What effect does heat have on molecules?

Heat increases the energy of molecules, causing them to move more rapidly. When the temperature of a substance rises, the added thermal energy accelerates the kinetic motion of its molecules. This increased movement can lead to changes in the state of matter, such as solid materials turning into liquids or gases as the molecules gain enough energy to overcome the forces holding them together. This is particularly evident in substances like ice, which melts into water and eventually evaporates into steam when heat is applied. Thus, the correct answer highlights the primary effect of heat on molecules, which is the enhancement of their movement.

The other options do not accurately describe the general effect of heat on all types of matter. For example, while heat can cause certain substances to become solid as they cool down, this is not a universal effect of heat itself. Similarly, heat does not cool molecules; rather, it raises their energy level, increasing their motion. The binding of molecules together also does not occur as a direct effect of heat; instead, heat generally causes them to move apart in states like liquids and gases.