1. Why do you think temperature is the measure of the "average"
energy of motion of all the particles within a substance?

Temperature is the measure of the average energy of motion of all the particles present in a substance because particles of a substance moves due to the presence of kinetic energy.

Explanation:

Kinetic energy is a type of energy that is present in an object due to its motion. When temperature of a substance is increased, the particles absorb heat energy from surrounding environment and starts motion. This motion of particles due to the absorption of heat energy is called kinetic energy. So that's why temperature is considered as a tool to measure average energy of a motion.