Growing & Observing Crystals

This activity is designed for students to understand the experimental process involving independent and dependant variables. Students will also further develop and refine their scientific thinking skills through observing, questioning, changing variables in an experiment, problem solving, and scientific writing. Students will understand the best process for growing crystals, along with the basic scientific process for answering a question/solving a problem. Students will also connect the ideas learned in the lab with those concepts read about in Geology—the sizing/shapes of various minerals that make up rocks. Vocabulary: variable, independent & dependent variables, control, crystal, mineral.

This activity is designed for an 8th grade classroom, 2 days (block classes of 88 minutes). The actual lab procedure, writing, etc can be completed in one day, then the crystals are left to grow—over the weekend works great—beginning on a Thursday or Friday and completing on Monday. The second day is for observing the grown crystals, writing conclusions, and discussing the experiment.
The type of salt used to grow the crystals will change the shape of the crystals you get. I usually use BOTH table salt and Epsom salt, as the table salt crystals are cubic and the Epsom salt crystals are needle-like. I use aluminum pie pans with black paper cut to fit the bottom, as the crystals stick to and stand out on the black paper.

This activity can be introduced and used in during the Geology unit within the Earth Science curriculum. It is a great way to include a hands-on lab, reviewing lab techniques, scientific writing, and variables during the Geology unit. I find that it's great to refer back to about what conditions allow for better crystal growth. Within Minnesota, a mining connection can be made and take this a step further with researching/discussing the mining of crystals/minerals/ores/etc that is done here in our state.
You can begin with the question, what variables affect how crystals form ("Grow")? Discuss as a class, or as a lab group, etc. Then walk through the procedure for the lab. Talk about what the students want to change as their one independent variable, what is the dependent variable, what are the constants. You can choose to allow the lab groups to decide on their own variable or assign it. It can work both ways. Here is one way to assign groups:
Lab 1: double the table salt
Lab 2: double the water
Lab 3: leave the pan with solution sitting out
Lab 4: put the pan with solution in a bag

Lab 5: double the Epsom salt
Lab 6: double the water
Lab 7: leave the pan with solution sitting out
Lab 8: put the pan with solution in a bag

You could also have a group use warm water versus cold water, etc. Students may also come up with other variables to change.
According to the group's variable, they must write a hypothesis before they begin. They can write individually, or as a group.
Then, allow the students to follow the directions, mixing the desired amount of salt and water, pouring it into the aluminum pie pan (with black paper), and place in desired location. After 3-4 days, or a weekend, allow students to observe their crystals, drawing what they look like, comparing to the control, and writing a conclusion. What do they see? Was their hypothesis supported or refuted? What are any similarities & differences with other groups? Would more time have given them more or bigger crystals? Then talk about communicating these results. Who would want to know this, why? What are the IDEAL conditions for growing crystals? Students can write this up as a simple lab, see attached lab sheets; or students could write up their own, formal lab report or paper about the experiment. It can be taken/adjusted to any level desired. crystal growth lab (Microsoft Word 36kB Aug2 09)

Again, this activity can be modified and changed to fit the needs of your students. The paper could be an optional or extra/challenge opportunity for select students. You can have them write the procedure, steps, materials, etc on their own or use the lab sheet. You can allow them to choose their variables or you choose for them.

Students can be assessed in various ways with this lab assignment, depending on how it is set up. You can provide a rubric at the start of the activity, outlining what you are requiring as part of the lab write up—for each student, or each group. http://rubistar.4teachers.org/index.php is a great website for creating rubrics! You can only collect their conclusions, looking to see what they learned, how they have thought through the process, related back to their original prediction/hypothesis, and made their results relevant. Can they explain why they got the crystals they did, etc. Or you can just collect the lab sheet from each student and assign points to each section. There are a variety of ways to asses what your students have learned from this investigation!

8.I.B.1 –scientific inquiry (making observations, investigations, changing variables, etc.)

8.III.A.3&6 –earth & space, earth structure & processes (rock cycle, identify & classify rocks & minerals)