Why do water molecules move slower as they are cooled?

• A. Because their kinetic energy decreases as temperature decreases.
• B. Because cooling increases the mass of the water molecules.
• C. Because cooling causes the molecules to gain potential energy.
• D. Because cooling increases the number of collisions between molecules.

Answer: (A)

Verified step by step guidance

1. Understand that the movement of molecules is related to their kinetic energy, which depends on temperature.
2. Recall that kinetic energy of molecules is given by the equation \(KE = \frac{1}{2}mv^2\), where \(m\) is mass and \(v\) is velocity (speed) of the molecule.
3. Recognize that as temperature decreases, the average kinetic energy of the molecules decreases because temperature is a measure of the average kinetic energy.
4. Since kinetic energy decreases, the velocity (or speed) of the water molecules decreases, meaning they move slower.
5. Note that cooling does not change the mass of the molecules, nor does it cause them to gain potential energy or increase collisions; it primarily reduces their kinetic energy.