When a liquid is cooled, its molecules undergo a series of transformations that alter its state of matter. In a liquid, molecules are closely packed, maintaining constant contact, yet possess sufficient energy to move and slide past one another, allowing the substance to flow and adapt to its container’s shape. This molecular freedom defines the liquid state. Cooling involves removing thermal energy from the substance, initiating changes at the molecular level.
Molecular Motion Slows
Molecules within any substance are in perpetual, random motion, directly related to their kinetic energy. In a liquid, this kinetic energy manifests as molecules constantly vibrating, rotating, and translating. As a liquid is cooled, thermal energy is systematically withdrawn, reducing the average kinetic energy of the molecules.
Consequently, molecules slow their movement and vibrations. While not uniform, the overall average speed and vibratory intensity decrease. This reduction in molecular motion sets the stage for structural rearrangements.
Intermolecular Forces Become Dominant
As molecular kinetic energy diminishes with cooling, intermolecular forces (IMFs) become more significant. These attractive forces, including London dispersion forces, dipole-dipole interactions, and hydrogen bonds, are always present. However, in warmer liquids, vigorous molecular motion often overcomes their influence.
With reduced molecular motion, attractive forces more effectively pull neighboring molecules closer. This leads to molecules being held in more cohesive associations. The balance shifts from kinetic energy dominating molecular behavior to intermolecular attractions determining their arrangement.
Molecules Arrange into Ordered Structures
As cooling continues and molecular motion decreases, molecules lose their ability to freely slide past one another. Attractive forces become strong enough to lock them into fixed positions. This process is known as freezing, where a liquid transforms into a solid.
For most substances, solidification involves molecules settling into organized, repeating patterns, forming a crystal lattice. Water molecules, due to hydrogen bonding, arrange into an open, hexagonal lattice when they freeze. This arrangement creates a rigid framework where each water molecule forms hydrogen bonds with four neighbors. This transition from a disordered liquid to an ordered, often crystalline, solid is a key molecular change during cooling.
Changes in Macroscopic Properties
Molecular changes during cooling manifest as alterations in the liquid’s macroscopic properties. Density is one change. For most substances, as molecules pack into a more ordered, compact solid structure upon freezing, their density increases.
However, water expands upon freezing, making ice less dense than liquid water. This is because water molecules form an open hexagonal lattice in ice, holding them further apart than in liquid water. Liquid water is densest at approximately 4°C, decreasing in density as it cools to 0°C and freezes.
Viscosity, a fluid’s resistance to flow, is another affected property. As a liquid cools, the increased influence of intermolecular forces makes it more difficult for molecules to move past each other. This increased resistance to flow results in higher viscosity. For example, substances like cooking oil or honey become thicker and flow less easily when cooled.