So, Newton's Law of Cooling has this constant k in the equation:
T-Ta = (T0-Ta)ekt
This k is based on the material properties of whatever's cooling. I wanted to compare k across liquids, so I chose liquids with different properties.
One of the liquids I chose was plain water. It wasn't distilled, but it acted as the "control" in the experiment. I could compare the k values of the other liquids to water and see how it reacted.
Then, I made a salt water solution of 1 part salt to four parts water (1/8th of a cup of salt in 1/2 a cup of water). I remembered from chemistry that adding solute to a solvent raises its boiling point and lowers its freezing point, because it interferes with the intermolecular reactions. I wanted to see how this influenced its cooling. As it turned out, salt water had a slightly lower k than regular water, indicating that the difference in temperature plays a slightly smaller role in how salt water cools.
I also used vegetable oil. I wanted a nonpolar liquid, but didn't want to inhale rubbing alcohol fumes during the experiment, so this seemed like the best choice. The oil not only heated much faster than the other liquids (I had to wait a while before it was at a temperature where I could start collecting data), but it also retained the most heat and had the smallest k value, less than half of that of regular water and about half of that of salt water.