According to the kinetic molecular theory, which phase of water has the greatest average kinetic energy per molecule?

• A. Solid (ice)
• B. Liquid (water)
• C. All phases have the same kinetic energy
• D. Gas (steam)

Answer: (D)

Verified step by step guidance

1. Recall that according to the kinetic molecular theory, the average kinetic energy of molecules is directly proportional to the temperature of the substance in Kelvin. The formula for average kinetic energy per molecule is given by \(\langle KE \rangle = \frac{3}{2} k_B T\), where \(k_B\) is the Boltzmann constant and \(T\) is the absolute temperature.
2. Understand that the phase of a substance (solid, liquid, gas) depends on temperature and pressure, but at a given temperature, the molecules in different phases can have different average kinetic energies.
3. Recognize that in typical conditions, the temperature of steam (gas phase) is higher than that of liquid water or ice, meaning the molecules in the gas phase have higher average kinetic energy.
4. Note that the kinetic molecular theory states that all molecules at the same temperature have the same average kinetic energy regardless of phase, but since the question implies comparing phases at their typical temperatures, the gas phase (steam) has the highest average kinetic energy.
5. Conclude that because steam is usually at a higher temperature than liquid water or ice, the gas phase has the greatest average kinetic energy per molecule.