Final answer:
As the temperature of a substance decreases, the particles move slower due to a decrease in kinetic energy, leading to a potential change in phase from gas to liquid or liquid to solid.
Step-by-step explanation:
When the temperature of a substance decreases, the particles move slower. This is due to a reduction in kinetic energy, which is directly correlated with temperature. In other words, the average kinetic energy of the particles decreases, which means that they have less energy to move around and thus their movements become more sluggish.
This slowing down of particle motion can lead to a change of phase from gas to liquid or from liquid to solid, as the particles move closer together and the attractive forces between them become more significant. The slower movement of particles does not mean they melt, grow in size, or become lighter. Instead, slowing down is a consequence of the decreased kinetic energy, implying a cooler state for the substance. As the temperature decreases further, substances typically progress from being a gas to a liquid and finally to a solid state, assuming they do not undergo sublimation.