21. Classify each of the following as an observation or a law:

a. When water boils, small bubbles form in the liquid and rise rapidly to the surface

b. Two grams of hydrogen combine with 16 grams of oxygen to form 18 grams of water.

c. Chlorine and sodium readily combine in a chemical reaction that emits much heat and light.

d. The properties of elements vary periodically with the mass of their atoms.