Alum Crystal Growth (Richard Hanley)
Alum Crystal Growth Crystals pictured here are from the Salton Sea
Longitudinal Research - alum crystal growth
Viewing the growth of crystals using the pickling spice alum (hydrated potassium aluminum sulfate).
Author(s)
Richard Hanley and his 8th grade physical science classes.
Question
What are the best conditions for growing large crystals?
Standards
CSS 3c. Students know atoms and molecules form solids by building up repeating patterns, such as the crystal structure of NaCl or long-chain polymers.
NGSS MS-PS1-1 Solids may be formed from molecules, or they may be extended structures with repeating subunits (e.g., crystals).
Experimental Design Large alum crystal from wikipedia
Students will dissolve 5mL of alum into 50mL of 80 degree water in a 100mL beaker. They will transfer the solution into a clean mason jar and decide as a group what conditions you believe will best aid in the growth of large crystals.
Controls
The mason jars, temperature of the alum solution (80 degrees), the amount of water (50mL), the amount of alum (5mL) and time (one week) are all independent variables.
Independent variable
Time
Dependent variables
The dependent variables depend on the decisions of the lab group (3-5 persons). They could include: placing a lid on the item or not, covering the jar with a coffee filter or not, placing the solution in light or in a cabinet, adding a Popsicle stick and string with a 'seed crystal' to begin the growth or not.
Series
Experimenting with which dependent conditions allow for the largest crystals given the independent variables.
Materials
100mL beaker, Alum, Measuring spoon, Hot plate, Thermometer, Glass stir stick, Mason jar with lid, Coffee filter, Popcicle stick, String, Masking tape, Sharpie marker (to label jars with), Cleared off counter top and Cleared out cabinet space.
Procedures
Add 50mL of water to a 100mL beaker and heat to 80 degrees Celsius. Add 5mL of alum (the pickling salt you add so that your pickles aren't limp and flabby but stiff and crispy) and stir the mixture together until all of the alum looks dissolved. Allow the solution to cool to room temperature and transfer it to a clean Mason jar and label it with your period and table group number. How students cover the crystals, if at all, whether they place a string with a seed crystal in it, or where they place the jar (out in the open or in a cabinet) is up to the individual group.
Sample data and Photographs
Two different tables with the same method of using a Popsicle stick and string with a small crystal tied on - known as a 'seed crystal' - to attempt to form large crystals.
Note how very small crystals formed only on the bottom of the jar and how no crystals formed on the string (in fact the 'seed crystal' dissolved into the solution).
Covered jars which are sealed, produced very few crystals as did completely uncovered jars.
These crystal did form a very cool hexagonal shape though!
This became a very popular way to use just the rim of the jar to secure a coffee filter so the solution could effectively evaporate out the water.
Note how everyone who chose to put their crystals in the cupboard also covered their solution.
Those lab groups which allowed their solutions to cool to room temperature before adding them to the clean Mason jar, who covered their solutions and who put them in the cupboard out of sunlight, grew the largest crystals. ReferencesChemistry page on About.com.
