Materials Included In Kit

Additional Materials Required

Safety Precautions

Avoid handling crystals with bare hands. 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 regulation 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.

Teacher Tips

• Growing alum crystals will take a varied amount of time. As growing conditions vary, so will the rate at which crystals grow. The following time line can serve as a general guide for planning lab time.

  Day 1: Steps 1–7
  Day 2: Step 8
  Day 3: Steps 9–13
  Day 4: Step 14–15
  Day 5: Steps 16–26

• A good seed crystal is the key to success in crystal growing. A good seed crystal is about ¼ inch 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 the string, and may float on the surface of the water instead of hanging in solution. If the seed crystal is too large, it may have a greater chance of having irregularities in its structure, or may have grown too fast, causing it to be cloudy.
• Constant temperature is very important for growing seed crystals. If the temperature in your classroom does vary, the beakers may be placed in a water bath to help them stay at a constant temperature.
• Instead of nicely separated seed crystals, a layer of solid may form on the bottom of the beaker. 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 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.

Answers to Questions

1. What would have happened if you had put your seed crystal in a saturated growing solution that was covered?

   The crystal would not grow because there would be no excess solute dissolved in solution to precipitate out onto the seed crystal and the solution would never become supersaturated since the water could not evaporate.

2. If you came back to school on a Monday and your alum 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 to dissolve the seed crystal. A constant temperature is very important for growing high-quality crystals.

3. If you let an alum crystal grow on the bottom of a beaker and let another alum 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 beaker 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 rates from side to side. The shapes of each of these two crystals are their crystal habits.

4. 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.

5. What is the habit for your alum crystal? Sketch the unit cell for your alum crystal. (If there are several variations to the unit cell, draw the simplest one.)

   Alum’s habit is octahedral. It’s unit cell is cubic.

   {12908_Answers_Figure_3}
6. Does your alum crystal 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 a crystal may or may not reflect its unit cell, depending on the growing conditions for the crystal. The habit changes due to changes in the rate at which each face grows. 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.

7. 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.

8. 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. Write your answer on the back or on a separate sheet of paper.
   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 beaker—this will take a few days. Once all the excess has fallen out of solution, the solution is saturated.
   2. Add a few grains of sugar to 30 mL of the saturated solution in a smaller 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. Reheat the original solution from step a to dissolve any unwanted solid that has precipitated. Hang the seed crystal in the solution from step a. 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

Growing crystals is fun and easy! In this activity, giant alum crystals will be grown from just a single seed crystal!

Concepts

• Crystal growing
• Crystal habits
• Crystal structure
• Saturation
• Solubility
• Supersaturation
• Unit cells

Background

Solutions and Solubility

In this lab, aluminum potassium sulfate (alum) crystals will 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. 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.

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:

• If more solute is added to an unsaturated solution, it will dissolve because the solution can accept more solute and still have a concentration less than the solubility.
• When additional solute is added to a saturated solution, the crystal cannot dissolve because the concentration of the original solution is already equal to the solubility. Neither will the crystal grow, since there is no excess solute dissolved in solution to precipitate out on the crystals. In this case, the crystal will remain the same size.
• If a crystal of solute is added to a supersaturated solution, it will not dissolve because the solute concentration of the original solution was already greater than the solubility. Instead, the crystal provides a lattice upon which the excess solid in solution (that which made the solution supersaturated) can precipitate. As a result, the crystal will grow until the solute concentration is equal to the solubility. The crystal of solute added 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.

The preceding scenarios apply when the solution is held at constant temperature—because as soon as the temperature changes, so does the solubility.

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—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 cell 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 alum crystals grown in this activity will have cubic unit cells, will be clear and colorless and will most commonly have octahedral crystal habits. However, if the growing conditions are varied, the crystal habit may be some shape other than octahedral. Comparing alum’s habit to the seven types of unit cells in Figure 1, it can be noted that octahedral is not one of the seven unit cell types. This difference between alum’s unit cell and its habit can be understood by looking at how crystals grow in more detail.

The variations seen in crystal shapes, such as those mentioned in the sodium chloride example and in 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 giant, single, high-quality crystal. A perfect crystal will have a geometric 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. 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

