White Powders
Starting
Introduce the chemistry of materials by showing the chemical spill video below:
https://www.youtube.com/watch?v=qNhhjFeuPFk
How can we know what white powder we are dealing with?
One way to pursue this is by understanding the nature of materials though testing. If we have a dozen different white powders, how can we tell them apart?
White powder Matrix Exploration
1. Demonstrate to students how to conduct several tests on a powder:
a) observe with a hand lens. Look for shape or texture of the crystals. Describe.
b) place a small amount on a non-porous surface. (glass or plastic plate) Place a drop of water on the substance. Does it dissolve quickly, slowly, not at all?
c) Place a small amount in a test tube or small cup. Add water, 10 ml. Use pH paper to report the acidity.
d) Place a drop of vinegar on a small amount on a glass or plastic plate. Does it bubble? If so, it reacts to an acid.
e) Place a drop of water on the end of a wire probe. Stick the probe into the flame. Does the powder melt? How does it react? Describe.
f) Leave the material in the flame until it is totally consumed or there is no change. What is the color of the flame?
g) Describe the residue
2. Create a data table to allow gathering of information from each white powder tested.
Complete the tests and matrix.
substance
alum
baking soda
epson salt
sand
salt
sugar
creme
borax
cane sugar
citric
cs
unown
ph
3
8
6
8.5
7.5
5
3.5
10
9
1
7
5/4
crystal
yes
yes
yes
no
no
no
none
no
no
yes
solubil
no
yes
no?
no
yes
yes
yes
yes
yes
yes
yes
yes
melt
no
yes
no
popped
yes
yes
no
yes
yes
yes
no
yes/no
burn
no
orange
no
no
orange bright
no
no
orange
orange
yellow
blue/pur
orange
acid
no
yes
no
no
no
no
no
no
no
no
no
Other Char
no
2 powders
o
no
yes/no
3. When finished, gather the class together and suggest that this matrix can now be used to determine an unknown spill. By testing an unknown it can be compared to the knowns. Where the results match we can deduce it is the same substance.
4. Give students an opportunity to test the unknown. Should we call out the HAZ-MAT Team?
Extensions
Chemists and chemical engineers study the characteristics of materials so they may know how to use them for various applications. Each of the white powders can be searched for special properties. If we use Alum as an example, here are its properties as reported by Wikipedia (https://en.wikipedia.org/wiki/Alum)
"Alums are useful for a range of industrial processes. They are soluble in water, have a sweetish taste, react acid to litmus, and crystallize in regular octahedra. In alums each metal ion is surrounded by six water molecules . When heated, they liquefy, and if the heating is continued, the water of crystallization is driven off, the salt froths and swells, and at last an amorphous powder remains. They are astringent and acidic."
These definitions may be difficult as they tend to use vocabulary that is unfamiliar to a student. In the above description, astringent means to constrict or shrink body tissue. Octahedra means the 8-sided shape of the crystal. And litmus refers to a pH test paper we use to determine acidity.
Such descriptions can be challenging, but many offer interesting historical and industrial uses of the material as well as chemical characteristics. Did you know that alum was used as a deodorant and to pickle cucumbers?
Developing a sense of the utility of various substances is an important branch of chemistry and engineering. Students can design other tests, read about characteristics of materials and report on the interesting things they find about how different chemicals behave. Reporting is an opportunity for critical reading, recapitulation, and writing. ELA standards can be applied in this area so that STEM standards might overlap with language arts goals.