Final answer:
An object's temperature correlates with the motion energy of its particles: the higher the average kinetic energy, the higher the temperature. Ice, water, and vapor exhibit different temperatures due to varying kinetic energies of water molecules in each state, with ice being the coolest and vapor the hottest.
Step-by-step explanation:
The statement "An object's temperature is equal to the average kinetic energy of its molecules" refers to how temperature is directly linked to how much energy the atoms or molecules of a substance have due to their movement. When considering water, its molecules have different amounts of kinetic energy in solid (ice), liquid, and gas (vapor) states, which leads to different properties for each state.
In the solid state as ice, water molecules are tightly packed in a lattice structure and can only vibrate around fixed points, hence they have lower kinetic energy. As a result, ice has a lower temperature. In the liquid state, water molecules have more kinetic energy allowing them to move freely around each other, which leads to a higher temperature compared to ice. Finally, as a gas (water vapor), water molecules move with the highest kinetic energy, bouncing off each other and their container, demonstrating the highest temperatures among the three states.
Essentially, with an increase in thermal energy, molecules move faster and the substance becomes hotter; conversely, a decrease in thermal energy slows molecular movement, making the substance cooler. At absolute zero, molecular motion ceases entirely, defining the lower limit of the temperature scale.