Course: Chemistry 20
Topic: Crystalline Structure Used for
Determining the Purity of Table Salt
A. Purpose
Name:
Unit: Chemical Bonding
Date:
Grade: 11
To use SEM crystalline images to determine the purity of table salt
Knowledge
Investigate the label on a consumer box of table salt with reference to the chemicals in the product; e.g., “salt, calcium silicate
[colourless monoclinic], potassium iodide [white cubic], sodium thiosulfate [colourless monoclinic]” on one box or salt
(colourless cubic), calcium phosphate (white amorphous), copper(I) iodide (beige cubic), magnesium carbonate (white
trigonal) and/or magnesium silicate hydrate.
The purpose of the iodides is to prevent thyroid gland problems (e.g., goiter) and the purpose of the other chemicals is
generally to prevent clogging of the NaCl salt (i.e., to facilitate free-flowing salt from a box and from a salt shaker).
Scientific Purposes
The scientific purpose of this investigation is to create a test for the qualitative analysis of crystalline samples of NaCl salt or something
else in table salt.
If you observe crystals with an SEM and the crystals appear cubic, then the crystals could be table salt (NaCl).
face-centered cubic model
If you observe crystals with the SEM and the crystals are not cubic, then table salt is a mixture. You can assume that if there are any
ingredients in table salt, other than NaCl, they are not going to have a crystalline structure like NaCl (i.e., the crystalline structures other
than NaCl will not be cubic). However if copper iodide or potassium iodide are listed in the ingredience, we know that potassium iodide
and copper(I0 iodide form cubic crystals, although the copper(I) iodide crystals are beige in colour.
monoclinic crystals
trigonal crystal
1. Introduction
a. How could you distinguish between NaCl salt and other crystalline salts?
b. Can you use the naked eye to see that table salt is a mixture rather than a pure substance?
c. Can you use a light microscope to see that table salt is a mixture rather than a pure substance?
d. What advantage would a scanning electron microscope be when looking at salt crystals?
2. Clarifying
You will be using King’s University website to view images of salt crystals using the SEM simulator for qualitative analysis
between samples of NaCl salt and other crystalline salts in table salt. The NaCl salt crystal is known from past investigations
and from the simplest 1:1 (Na1Cl1) theoretical structure as being (face-centered) cubic in nature
3.
Labwork
Go to the King’s University website where you will view two unknown samples labeled A and B
1.
2.
View each sample at different magnifications and sketch each sample and save the images for comparison. Look for cubic, trigonal and
monoclinic crystal structures.
Was the SEM useful for determining whether salt is a pure substance or a mixture?
4. Summary
In this investigation, the known difference in crystalline structure (i.e., the NaCl salt crystal is cubic) is used to look for other
components in table salt.
a. Could a magnifying glass or a laboratory microscope have been just as effective?
b.
What other tests might have been more economic?
c. What other tests might have been more time-efficient?
d. What other tests might have been less technologically dependent?
e. What other tests might have been more environmentally friendly?
f.
What other tests might have been less collaborative?
g. What other tests might have been less fun?
5. Homework
a. What kind of evidence is used to determine the external and the internal crystalline structure?
b. What kind of knowledge is gathered from the SEM—empirical or theoretical?
c. What kind of knowledge is used to portray the internal crystalline structure of the salt crystal—empirical or
theoretical?
d.
e. What alternatives are there to the SEM for the qualitative analysis of table salt for multiple components; e.g., how can
you determine if the table salt contains some of the components listed earlier in this lesson plan?