Placing sodium metal into water creates a bright light and a lot of heat – an explosion. Sodium metal disappears during this explosion. Is this a chemical change?

The answer for #3 is c but i don't know why

If anyone would be able to me out with these three questions it would be these are from the chem 2202 course.

What rule is used to determine how many covalent bonds an element can form? a. the number of covalent bonds is equal to six c the number of covalent bonds is equal to five minus the group number plus the group number b. the number of covalent bonds is equal to eight d. none of the above minus the group number select the best answer from the choices provided