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. Weigh out 100 g of solid alum and add it to a 1-L beaker.
2. Add 500 mL of distilled or deionized water to the 1-L beaker to dissolve the alum.
3. Place the beaker on a hot plate and heat the solution until all the solid dissolves. Stir the solution occasionally as it heats to speed up the dissolution process.
4. Once all of the solid has dissolved, turn off the hot plate. Carefully remove the beaker from the hot plate.
5. Cover the beaker with Parafilm M to prevent evaporation. Set the covered beaker aside to cool. Let it sit until it cools down to room temperature. This solution is a supersaturated solution. Label this beaker with the names of all group members.
6. Once the supersaturated solution has cooled to room temperature, remove the Parafilm M and add several small crystals from the stock bottle to the solution. Cover the beaker again and carefully swirl it to mix the contents. Note: Adding a few small crystals to a supersaturated solution is called seeding the solution. These few crystals will provide a place for the excess solid in the solution to deposit. Once all the excess falls out of solution, it is a saturated solution.
7. Set the covered beaker aside for two days. It is important to keep the beaker at a constant temperature. If the temperature in your classroom varies throughout the day, place the beaker in a water bath. The temperature of the water will be more constant than the temperature of your classroom, and this will keep the beaker at a more constant temperature.
8. Swirl the beaker to mix the contents daily. As the beaker sits, solid will deposit on the bottom and the solution will become saturated (instead of supersaturated).
9. After sitting for two days, the solid at the bottom of the beaker will have stopped growing. At this point, the solution is saturated. Carefully pour the clear solution into a clean, dry 1-L beaker. Make sure that the solution carries as little of the solid at the bottom of the beaker with it as possible. Cover the beaker containing the solution with Parafilm M to prevent any airborne contaminants from falling into the solution.
10. Scrape the deposited salt into the alum collection container provided by your teacher. Rinse the beaker, washing the solid into the alum collection container, until no solid remains in the beaker.
11. The solution from step 9 is the saturated alum solution.

12. Pour about 30 mL of the saturated alum solution into a clean, dry, 100-mL beaker. Do not cover this beaker.
13. As the solution evaporates, a few crystals will begin growing on the bottom of the beaker. These crystals are seed crystals. Do not disturb the beaker while seed crystals are growing. It will take at least a day before seed crystals appear.
14. If crystals do not begin growing within a day, add a few grains of solid from the alum stock bottle to the beaker with tweezers. In this case, addition of a few grains will cause seed crystals to grow on the bottom of the beaker.
15. Watch the formation of the seed crystals. It will take a day or two for good seed crystals to form.
16. Once the seed crystals have grown large enough to handle, but not so large that they touch one another, carefully remove several good seed crystals with a clean, dry pair of tweezers. Gently, place the seed crystals on a clean, dry paper towel. Do not touch the seed crystals with your fingers or get them wet as this could destroy them.
17. Remove and save all good seed crystals from the saturated solution. They will be needed if the first seed crystal doesn’t work. A good seed crystal is one which looks like a single crystal, not several crystals clumped together. It will have a nice shape, with clear-cut edges.

18. The saturated solution from step 9 may have grown a few seed crystals by now. Place the beaker on a hot plate and heat it until all the crystals dissolve. Stir the solution occasionally as it heats. Do not boil.
19. Once all of the solid has dissolved, turn off the hot plate. Carefully remove the beaker from the hot plate.
20. Wash the other 1-L beaker with hot water and let it air dry. Make sure it is dry enough so that the amount of water left in the jar is negligible. Avoid using paper towels because they could leave fibers on the side of the beaker, and a clean beaker is essential for crystal growing.
21. Pour the warm solution into the clean beaker. Place Parafilm M over the top of the beaker to prevent evaporation and contamination. Set the covered beaker aside to cool slowly. Let it sit until it cools down to just above room temperature (about 3 °C above room temperature). This solution is the growing solution—it is the solution in which your crystal will grow.
22. Wash your hands. By now your hands probably have very small crystals on them. These small crystals could contaminate your seed crystal.
23. 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 it is secure, tie the thread again 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.
24. Wrap the thread around a pencil, stirring rod, or stick long enough to lay across the top of the 1-L beaker without falling in. Leave enough thread below the pencil so that the seed crystal will hang about an inch above the bottom of the beaker.
25. Remove the Parafilm M from the beaker. Make sure that the growing solution is still about 3 °C above room temperature. If it is cooler, warm it briefly on the hot plate. If it is warmer, place the beaker in a bath of cold water until the temperature of the solution drops. In either case, stir the solution well to equalize the temperature throughout.
26. It is now time to plant the seed crystal in the growing solution. Hold the pencil with the seed crystal hanging from it. Place the pencil across the top of the beaker so that the seed crystal is submerged.

27. Do not disturb the crystal while it is growing.
28. It is important to keep the beaker at a constant temperature. If the temperature in your classroom varies throughout the day, place the beaker in a water bath. The temperature of the water will be more constant than the temperature of your classroom, and this will keep your beaker at a more constant temperature.
29. If the seed crystal falls off the string, simply retrieve it from solution with clean, dry tweezers or use another seed crystal and start over at step 23. The same growing solution may be used for the second attempt.
30. If crystals begin growing on the bottom of the beaker instead of on the seed crystal, carefully remove the seed crystal, and remove the crystals from the bottom with tweezers.
31. Keep the growing solution as clean as possible. Any debris that falls into the solution may act as a seed upon which excess solute from solution will deposit. In this case, crystals will begin growing on the debris instead of the seed crystal and they will not form a single alum crystal with an octahedral habit.

Student Worksheet PDF

12908_Student1.pdf