You are heating water in a beaker.   How would you describe the change in energy and speed of the water molecules as the water gets hotter? A. The energy and speed both increase.   B. The energy and speed do not change.   C. The energy increases while the speed decreases.   D. The energy stays the same while the speed increases.

Expert Answers

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The correct answer is A. As the temperature of the water in the beaker increases, both the energy and speed of the molecules increases.

Temperature, energy and molecular speed are all related. Temperature is the measure of average kinetic energy of molecules. The kinetic energy of an individual molecule is one half the product of its mass and the square of its velocity:

KE = 1/2 mv^2

As the water molecules absorb heat energy they move faster. Heat is conducted to the water molecules by collisions with faster moving particles. The speed and therefore the temperature of the water molecules continues to increase until the boiling point is reached. At that point the faster moving molecules escape into the gas phase so their speeds are no longer part of the average. The avarage speed kinetic energy of the molecules still in the liquid phase remains constant as the liquid boils so no increase in temperature is observed.

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