19. Gas leaking is A. physical property
B. physical change
C. chemical change
D. chemical property

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

Imagine that you push on a large rock. at what point does your effort change the rock’s mechanical energy?

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