As water is heated, how does the mean speed of its molecules change?

• A. The mean speed of the molecules increases.
• B. The mean speed of the molecules fluctuates randomly.
• C. The mean speed of the molecules decreases.
• D. The mean speed of the molecules remains constant.

Answer: (A)

Verified step by step guidance

1. Understand that the mean speed of molecules in a substance is related to their kinetic energy, which depends on temperature.
2. Recall the kinetic molecular theory, which states that the average kinetic energy of molecules is proportional to the absolute temperature (in Kelvin). This can be expressed as \(\frac{1}{2} m \overline{v^2} = \frac{3}{2} k_B T\), where \(m\) is the mass of a molecule, \(\overline{v^2}\) is the mean square speed, \(k_B\) is Boltzmann's constant, and \(T\) is the temperature.
3. Recognize that as the temperature of water increases, the average kinetic energy of its molecules increases, which means their mean speed increases.
4. Note that the mean speed is related to the square root of temperature, so the relationship is \(\overline{v} \propto \sqrt{T}\), meaning the mean speed increases as temperature increases.
5. Conclude that heating water causes the mean speed of its molecules to increase, reflecting the increase in molecular motion with temperature.