Materials Included In Kit

Additional Materials Required

Safety Precautions

Sodium borate is slightly toxic by inhalation and ingestion. Use caution when working with heat and glassware. Handle the chenille wires carefully as the ends of the wires may be sharp. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. It is not recommended that students be allowed to take crystals home. 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.Leftover sodium borate solution may be disposed of down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b. Excess solid sodium borate may be stored for future use in a Flinn Chem-Saf™ Bag or may be disposed of according to Flinn Suggested Disposal Method #26a.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students with each student making one ornament. This laboratory activity can reasonably be completed in one 45- to 50-minute class period, with the drying and examination of the finished ornaments taking place the next day. The prelaboratory assignment may be completed before coming to lab, and the worksheet may be completed the day after the lab.
• If enough hot plates are not available, an option for hot water is to use a large hot pot or coffee percolator. Dispense the hot water into the beaker just prior to adding the premeasured sodium borate. A laboratory microwave is another option. Be sure to use beaker tongs or wear heat-resistant gloves when handling glassware filled with hot water.
• Have several weighing and dispensing stations around the lab to ease congestion. Three should be sufficient for 15 groups of students.
• Borosilicate glass beakers or canning jars of varying sizes may be used. Adjust the amount of water and sodium borate accordingly, keeping a 1:6 ratio of sodium borate to water. The advantage of using larger beakers is that fewer are needed since lab partners may share one.
• The sodium borate crystals will not take up food dye and therefore coloring the solution will have no effect. The color of the chenille wire will determine the color of the ornament. Students may wish to cut and trade smaller pieces of different colored wires to expand the creative possibilities.
• If crystals have grown on the bottom or sides of the beaker, simply reheat the solution to dissolve the borax before disposal.

Teacher Tips

• This is a wonderful integrated activity that combines science and art and may be used as a science activity for any holiday or as a component for a unit on crystallization, solutions and solubility, or even mineral crystal shapes.
• Use the crystal ornaments to decorate your classroom or lab. It is not recommended to allow students to take chemicals home.
• If time permits, turn this into an inquiry activity. Allow students to design and carry out an experiment to test the variable of concentration or temperature of the sodium borate solution and observe the effect on the formation of the crystals.
• Have students look at salt (sodium chloride) crystals under magnification to compare their cubic shape to the monoclinic sodium borate crystals.

Answers to Prelab Questions

1. Read through the Background and Procedure sections. What is the solvent in this activity? What is the solute?

   The solvent is DI water. The solute is sodium borate (borax).

2. What is the purpose of heating the water before adding the solid sodium borate?

   Increasing the temperature of the water increases the solubility of the solute, so more sodium borate can be dissolved. As the solution cools to room temperature, the solubility decreases, and some of the sodium borate will precipitate out of solution, forming crystals.

3. Read the Safety Precautions. Identify the safety hazards associated with this activity and list the precautions necessary to avoid those hazards.

   Sodium borate is slightly toxic by inhalation and ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Exercise caution when handling hot glassware—use beaker tongs or wear heat-resistant gloves. The chenille wires have sharp ends—handle carefully.

Sample Data

Sketch a few of the crystals as observed under magnification.

Answers to Questions

1. Describe two ways an unsaturated solution could become saturated.

   An unsaturated solution could become saturated by adding more solute until no more can be dissolved. The solution could also become saturated if the temperature of the solution decreased.

2. Examine several crystals clustered together. Explain why the crystals are not identical.

   The variations seen in the crystal shapes 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. Some crystals overlap as they grow. The growth of some crystal faces is also affected by the chenille wire.

3. Based on your observations of the crystals, which of the seven unit cell types do you think is the “building block” of sodium borate crystals? Hint: Sodium borate crystals take on a prismatic form as they typically grow longer in one direction than in the other two.

   Sodium borate crystals are monoclinic. Note: This may be difficult for students to determine. Monoclinic crystals have three unequal axes, two of which are right angles.

4. Describe two important factors in the way this experiment was carried out that led to the formation of large crystals.

   The solution was allowed to cool slowly and the crystals were given room to grow.

5. Geologists describe igneous rocks as extrusive (formed from lava on the Earth’s surface) or intrusive (formed from magma deep within the Earth). In which type of rock would you expect to find larger mineral crystals? Why?

   Intrusive igneous rocks would more likely have larger mineral crystals because the rocks formed from magma within the Earth would have cooled more slowly and with less variation in temperature than rocks formed from lava at the surface. Both of these factors result in larger crystals.

References

Borax Snow Crystals, Indiana Alliance of Chemistry Teachers, 2009 HASTI Presentations, http://www.chem.purdue.edu/sciexpress/IACT%20Webpage/HASTI.htm (accessed June, 2010).

Introduction

Add some sparkle to your next holiday celebration by making ornaments of crystals. Simply dissolve a common household chemical in hot water, suspend a shaped chenille wire into the solution and wait. The next day remove a beautiful crystal-covered ornament from the solution!

