Materials Included In Kit

Additional Materials Required

Safety Precautions

Chrome alum is a body tissue irritant. Copper(II) sulfate is moderately toxic by ingestion and inhalation and is a skin and respiratory irritant. Nickel sulfate is moderately toxic by ingestion and a known carcinogen as a dust; avoid inhalation of this material; use and dispense in a fume hood. Use caution when handling hot glassware. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding.The crystals may be saved or disposed of after completion of this activity. If saved, students should avoid handling the crystals directly with their bare hands, both for their own safety and to keep the crystals looking good. It is not recommended that students be allowed to take the crystals home. Dispose of alum crystals and solutions according to Flinn Suggested Disposal Method #26a and #26b, respectively. Dispose of chrome alum crystals and solutions according to Flinn Suggested Disposal Method #27f. Dispose of copper(II) sulfate crystals and solutions according to Flinn Suggested Disposal Method #26a and #26b, respectively. Dispose of nickel sulfate crystals and solutions according to Flinn Suggested Disposal Method #27f.

Lab Hints

• Constant temperature is very important for growing seed crystals. If the temperature in your classroom varies, the jars may be placed in a water bath to help them stay at a constant temperature.
• If the seed crystal falls off of the thread, simply retrieve it from the solution with a clean, dry tweezers or use another seed crystal and start over at step 27.
• If the crystals begin growing on the bottom of the jar instead of on the seed crystal, carefully remove the lid and remove the crystals from the bottom with tweezers.
• A quality seed crystal is the key to success in crystal growth. A good seed crystal is ¼" to ⅛" long. It must be a single crystal so that the crystal growing from it will also be a single crystal. If the seed crystal is too small, it will dissolve more easily once it is hung in solution, be more difficult to tie on a string, and may float on the surface of the solution. If the crystal is too large it has a greater chance of having irregularities on its structure, or may have grown too fast causing it to be cloudy.
• Instead of nicely separated seed crystals, a layer of solid may form on the bottom of the jar. This may happen because some solid was accidentally poured into the solution causing the precipitate to form in a big clump instead of individual crystals. Another possibility is that the temperature in the classroom was not constant. In either case, simply redissolve the solid layer by heating the solution and starting over.
• Make sure the jars or beakers used for growing crystals are clean and free from scratches. Scratches may cause too much precipitate to form, or may cause the seed crystals to take on an imperfect shape.
• It is more difficult to grow seed crystals from some solids than others (see tips for each crystal above). Alum is the easiest to grow, while nickel sulfate is the most difficult. You may want to assign crystals to student groups based on the level of the students and the ease of crystal growth.
• In general, the amount of solid to add to 100 mL of water to prepare the initial solution for growing crystals is about twice that of the solubility. Crystals of other solids, such as potassium ferricyanide, can be prepared following this general guideline.
• The method of growing crystals used in this activity is called the sealed jar method because crystals are grown in a sealed jar. In this method, the size of the crystal produced cannot be larger than the second amount of solid that was added to the solution (step 20 of the standard procedure). Crystals can also be grown by the evaporation method. With the evaporation method, the crystals are grown in an open jar instead of a sealed one. The evaporation method makes growing larger crystals possible because, as the water evaporates from the beaker, the amount of solid that can be dissolved in the water is constantly decreasing. All of this extra solid can be deposited on the crystal. For example, when growing alum crystals, 20 g of starting material is used, and 4 g is added in step 20. With the sealed jar method, the largest mass the crystal can attain is 4 g. But, with the evaporation method, the crystal could approach 20 g. The evaporation method is not used in this activity because the rate of evaporation is hard to control. Since evaporation occurs at the surface of the solution, supersaturation tends to be greatest there. Unwanted seeds may form at the surface and drop on the growing crystal. In addition, droplets of solution splashed on the sides of the jar will have evaporated to dryness. This residue of crystalline dust provides numerous sites where unwanted crystals may start growing.

