I combined different materials. The first combination was a mixture of salt, pepper, and water. The second was of water and an Alka-Seltzer tablet....

The observation in each case depends on the properties of the materials.

Mixture 1: Salt and Pepper in Water

Salt is soluble in water, while pepper is not. In fact, pepper is lighter than water and hence floats over it. Thus, when we mix the three components together, salt will dissolve in water and will disappear, while the pepper will float over the water surface.

These components can be separated by filtering pepper out and then evaporating water to obtain salt.

Mixture 2: Water and a tablet of Alka-Seltzer

Alka-Seltzer contains sodium bicarbonate, aspirin and citric acid. When dissolved in water, sodium bicarbonate and citric acid react with each other and produce carbon dioxide. This generation of carbon dioxide is evident as a white foam coming out of the container (which may actually overflow as well) in which you have mixed water and Alka-Seltzer tablet.  

Hope this helps.