out of the solution and set them on a paper towel or some used newspaper to dry. Borax is water soluble so don’t let them get wet after they dry off.

Report

Answer the following questions in a typed report. Use a numbered question and answer format. Much of it can be answered by carefully reading the introductory text in your lab handout. Other questions depend on your lab experiments, the results of those experiments, and your interpretation of those results. In order to deepen your understanding of the material plan to view the material found at the links in some of the questions below.

1. Why is water called the universal solvent? Besides re-reading the introductory text, also read this short article: http://water.usgs.gov/edu/solvent.html (or use this shortened URL to go to the same page: http://goo.gl/yXAK11).
2. What does it mean to say that a solution is saturated? See this video for additional explanation: https://youtu.be/JQRIlS9lNdg
3. Use the same video as a source to help you to define what a supersaturated solution is.
4. Which of the two lab activities you did involve a solution that was saturated at a high temperature? Was the solution super-saturated when it cooled off? Explain.
5. Where is borax found and what are some of its many uses? Also, what will happen to the color of your crystals when you put them away to store them until next year? Read about it here: http://www.minerals.net/mineral/borax.aspx (or use this shortened URL to go to the same page: http://goo.gl/wrPwbs).
6. What is the usual shape of salt crystals and how do they grow? For a useful link about naturally-occurring salt crystals, take a look at this page: http://www.minerals.net/mineral/halite.aspx (or use this shortened URL to go to the same page: http://goo.gl/e0YX7R).
7. For this question insert photos of your 1. ‘magic tree’ and 2. your modified trials.
8. For this question insert photos of your own homemade borax ornament(s).
9. Based on your in-class experiments, what is the point of having salt in the mixture used to make the ‘magic trees’? For a trial with just salt and water, describe what the crystals looked like and explain why they looked that way. Read the introduction to the lab for help with this.
10. How do the ammonia and the Prussian Blue pigment work together to make the ‘magic trees’ work? Cite relevant results from trials which left out one or the other or both of these ingredients. Also, read the introduction to the lab for help with this.
11. What is dendritic crystal growth and how was it demonstrated in the in-class portion of the lab? Read the introduction to the lab for help with this.
12. In your opinion, can science be artistic or creative? Can creative work use the benefits of scientific knowledge? How?

Grading

1. Bode, A. A. C.; Vonk, V.; van den Bruele, F. J.; Kok, D. J.; Kerkenaar, A. M.; Mantilla, M. F.; Jiang, S.; Meijer, J. A. M.; van Enckevort, W. J. P.; Vlieg, E., Anticaking Activity of Ferrocyanide on Sodium Chloride Explained by Charge Mismatch. Crystal Growth & Design 2012, 12 (4), 1919-1924.
2. Davidson, C. F.; Slabaugh, M. R., Salt Crystals-Science behind the Magic. Journal of Chemical Education 2003, 80 (2), 155.
3. Dorazio, S. J.; Brückner, C., Why Is There Cyanide in my Table Salt? Structural Chemistry of the Anticaking Effect of Yellow Prussiate of Soda (Na4[Fe(CN)6]·10H2O). Journal of Chemical Education 2015, 92 (6), 1121-1124.
4. staff, J. C. E., How Does Your Garden Grow? Investigating the "Magic Salt Crystal Garden". Journal of Chemical Education 2000, 77 (5), 624A.
5. Katz, D. A. Magic Tree: An Explanation. http://www.chymist.com/Magic%20tree.pdf (accessed April 27, 2016).

Here are some photos of the resuls of various modifications of the recipe for a ‘Magic Tree’.


This image shows typical results for the standard recipe as given in the procedure. One of the trees shows the use of some blue food coloring.


This cone shows crystal growth in the usual cubic shape of salt crystals. No ammonia or laundry bluing were used in this dish.


In this dish laundry bluing was used but no ammonia. The blue color was retained and some of the dendritic crystal growth can be observed. It is not as extensive as the trees shown in the first picture above. Without ammonia, less of the laundry bluing was soluble and so less of it was able to interfere with the crystallization of the salt. Still, this shows it is not completely insoluble in water.


This image shows how much more dendritic crystal growth you can get by attaching pieces of reinforced paper to the cone. The attached pieces of paper were cut to have a sharp point. Edges and points encourage evaporation and therefore crystal growth.