Teacher Tips

• Note: The procedures for alum, copper(II) sulfate, and nickel sulfate, are all the same, except that different amounts of solid are added in steps 1 and 20. The procedure for chrome alum deviates slightly from the other procedures. A summary of the differences for chrome alum are listed below. Each of the procedures is printed on a single sheet of paper (front and back) so that each student group can have their own procedure.
• Alum: Alum grows well and is the easiest crystal in this set to grow.
• Chrome alum: Chrome alum is very expensive! Therefore, only 60 g have been included in this kit. Students will have to share this 60 g. It is recommended that the three groups growing chrome alum work together in Part A to prepare the saturated solution. In this case, follow the procedure as written. Instead of each group growing their own saturated solution, they will grow one solution together as a group. This saturated solution is then divided into three 30-mL portions so that each group can grow their own seed crystals. Chrome alum grows well, but a saturated solution is so dark that the crystals growing inside cannot be seen. To remedy this, the following modifications have been made to the general procedure for growing crystals (these modifications are included in the student procedure section). First, seed crystals are grown following the standard procedure, but then they are grown in a different type of growing solution than the other crystals. To prepare the growing solution, take a solution of chrome alum and slowly pour it into a growing solution of ordinary alum. The two solutions can be mixed in any ratio, but for convenience add about 10 mL of chrome alum to 90 mL of alum. This ratio should provide a solution that is clear enough to see through. The resulting crystal in this case will actually be a mixed alum crystal—the crystal lattice will contain chromium ions, aluminum ions, potassium ions, and sulfate ions. Since both alum and chrome alum have an octahedral habit, the mixed crystal will also be octahedral. Because the two are mixed, the resulting crystal will be more of a lavender color than a deep purple.
• Copper(II) sulfate: Copper(II) sulfate grows well and is easy to grow.
• Nickel sulfate: Nickel sulfate has two hydrated forms: the hexahydrate and the heptahydrate. In preparing seeds at room temperature, even though the heptahydrate will usually come out of solution, the hexahydrate can be encouraged to fall out of solution by adding grains of the hexahydrate (this is the prescribed procedure in this activity). If the seeds are chunky, the hexahydrate has been formed; if the seeds are long, the heptahydrate has been formed instead. The shapes of the two hydrates differ. If the heptahydrate is formed, the resulting crystal will be orthorhombic, while the hexahydrate will form tetragonal crystals. The colors will also be different. The hexahydrate is a blue-green while the heptahydrate is more of a true green. Seeds of nickel sulfate are the most difficult to grow in this set. A layer of solid may form on the bottom of the jar instead of forming nicely separated seed crystals. If this happens, simply redissolve the solid by warming the solution and start over.

Answers to Questions

1. What would have happened if you had put your seed crystal in a saturated growing solution instead of a supersaturated growing solution? The crystal would not grow because there would be no excess solute dissolved in solution to precipitate out onto the seed crystal.
2. Why is it important that your crystal remain at a constant temperature while it is growing? A constant temperature is very important for growing high-quality crystals. If the temperature varies during crystal growth, the solubility of the solution changes. If the solubility increases, then the crystal may begin to dissolve since the solution can now accept more solute in solution. If crystals are grown too fast—for example, if the solutions are cooled too quickly after heating—the crystals will be cloudy in appearance. The milkiness is due to defects in the crystal structure. Crystals grown slowly have time to make sure that all lattice points contain an atom and that all atoms are in their proper position in the crystal lattice.
3. If you came back to school on a Monday and your crystal was dissolved, what might have happened over the weekend? If the temperature of the classroom increases over the weekend, or even overnight, the solubility of the growing solution will increase. If the solubility increases, the solution will then be able to dissolve more solute. It will begin by dissolving the seed crystal.
4. If you let a crystal grow on the bottom of a jar and let another crystal grow from a seed crystal hanging by a string, how would the shapes of the two final crystals differ? The crystal grown on the bottom of the jar will be flattened compared to the crystal grown from a hanging seed crystal. The crystal growing on the bottom will be flattened because all of its growing surfaces experienced different growth rates while the hanging crystal will experience more constant growth rate from side to side. The shapes of each of these two crystals are their crystal habits.
5. If a crystal is cloudy, what may be the cause of the cloudiness? Crystals that are grown too fast will be cloudy rather than crystal clear. The cloudiness is due to defects in the crystal structure—lattice points containing no atom or the wrong atom.
6. What is the habit for your crystal? Sketch the unit cell for your crystal. (If there are several variations to the unit cell for your crystal, draw the simplest one.) The habits for each crystal may be the same as the most common habit, or they may be different—it all depends on the growing conditions. Table 1 gives the most common habit for each crystal and its unit cell. A sketch of each type of unit cell is shown in Figure 1. (The simplest unit cell should be drawn if there are variations of the unit cell for that particular crystal structure.)
7. Does your crystal’s habit reflect the shape of its unit cell? If not, why might the actual crystal be a different shape than its unit cell? The shape of each of the crystals may or may not reflect their unit cell, depending on the growing conditions for each crystal. The habit changes due to changes in the rate at which each face grows. For example, with alum, if the cubic faces grow fastest, the resulting habit will be octahedral. But, if the octahedral faces grow faster than the cubic faces, the resulting habit will be cubic. In general, a crystal’s shape is determined by its slowest growing faces.
8. If you were to break your crystal in half by tapping the sharp edge of a razor blade on the crystal, what do you think the two broken pieces would look like? Hint: Would the crystal break parallel to one of the faces? The process of breaking crystals parallel to some of its faces is called cleaving and it is unique to crystals. Cleavage planes are always related to the symmetry of a crystal and the arrangement of the atoms, ions, or molecules in that crystal—they are generally parallel to two faces of the crystal.
9. Imagine you have a friend who wants to grow sugar crystals to make rock candy. Briefly outline the instructions you would give your friend for growing sugar crystals. (Don’t worry about the specific number of grams of sugar—just outline the basic procedure.)
   1. Warm a sugar solution and add more sugar until no more will dissolve. Remove from heat and let cool to room temperature. Cover. Let it sit (covered) until all the excess sugar has precipitated out of solution onto the bottom of the jar—this will take a few days. Once all the excess has fallen out of solution, the solution is saturated.
   2. Pour a portion of the saturated solution into a small beaker. Add a few grains of sugar to the beaker. Wait for a day or so. Do not cover. Seed crystals will begin to form on the bottom of the beaker.
   3. Remove all seed crystals from the beaker. Tie the best seed crystal to a piece of string. Add more sugar to the original saturated solution in the jar from step 1. Heat to get it to dissolve. Cool the solution to room temperature. Once cool, hang the seed crystal in the solution. Cover. Wait.
   4. Watch the sugar crystal grow. Try to keep the solution and growing crystal at a constant temperature.

