Lesson 3/Week 3: Crystals
Establish what a pattern is: draw a series of three (or more) incomplete patterns on the white board and ask the kids to finish each one. You can either invite the kids up to finish each pattern or you can poll them and then fill it in yourself.
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"Remember in our first class we talked about how lots of molecules together make up things that we can see and we talked about solids, liquids and gases. Well today we're going to talk about solids whose molecules form a pattern, or crystals. Not all solids have a pattern, their molecules are scattered all about randomly, but crystals follow a regular, repeating pattern".
"It's possible to "grow" crystals, and when we grow then, we are separating all the building block molecules into individual little pieces in water, and letting them falling into their natural repetitive pattern once the water evaporates, or "dries up"."
Types of Crystals:
- Ionic (salt)
- Molecular (sugar, ice), where the molecule's actual shape can be seen once the crystal has formed; we were lucky enough to still have snow on the ground, so I also used this as an example.
- Covalent (diamond and other gems), Some elements can make more than one crystalline form. For example, Carbon has different forms can be graphite like in a pencil or added to steel to make it stronger, OR it can be diamond like in jewelry
- Metallic (atoms of metals)
Crystals have different shapes and properties.
Activity: Show different pictures on laptop of crystals and of salt, water, salt + water molecules.
Crystals can only grow from a saturated solution. When a liquid is saturated, nothing more of what you add to it will dissolve. For example, you can dissolve sugar by adding it to a cup of hot tea. If you stir it well, the sugar crystals will disappear and you won't see them anymore. BUT if you keep adding huge amounts of sugar, after awhile no more will dissolve and all you'll see is crystals from the extra sugar you added. That is when the solution/liquid/tea has become what is called saturated.
Experiment: Growing Glow-In-The-Dark Crystals:
- While stirring a cup of boiling water, slowly pour in Epsom Salts, adding about 1/4 cup at a time. It will hold quite a lot. As soon as the solution won't turn clear with stirring, it is time to stop.
- Take one or two pretty, colorful pipe cleaners. Bend it into some shape at one end, leaving the other end for a "hanger."
- Paint it with GITD paint and allow to dry for 15-30min.
- Hang it over a pencil suspended over a STRAIGHT SIDED container, or you won't be able to get your crystal out.
- Pour the Epsom Salt solution over the pipe cleaner in the container. Set it in the warmest room in your home where it won't be disturbed. In about 3 weeks, the water will have evaporated, leaving long, lovely crystals behind on the pipe cleaner.
Experiment: Making Rock Candy***
Explanation: When growing sugar crystals, the sugar in solution is not "happy" so it comes out of the water-sugar solution because there's too much sugar and not enough water to balance it out. When it comes out of the solution, it's called a precipitate. The more water that evaporates or dries up out of the cup/solution, the more sugar will come out, too, making the crystals grow, because there's already not enough water to make the sugar "happy".
- Heat 2 cups of water to boiling.
- Completely dissolve 4 cups of sugar and stir using a wooden spoon until solution is clear.
- Pour the cooled solution into a glass jar & cover the jar with a paper towel.
- Wrap a pipe cleaner around a bamboo skewer, with the tip removed, leaving about 1 inch in between the bottom of the cup and the pipe cleaner. It should not touch the bottom.
- Dip the pipe cleaner into the sugar solution, remove it, lay it on a piece of waxed paper, straighten it out, and let it dry for a few days.
- Gently suspend the pipe cleaner in the solution and let it sit for about 7 days. Be careful not to move or touch it during this time, except to observe daily crystal growth.
***I prepared a kit ahead of time for each student and handed it out at the end of class for them to take home. It consisted of a plastic ziploc sandwich bag 1/3 of the way full of sugar-saturated and colored (with food coloring) solution I made ahead of time at home, a 9oz plastic cup, a half of a bamboo skewer and a white pipe cleaner cut in half.
I placed the bagged solution inside of the cup to avoid leakage, then tucked the skewer and the pipe cleaner in the cup next to it. I placed each kit into a brown paper bag and stapled an instruction sheet for parents to the outside of each bag. I made sure to demonstrate the process for the kids in class so that they could guide their parents on how to assemble the kit at home, if they were unsure or got stuck.***
Last, show them the video at this link so they can see what their crystals will look like at home. http://www.youtube.com/watch?v=WQdXbf8huuQ