Concepts

• Crystal formation
• Solutions
• Solubility

Background

Solutions and Solubility

In this lab, sodium borate (borax) crystals will be “grown” in saturated growing solutions. To understand how these crystals can possibly “grow” out of a solution, the concept of saturation must first be addressed.

A solution is a mixture of two or more pure substances that is homogeneous or uniform throughout. The substance that is being dissolved is called the solute, and the substance that does the dissolving is called the solvent. Solubility is the amount of solute that will dissolve in a given amount of solvent at a particular temperature. Solubility generally increases with increasing temperature when dissolving solids into liquids. 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 can be dissolved in the solution. However, if a saturated solution is heated, the solubility of the solute may increase, making it possible to dissolve more solid in that same solution. If additional solid is added to the hot solution and then cooled, often the extra solid will precipitate or crystallize out of solution.

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 a repeating, orderly 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. Sodium borate is an ionic compound. Sodium and borate ions are arranged into a regular three-dimensional pattern resulting from a net balance of attractive and repulsive forces. This arrangement forms an extended network, constructed by repeating the unit cell pattern over and over again in all three dimensions and the crystal “grows.”

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.

Although the unit cell for a particular solid will always be uniform, variations in crystal shapes 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 one example of how the different growth rates of the different types of faces can affect the overall shape of a crystal. While an alum crystal forms an octahedral shape, 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 large crystals. If the temperature varies during crystal growth, the solubility of the solute changes. If the solubility increases, then the crystals may begin to dissolve since the solvent can now accept more solute in solution. Another factor affecting the quality and size of crystals is the rate at which they are grown. Slow growth results in larger quality crystals. If crystals are grown too fast—for example, if the solutions are cooled too quickly after heating—the crystals will be smaller and cloudy in appearance. Crystals also need room to grow and may be smaller or overlap if there is a limited area for growth.

Experiment Overview

The purpose of this activity is to create a crystal ornament by suspending a shaped chenille wire into a saturated solution of sodium borate. As the solution cools, sodium borate will slowly precipitate out of the solution, forming crystals on the chenille wire.

Materials

Prelab Questions

1. Read through the Background and Procedure sections. What is the solvent in this activity? What is the solute?
2. What is the purpose of heating the water before adding the solid sodium borate?
3. Read the Safety Precautions. Identify the safety hazards associated with this activity and list the precautions that must be followed to avoid those hazards.

Safety Precautions

Sodium borate is slightly toxic by inhalation and ingestion. Use caution when handling hot glassware. Use beaker tongs or wear heat-resistant gloves. Handle the chenille wires carefully as the ends of the wires may be sharp. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

1. Obtain one chenille wire. Bend the wire into desired ornament shape. Note: Chenille wires may be cut with scissors to shorter lengths. Be careful when bending or twisting the wire as the ends are sharp.
2. Cut a 9" piece of string and tie it to the shaped wire so the ornament will hang in balance.
3. Obtain a 400-mL beaker and label it with your initials.
4. Holding the string, lower the ornament into an empty 400-mL beaker to make sure the ornament will fit completely in the beaker without touching the sides or bottom. If the ornament does not fit, adjust the shape to make it smaller. Remove the ornament from the beaker and set aside for step 11. Note: Two smaller ornaments may fit into a 400-mL beaker if they do not touch each other any part of the beaker.
5. Add 300 mL of distilled or deionized water to the beaker and place on a hot plate.
6. Heat the water to just below boiling, at least 80 °C.
7. While the water is heating, place a weighing dish on a balance and tare the balance.
8. Weigh 50 g of sodium borate in the weighing dish and then set aside for step 10.
9. When the water temperature has reached 80–85 °C, use beaker tongs to carefully remove the beaker from the hot plate and place it on a heat resistant surface. Note: Turn off the hot plate if you are the last group using it.
10. Carefully add the 50 g of sodium borate to the hot water and stir with a stirring rod until dissolved.
11. Holding the ornament by the end of the string, lower it into the sodium borate solution until the ornament is completely covered by the solution but is not touching the bottom or sides of the beaker.
12. Wrap the excess string around a wood splint and place the splint across the top of the beaker to keep the ornament suspended in the solution (see Figure 3).
    {12152_Procedure_Figure_3}
13. Allow the solution to cool slowly overnight and the crystals to “grow” on the ornament.
14. The next day, carefully remove the ornament from the beaker and place on a folded paper towel to dry.
15. Look at the crystals with a strong magnifying lens or stereoscope and try to identify the shape (see Figure 1 from the Background section as a reference). Sketch several examples of the crystal shapes and record your observations on the Crystal Ornaments worksheet.
16. Consult your instructor for appropriate disposal of the leftover sodium borate solution.

Student Worksheet PDF

12152_Student1.pdf