References

Ellis, A. B.; Geselbracht, M. J.; Johnson, B. J.; Lisensky, G. C.; Robinson, W. R. Teaching General Chemistry, A Materials Science Companion; American Chemical Society: Washington, D.C., 1993; p 143.

Heslop, R. B.; Robinson, P. L. Inorganic Chemistry: A Guide to Advanced Study; Elsevier: New York, 1960; pp 136–137.

Holden, A.; Morrison, P. Crystals and Crystal Growing; MIT: Cambridge, MA, 1995.

Kotz, J. C.; Joesten, M. D.; Wood, J. L.; Moore, J. W. The Chemical World: Concepts and Applications; Harcourt Brace: Orlando, FL, 1994; pp 603–619, 649–651.

Introduction

Grow your own crystals! In this activity, four crystals—each with its own distinct color and shape—will be grown from a single seed crystal in a supersaturated growing solution.

Concepts

• Crystal growing
• Crystal structure
• Saturation vs. supersaturation
• Solubility

Background

Solutions and Solubility

Crystals can be “grown” in supersaturated growing solutions. To completely understand how these crystals can possibly “grow” out of a solution, the concept of saturation and supersaturation must first be addressed.

A solution is a homogeneous mixture of two or more substances. Generally, solutions are thought of as solutes dissolved in a solvent—usually water. The solubility of a given solute is the largest amount of that solute that will dissolve in a specified volume of solvent at equilibrium and at a particular temperature. Solubility is strongly temperature dependent; it generally increases with increasing temperature. Solubility also depends on the substance being dissolved. Some salts are very soluble in water, while others are only slightly soluble.

A solution is said to be unsaturated if its solute concentration is less than its solubility. Another way to think about it is, if more solid can be dissolved the solution is unsaturated. When a solute’s concentration is equal to its solubility, the solution is said to be saturated. At that temperature, no more solid, not even one small grain, can be dissolved in the solution. However, if a saturated solution is heated, its solubility may increase, making is possible to dissolve more solid in that same solution. If additional solid is added and then the solution is cooled, it might be expected that the extra solid would precipitate out of solution. This does not always happen though. Instead, the extra solid may remain dissolved in solution even though its concentration has exceeded its solubility. In this case, the solution is said to be supersaturated. A supersaturated solution is one that is more concentrated than a saturated solution at the same temperature (see Table 1). To clarify the difference between unsaturated, saturated, and supersaturated solutions, consider what happens when additional solute (in the form of a crystal) is added to each type of solution. The scenarios in the table below apply when the solution is held at constant temperature—because as soon as the temperature changes, so does the solubility.

