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Solubility Science: How to Grow the Best Crystals

A chemistry challenge from Science Buddies
bring science home borax crystals



George Retseck

Key concepts
Chemistry
Solubility
Saturation
Crystals
Purification

Introduction
Have you ever wondered how crystals are made? Crystals come in all different shapes and sizes. The purest and cleanest crystals, however, are usually also the ones that grow to be the largest in size. In this activity you'll compare the size and shape of crystals grown in different temperatures. With just water and borax, a household cleaning product, you can discover the method for growing large, pure crystals!

Background
Chemical reactions are constantly happening all around you—and inside of you. For instance, a chemical reaction can turn metal into reddish-brown rust (the iron in the metal is reacting with the oxygen in the air or water, and the end product is what we recognize as rust). Chemists perform chemical reactions to change one chemical compound into another. Sometimes when multiple products are formed the chemist may want to separate one compound from the others. One way this can be done is using a process called recrystallization, where a solution of compounds can be dissolved in hot water and then cooled. As it cools, one substance crystalizes (becomes crystals), and can be removed from the rest of the liquid, which holds the other compound.

Why do crystals appear as the mixture cools? It has to do with solubility, or the largest amount of something that can be dissolved in something else, such as dissolving the powdered cleaning product, Borax, in water. The solubility of most solids increases with temperature. In other words, more Borax may be dissolved in hot water than cold water. So if a hot, saturated mixture is cooled, there's more Borax than can be contained by the colder water, and so Borax may fall out of the mixture, forming crystals.

Materials
• Large bowl
• Ice cubes
• Water
• String
• Scissors
• Two pencils
• Two identical jars or large drinking glasses
• Cooking pot
• Borax, also called 20-Mule Team household cleaner. It can be found in the cleaning aisle of many grocery stores. (Use caution when handling cleaners—they can harm skin and eyes, and should not be inhaled.)
• Measuring tablespoon
• Plastic wrap

Preparation
• Fill the large bowl half full of ice cubes and then add water until the bowl is about three quarters full.
• Cut two pieces of string (they should be at least as long as the height of the jars or large drinking glasses). Tie the end of one string around each of the two pencils. Adjust the strings' lengths so that when the pencil is laid across the top of one of the jars or large drinking glasses, the end of the string hangs down to just above the bottom of the jar. Make the strings equal length.
• Borax is harmful if swallowed, inhaled or contacts eyes, and on rare occasion touching it can result in rashes. Caution and adult supervision is advised when handling it.

Procedure
• Fill a cooking pot with enough water to fill both jars nearly full. Then bring that water to a boil on the stove. Once the water is boiling, turn the burner off so that the water stops boiling. (Because Borax is harmful if inhaled or contacts eyes it is advised to not dissolve it with boiling water.)
• Add one tablespoon of Borax to the water and stir until it dissolves. Continue to add one tablespoon at a time until no more dissolves. You will probably need about three tablespoons of Borax for each cup of water. How does the saturated solution look?
• Carefully pour equal amounts of the saturated Borax solution into the two jars. Each jar should be about three fourths full.
• Lay a pencil across the top of each jar so that the string hangs down into the saturated solution.
• Cover the top of the jars with plastic wrap.
• Leave one jar undisturbed on a countertop or table at room temperature. Place the second jar in the bowl full of ice that you prepared. If needed, adjust the water level in the bowl so that the water reaches at least three fourths the way up the jar, but is not so high that it goes into the jar.
• Do not disturb the jars for at least five hours. Check the bowl of ice regularly and add ice if it has melted.
• Check on the jars about once an hour to see how the crystals are forming. It may be difficult to observe the jar in the bowl—try looking at the string through the plastic wrap cover. Do you see crystals forming on the side of one of the jars? Do crystals form in one jar before the other?
• After at least five hours carefully remove the pencils and observe the crystals on the strings. How do the size, shape and number of crystals on each string compare with one another? Why do you think this is?
Extra: In this activity you examined Borax crystal formation at two different temperatures, but you could try other temperatures as well; one way is to put one of the jars in the refrigerator. How does allowing the Borax mixture to cool at a different temperature affect crystal formation?
Extra: Try making crystals out of other materials, such as sugar or salt. How well do crystals form using other mixtures with water?
Extra: You did this activity for at least five hours. How do your results change if you grow your crystals for a longer period of time? Make sure to keep adding ice cubes to the water bath to keep it cool throughout the activity.

Observations and results
Did smaller, more abundant crystals form in the jar and on the string in the bowl of ice water, whereas larger, fewer, better-shaped crystals formed in the jar at room temperature?

As the hot, saturated mixture of Borax and water cooled, there was more borax than could be contained by the colder water, and so this borax fell out of the mixture and formed crystals. A crystal is made of molecules of a product that have come together in a specific repeated pattern. When the molecules of the crystal come together, other products that are often considered impurities, or the unwanted products of the chemical reaction, do not fit well into the structure, much like the wrong piece of a puzzle does not fit. If the crystals form slowly enough, the impurities will be rejected because they do not fit correctly, and instead will remain in the water. This is why the crystals in the room-temperature jar should have been larger and more cube-like. But if a solution is cooled too quickly, there isn't time to expel the impurities and instead they become trapped within the crystal structure and the pattern is disturbed. Consequently, the crystals in the bowl of ice water should have formed more quickly and in greater numbers, but were smaller and less cubelike in shape because they had more impurities.

More to explore
Crystallization , from the Department of Chemistry and Biochemistry at the University of Colorado at Boulder
How to Grow Great Crystals: Tips, Tricks and Techniques , from About.com
Crystal Chemistry ( pdf ), from the Royal Society of Chemistry
Crazy Crystal Creations: How to Grow the Best and the Largest Crystals , from Science Buddies

This activity brought to you in partnership with Science Buddies
ScienceBuddies

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