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Jan. 25, 2010

Let’s Grow Some Crystals!

by Science Friday Education

Winter weather means more than sledding and snowmen. Next time there’s a snowstorm, grab a magnifying glass and try snowflake hunting. Snowflakes are made of crystals: bullet rosettes, stellar plates and capped columns are just a few of the varieties of snowflake crystal you can find in your backyard. (If it never snows where you live, you can find photos of snowflake crystal structures online.) Kenneth Libbrecht, physicist at California Institute of Technology (Caltech) and snowflake expert, shares secrets of the snowflake.
 
Grade Level: 6th – 8th grade
Subject Matter: Chemistry
National Standards: NS.5-8.1, NS. 5- 8.2, NS.5-8.7
 
Overview
A crystal is a natural solid made up of a repeated pattern of molecules connected together. Crystals can form through the slow cooling of molten material (gemstones), or when a warm gas such as oxygen cools down (snowflakes), or when a liquid that contains dissolved minerals cools very slowly (salt).
In this activity, students will observe various substances and determine whether these substances are crystals based on their physical properties. They will also learn one of the processes for crystal formation and how to create their own crystals by cooling a supersaturated solution.
 
Activity Materials
White pipe cleaners, at least two for each student
Borax powder
Salt
Baking soda
Yeast
Sand
Wide-mouthed jar, one for each student
Pencils, one for each student
Water
Food coloring
Scissors
Magnifying lens
Craft stick
Package of black construction paper
Plastic spoon
Oven mitts or gloves – a pair for each student
Hot water
 
Vocabulary
Crystal: a solid containing an internal pattern of molecules that is regular, repeated, and geometrically arranged.
Molecule: the smallest physical unit of a substance that can exist independently. A molecule is made up one or more atoms held together by chemical forces.
Supersaturated solution: a solution that has been heated in order to dissolve more material than would be possible at room temperature.
 
What To Do
  1. Begin the lesson by having students watch the Science Friday Video, “Snowflake Safari”. Ask students if they know any other examples of crystals besides snowflakes. Tell students that they are going to conduct two experiments. The first experiment will help them understand and define what a crystal is. In the second experiment, students will be able to create their own “designer” crystals.
Experiment 1
  1. Hand out one sheet of black construction paper to each student. Have students fold and cut the construction paper into four evenly squared pieces.
  2. Have students place one small spoonful each of salt, sugar, baking soda, and yeast on each of the four separate squares of black paper. Then students can observe each substance with a magnifying lens. How are they similar to or different from one another?
  3. Tell students that a crystal is a solid substance with a fixed and repeated pattern. Crystals will retain their pattern even when they are broken into pieces. Have students use a craft stick to crush each substance and then observe the substance again with the magnifying lens. Out of the four substances, which are crystals and which are not crystals? Have students explain their reasons.
 
Experiment 2
N.B.: This activity requires the use of hot water. An adult should pre-heat the water in a microwave or on a hot plate, and then pour the hot water into a wide-mouthed glass jar for each student who is conducting the experiment. Review safety procedures with students and the importance of using oven mitts or gloves before touching the glass jar.
  1. Have students slowly add and mix spoonfuls of borax powder into the hot water (about 3 tablespoons per cup of water). Ask students to describe the difference in the water before and after adding the borax powder. Tell students that they are creating a supersaturated solution. What do they think that means?
  2. Give students the option of adding a few drops of food coloring to their solution for color. Ask students what color the resulting crystals will be without any food coloring added.
  3. Hand out a white pipe cleaner to students and instruct them to bend the pipe cleaner into a shape of their choice (heart, star, circle, letter, square, etc.)
  4. Hand out a second pipe cleaner and have students wrap the tip of the second pipe cleaner around the shaped pipe cleaner so that the shaped pipe cleaner looks like a dangling ornament.
  5. Wrap the tip of the other end of the second pipe cleaner around the middle of a pencil, and place the shaped pipe cleaner inside the jar with the pencil resting on top of the jar to hold it in place. Make sure the shape is not touching the bottom or sides of the container and that the shaped portion of the pipe cleaner is completely immersed in the solution.
  6. Carefully place the container in a secure area for the next few days. Ask students to predict what will happen as the solution cools down.
  7. Have students observe the solution and record their observations over the next 1-2 days. Depending on the room temperature, crystals may begin forming within a few hours. Ask students to explain why these changes in the solution are happening.
  8. Have students gently remove their crystallized pipe cleaner from the jar by the second day, regardless of any solution that remains. Have students use the magnifying lens to observe their crystal formations. Compare and contrast the crystal formations from the other students. What variables could have caused some of the crystal formations to be different?
 
What’s happening?
Crystals are solids that are formed by a regular, repeating pattern of molecules connected together. Most crystals come in geometric shapes with sharp, straight edges and smooth sides. Salt crystals have a cubic shape while sugar crystals have a flattened rectangular shape. When a crystal breaks, it will break cleanly and smoothly into smaller versions of the same geometric shape.
 
Crystals can form when a supersaturated liquid that contains a dissolved mineral cools. In this activity, a supersaturated solution was made using hot water and borax (a soft crystal). The hot water caused the water molecules to move further away from each other so that more of the borax could dissolve into the solution. Once the solution reaches a point where it cannot dissolve any more borax, it becomes supersaturated. As the solution cools, the water molecules come closer together again causing the forming borax crystals to cling to the pipe cleaner.
 
Topics for Science Class Discussion
  • Will substituting sugar or salt for borax yield the same crystal formations?
  • What determines the color of crystals found in rocks?
  • How is the crystal-forming process that occurs from slowly cooled molten material different from the method used in this activity?
  • How and why are crystals used in electronic devices?
Extended Activities and Links
  • Experiment with different variables using the same activity. What happens if the cooling rate of the solution is changed by refrigeration?
  • Have students research the various types of crystal shapes (lattices) and build models using crafts materials such as toothpicks, clay, marshmallows, gumdrops, etc. Have students explain the atomic structure of each crystal and how it forms that specific shape.
  • Preserve your own snowflake or view snowflake images online at Kenneth Libbrecht’s website:
  • Measure the density of water, ice and snow and submit your data online:
About Science Friday Education

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