The crystal of solute added to a supersaturated solution is called a seed crystal. It is planted in the supersaturated solution and grows into a bigger crystal. The supersaturated solution in which the seed crystal grows is called a growing solution. Supersaturated solutions are fragile solutions. Not only will adding a crystal of solute cause them to precipitate, but disturbing the solution in other ways, such as stirring or scratching the walls of the container, may also cause the excess dissolved solute to precipitate out of solution until only a saturated solution remains.

Unit Cells

The macroscopic regularity in the shapes of ice crystals, snowflakes, crystalline salts, and gemstones suggests that crystals must possess some sort of atomic-level regularity. This regularity is called a crystal lattice, and every crystal is built upon one. A crystal lattice is an orderly, repeating arrangement of atoms, molecules, and ions. The specific repeating pattern unique to each crystal lattice is called a unit cell, the smallest repeating pattern that reflects the macroscopic shape of the crystal. In general, crystals are extended networks, constructed by repeating this unit cell pattern in all three dimensions.

Seven types of unit cells occur in nature. They include cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal and rhombohedral. Several of these types of unit cells have variations. The base unit cell plus its variations make up the unit cells for a given crystal system. The seven types of unit cells, their variations, and associated crystal structures are sketched in Figure 1. The most familiar unit cells are cubic unit cells—they have equal length edges that meet at 90° angles. There are three variations on the cubic unit cell: simple cubic, body-centered cubic, and face-centered cubic. Simple cubic unit cells place an atom at each corner of a cube. When an extra atom is positioned in the center of the cube, the unit cell is called body-centered cubic. The face-centered cubic unit cell places an extra atom in the center of every face of the cube. Similar variations exist for the other types of unit cells as shown in Figure 1.

Crystal Habits
Although crystals of a specific substance do exhibit regularity, their shapes are not always exactly identical. The conditions in which a crystal is grown affect the relative sizes of the faces as well as the number of faces that appear; however, a particular substance will generally display a characteristic shape or group of shapes. The characteristic shape a crystal assumes is called its crystal habit. Consider a sodium chloride crystal in a growing solution. If the crystal is suspended in solution and the solution is stirred, it will grow into a cubic-shaped crystal. But, if the crystal is allowed to just sit and grow on the bottom of the container, the resulting crystal will be a flat, square tablet. In each situation, the shape assumed by the sodium chloride crystal is its habit.

The unit cells, most common crystal habits, and colors of each crystal grown in this activity are listed in Table 1. If the growing conditions are varied, a crystal’s habit may deviate from that listed in Table 2. When the structures of the above crystals are considered, an apparent discrepancy is evident. The structures of alum and chrome alum are listed as octahedral, but octahedral is not listed as one of the seven unit cell types. This discrepancy can be resolved by looking at how crystals grow in more detail.

The variations seen in crystal shapes, such as those mentioned in the sodium chloride example above and the octahedral habit of alum crystals, occur because the growing solution’s concentration varies from one point to another around the crystal. If a particular face of the crystal is surrounded by solution that is more concentrated, it will grow faster than other faces which are surrounded by less concentrated solution. In addition, the different types of faces have different inherent growth rates. The specific shape of the crystal that forms is determined by the rates at which its various faces grow.

Alum is a good example of how the different growth rate of the different types of faces can affect the overall shape of a crystal. While alum’s habit is octahedral, it is actually composed of several structures superimposed on each other. Figure 2 shows an alum crystal in various stages of development. Other factors also affect crystal growth. One of the most important factors is the temperature at which crystals are grown. A constant temperature is very important for growing high-quality crystals. If the temperature varies during crystal growth, the solubility of the solute changes. If the solubility increases, then the crystal may begin to dissolve since the solution can now accept more solute in solution. Another factor affecting the quality of crystals is the rate at which they are grown. Crystals should be grown as slowly as possible. They can be grown slowly by making sure the solution does not evaporate too rapidly and by keeping the temperature constant so the solubility remains constant. If crystals are grown too fast—for example, if the solutions are cooled too quickly after heating—the crystals will be cloudy in appearance. The milkiness is due to defects in the crystal structure. Crystals grown slowly have time to make sure that all lattice points contain an atom and that all atoms are in their proper position in the crystal lattice.The goal in this activity is to grow a single, high-quality crystal. A perfect crystal will have a nice shape; it will be very symmetric with parallel edges. Look out for clumps of crystals growing together—the goal here is to grow a single crystal. Perfect crystals are also “crystal clear.” Try to avoid the milkiness that results from growing crystals too fast. Growing the perfect crystal is not easy and may not result from the first attempt at crystal growing. However, even imperfect crystals are beautiful and well worth the effort involved. As the old saying goes, “If at first you don’t succeed... try, try again!”

