Which of the following will occur as the speed of the molecules in a substance increases? A. The force of attraction between the molecules will decrease. B. The substance will become cooler. C. The substance will begin to take up less space. D. The force of attraction that draws objects to Earth will increase
The correct answer and explanation is:
The correct answer is A. The force of attraction between the molecules will decrease.
As the speed of molecules in a substance increases, their kinetic energy also increases. This increased motion causes the molecules to move more vigorously and spread farther apart from each other. Because the force of attraction between molecules depends on how close they are, when molecules move faster and farther apart, the intermolecular forces become weaker.
When a substance is heated, its molecules gain kinetic energy and vibrate or move faster. This typically causes the substance to expand as the molecules need more space to move. For example, when ice melts into water, the molecules move faster and the structure loosens, reducing the attractive forces between them. If the speed of molecules continues to increase, the substance may eventually transition into a gas, where molecules move freely and intermolecular forces are very weak.
Option B, “The substance will become cooler,” is incorrect because increasing molecular speed corresponds to an increase in temperature, not a decrease. Temperature is a measure of the average kinetic energy of the molecules, so faster molecules mean higher temperature.
Option C, “The substance will begin to take up less space,” is incorrect because as molecular speed increases, most substances tend to expand and take up more space due to increased molecular motion.
Option D, “The force of attraction that draws objects to Earth will increase,” is unrelated to the molecular speed of a substance. The force of gravity depends on mass and distance, not on molecular speed.
In summary, increased molecular speed results in weaker intermolecular attraction because the molecules move apart and collide more forcefully, reducing the strength of the forces holding them together. This explains why heating a substance usually causes expansion and phase changes like melting or evaporation.