The temperature of a sample of liquid water changes from 50°C to 30°C. Which statement best explains the change that must
happen at the molecular level?

kinetic energy decreases.

Explanation:

When molecules moves more  slowly their kinetic energy decreased which results in decrease in temperature of a solution.

Kinetic energy is the movement of molecules present in the solution and if it increases the molecules movement will also increased which results in high energy production and temperature will increases rapidly but if this movement is slow temperature will decreases and kinetic energy of the solution too.

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