Materials

Safety Precautions

Chromium potassium sulfate is a body tissue irritant. Copper(II) sulfate is moderately toxic by ingestion and inhalation and is a skin and respiratory irritant. Nickel sulfate is moderately toxic by ingestion and a possible carcinogen as a dust; avoid inhalation of this material; use and dispense in a fume hood. Avoid handling crystals with bare hands. Use caution when handling hot glassware. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron.

Procedure

Part A. Preparation of a Saturated Solution

1. Refer to Table 3 (Part A) to obtain the appropriate amount of specific solute assigned to your group by the instructor.
2. Add 100 mL of distilled or deionized water to a clean 250-mL beaker.
3. Add the solute to the beaker of water and stir.
4. Place the beaker on a hot plate and heat the solution until all the solid dissolves, stirring intermittently. Do not allow the solution to boil.
5. Turn off the hot plate and carefully remove the beaker using beaker tongs.
6. Once the solution in the beaker has cooled slightly transfer by pouring into a clean glass jar. Tighten the lid on the jar to prevent evaporation.
7. Allow the supersaturated solution to cool to room temperature and add several small crystals of the stock solute. Note: Adding a few small crystals to a supersaturated solution is called seeding the solution. These crystals provide a location for excess solid in the saturated solution to deposit.
8. Set the jar somewhere it can remain at a steady room temperature for two days. Shake the jar daily so that the solid will deposit on the bottom of the jar. As the solid precipitates out, the solution becomes saturated.
9. After two days transfer the solution into a clean, dry beaker. Caution: Try and transfer as little solid as possible. Cover the beaker with a watch glass.
10. Scrape the deposited salt into the designated collection container provided by your instructor.
11. Rinse the jar washing the solid into the specified waste container until no solid remains in the jar.
12. Wash the jar with hot water and let it air dry until there is negligible water remaining. Do not dry with a paper towel as they leave fibers that inhibit crystal growth.
13. Transfer the saturated solution from the beaker back to the original clean jar and cap with the lid.

Part B. Preparation of a Seed Crystal

14. Pour 30-mL of the saturated solution into a clean dry 100-mL beaker.
15. Allow the solution to sit uncovered for one day or until the development of seed crystals. If crystals do not appear after one day add a few grains of the original solute. This will seed the other crystals to grow on them.
16. Monitor the jar over the next few days checking for the formation of seed crystals.
17. Once seed crystals are large enough to handle, but not clumped together, carefully remove several with a clean tweezers and place them on a paper towel. Caution: Handle crystals very carefully! Do not touch them with your fingers as they are delicate and could be destroyed.

Part C. Preparation of the Supersaturated Growing Solution

18. Weigh out the specified amount of solid as indicated in Table 3 (Part C) and place in a 250-mL beaker. Note: If your group was assigned chrome alum, in this step you should obtain alum not chrome alum.
19. Transfer the solution from step 13 from the jar to the beaker.
20. Repeat steps 4 and 5.
21. Wash the jar with hot water and let it air dry until there is negligible water remaining. Do not dry with a paper towel as they leave fibers that inhibit crystal growth.
22. Pour the solution from the beaker back into the glass jar. Place a watch glass over the top of the jar. Let it sit until it cools to just above room temperature. This is the growing solution where the crystal will grow.
23. Place the lid of the jar on an index card and trace around the lid.
24. Using scissors, cut out the circle.
25. Using a pen punch three small holes near the center of the circle (see Figure 3).
    {12941_Procedure_Figure_3}
26. Ensure that your hands are clean before performing the following steps as contamination easily occurs.
27. Tie a knot around your best seed crystal. To do this, make a loop at one end of a 1-foot piece of thread. Place the seed crystal in the loop and tighten. Once secure, tie the thread once more to form a knot. Cut off any excess thread after the knot with a pair of scissors, keeping the long end of the thread intact.
28. Work the opposite end of the thread through the three punched holes (see Figure 4).
    {12941_Procedure_Figure_4_Seed crystal tied to a piece of thread, hung from a paper disk}
29. Make sure the growing solution is about 3 °C above room temperature. If it is cooler, warm it briefly on a hot plate. If it is warmer then place it in a cool water bath. Stir the solution well to equalize temperature throughout.
30. Plant the seed crystal in the growing solution by holding the index card with the seed crystal hanging from it. Place the index card on top of the jar so that the seed crystal is submerged. Attach the lid onto the jar by securing the index card between the lip of the jar and the lid.

Student Worksheet PDF

12941_Student1.pdf