4. Use the particle theory to explain density.​

Density is the total number of particles in a given volume

Explanation:

As per the particle theory, density is defined as total space occupied by certain number of particles of any given element with in a defined volume is termed as density.

Density is not the denseness of a single unit but the space occupied by many single units with in a given volume V

Hence, density is the total number of particles in a given volume.

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.