Introduction to Crystals

In this hands on exercise, students look at symmetry and self-organization of crystals.

Educational objective

Show how the world we see is built from atoms.

Audience

This material was presented to a group of elementary school students (grades 3-5).

Macro Crystals 1

Show crystal structure forming 'in the large'.

    Materials required

    Lots of similar sized balls and a box. There should be enough balls to cover a little more than one layer. An odd shaped box is useful.

    Tape to mark the structures found.

    Presentation

    Have the kids shake the box (they like this), slowing down. Patterns should emerge in the box.

    This can be repeated with a few balls of different shapes added, or other things to cause defects in the crystal.

Letter Symmetries

Show that there are several kinds of symmetry.

    Materials required

    1. Various letters and short words printed out, such as MOM (bilateral), K (horizontal), H (vertical), X (both).
    2. Mirrors. We used pieces of aluminum flashing which edges taped for safety.

    Presentation

    Discuss the kinds of symmetry and let kids find the symmetries within words and letters.

Alum

We did not grow alum crystals this session, but our group has had great success with them in the past.

    Materials

    Preparation

    Tiny alum seed crystals attached to threads (alum grows quickly enough to demonstrate in a two hour class period).

Sodium Acetate

Rapid crystallization of sodium acetate. This demo shows that crystallization can be rapid and it can give off heat.

    Materials required

    1. Sodium acetate, or vinegar and baking soda.
    2. Glasses, spoons, resealable bags.
    3. REI(TM) sells reusable heat packs made with sodium acetate. The local stores only carried the single use variety (made from iron and charcoal), but the reusable ones may be available from the web.

    Preparation

    Look for videos of sodium acetate on the web.

    We made our own sodium acetate solution and put it in resealable snack bags. We purchased pure sodium acetate from a chemical supplier; we could have boiled the residue from a vinegar+baking soda demonstration. Getting a sodium acetate preparation that was stable enough to remain a liquid during transportation and setup, but supersaturated enough to crystallize in class was a challenge.

    To make the sodium acetate we microwaved a glass of water+sodium acetate as well as a glass part full of water to heat the glass). Stir the hot sodium acetate mixture until it is all dissolved. Pour out the hot water so you have an empty heated glass. Pour the sodium acetate mixture down a spoon into the pre-warmed glass. Cool the solution until it can be easily handled and poor it into the resealable snack bags. Cool to room temperature. Test one package by opening the bag and touching with your finger --- this should be enough to crystallize the whole mixture. Test another package by handling it normally without opening. It should not spontaneously crystallize. You should keep some of the slower crystallizing samples since they are more interesting to watch, and quick growing samples for the wow factor. Preparing these samples was frustrating, and involved many cycles of heating and cooling, adding water or powder, to get the correct saturation.

    Presentation

    Hand the packages of liquid to the students.

    The children handled the packages roughly and most of them spontaneously crystallized. One of them we needed to open and touch with a finger to get it to crystallize.

    This demo was a great success with both parents and children, be prepared to explain what is happening and why it got warm.

Crystal samples

Are there crystals in the world?

    Materials required

    1. Crystal examples (we had a variety of minerals picked up during our travels) and non-crystals.
    2. Sugar, epsom salt, sodium acetate, alum, copper sulphate (pretty but poisonous!) or other to grow your own crystals in advance.

    Preparation

    Epsom Salt crystals are easy to make. Simply boil a pot of epsom salts purchased from your local pharmacy, dissolving as much as you can into the solution. Let cool a bit. Tie a string around a largish crystal from the box as your seed. Hang the string in the solution overnight.

    Rock candy is also easy: boil a sugar solution and dangle a string in it. Over the next week large sugar crystals will grow. Use food colouring for variety.

    Presentation

    Hand the examples to different students in the class and have them comment on the structures they see. At the end of the class

Macro Crystals 2

Show different symmetries using stacks of balls.

    Materials

    1. 3 dozen balls (we used tennis balls).
    2. wooden strips for frames.
    3. heavy tape for hinges.

    Preparation

    Create a square array of balls and build a frame to go around it. Repeat.

    Presentation

    Show various symmetries for crystals by stacking the frames and shifting one relative to the other. There are a lot of them.

    Note in particular that for the triangular close-packing there are two ways you can stack the third layer, one which allows a stick to go right through all the layers and another which doesn't.

Build your own crystals

The classic gummy candies and toothpicks.

    Materials

    1. Soft candies (firm but not brittle is best).
    2. Toothpicks.

    Presentation

    Build structures and look for repeat pattern. Which of them are crystals?

Carbon magic

Carbon forms both the hardest and softest of materials.

    Preparation

    Large scale ball and stick crystal structures for graphite and diamond.

    We used Zome(R)

    Presentation

    Show diamond and graphite structures.

    Show how at different angles you can see different shapes through the diamond crystal.

    Talk about the different facets you can cut off.

Watching Crystals Grow

There are wonderful movies on the web showing gold, alum and H,2,O crystal growth. Choose your favourites.

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