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Grades 6-8 Chemistry: How to Grow the Best and the Largest Crystals

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Crystals come in all different shapes and sizes. However, the purest and cleanest crystals are usually also the ones that grow to be the largest in size. In this science fair project, you will compare the size and shape of crystals grown in three different temperature conditions: room temperature, in the refrigerator, and in an ice bath. With just water and borax, a household cleaning product, you can discover the best recrystallization method for growing large, pure crystals.

Objective

To find the best temperature conditions for growing the largest, purest crystals.

Introduction

Chemists perform chemical reactions as a way to change one thing into something different. For example, when you leave your bike out in the rain, you might notice that the shiny metal turns reddish-brown. This happens because rust forms through a chemical reaction between the metal and the oxygen in the air. Sometimes, chemical reactions form more than one product, though, and chemists need a way to separate and remove the product they want from all of the other material. One way they do this is with a process called recrystallization. The scientist dissolves the mixture of products in hot water, and then lets the mixture cool. As the mixture cools, the desired product slowly appears as crystals, which can then be removed from the rest of the liquid.

Why do you think the crystals appear when the solution is cooled? It has to do with the fact that every solid that can be dissolved in water has a solubility, which means the largest quantity of the solid that can be dissolved in the water to make a clear solution. When the water starts getting cloudy and you can see solid particles floating around, that means no more solid can dissolve into the water and the solution (water and solid mixture) is saturated. But, the solubility of most solids increases as the mixture is heated, so more of the solid can be dissolved in hot water than in cold water. For instance, imagine you are making a cup of tea—you might notice that you can dissolve more sugar in hot tea than in iced tea. Give it a try and you’ll probably see sugar crystals at the bottom of the iced tea glass, even after you’ve stirred it.

When a hot saturated solution is cooled, however, there is suddenly more solid in the solution than can normally be contained by the cooler water. Because it can no longer stay dissolved in the water, some of the bits of solid fall out of the solution. As they do this, they bump into each other, stick together, and form larger and larger pieces, called crystals. A crystal is a solid made of molecules (tiny little pieces too small to even be seen by most microscopes) that have come together in a specific repeated pattern, like in Figure 1, below. Going back to the tea example, if you made a saturated solution of hot tea and sugar and then let it cool, under the right conditions, you’d be able to see small sugar crystals forming.

Cubic crystal structure
Figure 1. Crystals, like the cubic crystal above, are solids with a specific repeated pattern.

When the molecules of the crystal come together, impurities (which are the unwanted products of the chemical reaction) do not fit into the structure, much like the wrong piece of a puzzle doesn’t fit. So, if the crystal forms slowly enough, the impurities will be rejected because they do not fit correctly, and instead, remain in the solution and float away. But if a solution is cooled too quickly, there isn’t time to reject the impurities and instead, they become trapped in the crystal structure and the pattern is disturbed.

Do you think crystals that are cooled quickly will look different than those that are cooled slowly? Think about the effect that the speed of cooling might have on crystal size and clarity. With this science fair project, you can discover the answer by growing your own crystals out of borax (a home cleaning product) in different temperature conditions.

Terms, Concepts and Questions to Start Background Research

- Chemical reaction

- Recrystallization

- Solubility

- Particle

- Solution

- Saturated solution

- Crystal

- Molecule

- Impurity

- Clarity

- Supersaturated

Questions

- How do you make a saturated solution?

- Why does slower cooling result in purer crystals?

Materials and Equipment

- Lab notebook

- Large bowl

- Ice, enough to fill large bowl at least three times

- Water

- Thermometer

- String

- Scissors

- Pencils (3)

- Identical jars or large drinking glasses (3)

- Pot with a lid

- Borax, found in the laundry aisle of most grocery stores (1 box)

- Tablespoon

- Plastic wrap, wax paper, or aluminum foil

Experimental Procedure

1. In this science fair project, you’ll be recrystallizing borax under three different temperature conditions: in a refrigerator, at room temperature, and in an ice bath. Before you begin, make a hypothesis, based on your background reading, about how the crystals grown under each of these conditions will look. Write your hypothesis in your lab notebook.

2. Prepare an ice bath by filling the large bowl half full of ice and then adding water until the bowl is three-quarters full.

- Place the ice bath on a countertop or on a table, where it can be left undisturbed for at least 5 hours.

3. As soon as the ice bath is prepared, use the thermometer to take the temperature of the ice bath, of the refrigerator, and of the room (do this by putting the thermometer on the countertop or table), and record the temperatures in your lab notebook.

4. Cut three pieces of string and tie one around each pencil. The string pieces should be of equal length and should be long enough that when the pencil is laid across the top of the jar, the end of the string hangs down to just above the bottom of the jar.

5. Bring enough water to fill each jar three-fourths full to a boil in a pot, with adult supervision.

6. Add 1 tablespoon (Tbsp.) of borax to the water, and stir until it dissolves. Repeat, 1 Tbsp. at a time, until no more borax will dissolve. This is your saturated solution.

7. With an adult’s help, pour equal amounts of the saturated solution into the three jars. The jars should be about three-fourths full.

8. Lay a pencil across the top of each jar so the strings hang down into the saturated solution.

9. Cover the jars with plastic wrap, wax paper, or aluminum foil.

10. Place one jar in the refrigerator, leave one undisturbed on a countertop or table at room temperature, and put one in the ice bath you prepared.

11. Leave the jars alone for a minimum of 5 hours, or until crystals form (whichever is longer), and be sure not to disturb them. Check the ice bath regularly to make sure that the ice has not melted. Add ice, as necessary.

a. If crystals form under one condition before they do in the others, note that in your lab notebook and let all three conditions continue for another hour to see if any crystals form in the other conditions.

b. Record in your lab notebook the total amount of time (from step 9 to step 11) that you let the crystals form.

12. Carefully remove the pencils, one at a time, and note the size, shape, and number of crystals obtained from each solution. Are there any differences? Why do you think this is so? Record your observations in a data table, like the one below.

Cooling Condition Trial 1
Temperature Time of Crystal Formation (in hours) Number Size Other Observations
Ice bath
Refrigerator
Room temperature

13. If you are presenting your project in a science fair, save the strings and display them at the fair. Be sure to keep track of which string belongs with which solution.

14. Repeat steps 1–13 at least two more times to make sure that your results are accurate and repeatable. How do your results compare to your hypothesis?

Variations

- Try this experiment with other materials, such as sugar or salt.

- Try comparing how crystals form in a substance, both with and without impurities. For example, try crystallizing iodized versus uniodized salt.

- How do your results change if you grow your crystals for a longer period of time? Note: make sure you keep adding ice to the water bath to keep it cool throughout your experiment.

- Is recrystallization a good purification method? What kind of impurities can and cannot be separated using this method? Hint: Try adding impurities, like sand or food coloring, to the borax before you dissolve it in the water.

- For more science project ideas in this area of science, see Chemistry Project Ideas.

Bibliography

- University of Colorado, Boulder, Chemistry and Biochemistry Department. (2003). Lab Techniques: Crystallization. Retrieved July 31, 2009, from http://orgchem.colorado.edu/hndbksupport/cryst/cryst.html

- Helmenstine, Anne Marie. (2009). How to Grow Great Crystals: Hints, Tips, and Techniques. Retrieved July 31, 2009, from http://chemistry.about.com/cs/growingcrystals/a/aa012604.htm

- Royal Society of Chemistry. (2004). Crystal Chemistry. Retrieved August 6, 2009, from http://www.rsc.org/education/teachers/learnnet/aflchem/resources/41/41%20resources/41-2%20Interview.pdf