Hello,
This is a great question. The best way to figure out if other substances will work is to try it. Since this is a science project, experimentation is encouraged!
When added to water, the majority of impurities/substances will lower the freezing point. Table salt (with the chemical formula NaCl or sodium chloride) has a particularly strong effect because it is soluble in water; this means that the salt produces Na+ and Cl- ions in the water, which prevent water molecules from getting close enough to freeze.
Other substances will have an effect on the freezing point as well, but to a different extent. Other salts, like CaCl2 or MgCl2, may have a greater effect than NaCl because they will produce even more ions in water. Pepper, on the other hand, is a more complicated substance because it comes from a plant and therefore is made of many different chemicals; some of these chemicals are insoluble, and some are only slightly soluble. This means that you may not achieve the same effect if you were to do this activity with pepper.
I suggest that you try the activity with sugar. You are much more likely to achieve results with sugar than with pepper since the sugar dissolves in water.
Let us know if you need more clarification or if you have any other questions, and don't be scared to have fun with this activity!
Emma
