A substance’s physical state is determined by the balance between the kinetic energy of its molecules and the attractive forces holding them together. Matter exists in three states—solid, liquid, and gas—each characterized by a distinct molecular arrangement. A phase change is the physical process where a substance transitions from one state to another, such as ice turning into water or water turning into steam. These transitions occur at specific, characteristic temperatures known as the melting point (MP) and the boiling point (BP).
The Role of Latent Heat in Phase Transitions
When heat is continually added to a substance at its melting point, the temperature stops rising despite the constant application of energy. This added thermal energy does not increase the molecules’ translational kinetic energy, which is measured as temperature. Instead, the energy is converted into potential energy, stored as the substance breaks the strong intermolecular forces maintaining the current phase.
This hidden energy absorbed or released during a phase change at a constant temperature is known as latent heat. For melting, the energy absorbed is the Latent Heat of Fusion, required to disrupt the solid’s ordered structure. For boiling, the energy needed to fully separate the liquid molecules is the Latent Heat of Vaporization. Only after the phase transition is complete will additional heat begin to increase the kinetic energy and, consequently, the temperature of the new phase.
Molecular Dynamics During Melting
In the solid state, molecules are locked into a rigid structure, such as a crystalline lattice, where their motion is restricted to vibration around fixed positions. Strong attractive intermolecular forces hold the molecules in place despite this vibrational motion. As heat is applied, the increasing kinetic energy causes the molecules to vibrate more intensely.
At the melting point, the energy of these vibrations becomes high enough to overcome the attractive forces maintaining the fixed structure. The absorbed Latent Heat of Fusion provides the potential energy necessary to disrupt this long-range order. Once melted, the molecules gain translational freedom, allowing them to slide past one another while remaining closely packed and held by residual attractions.
Molecular Dynamics During Boiling
Liquid molecules possess a greater degree of movement than solids, constantly moving and colliding while still being constrained by intermolecular forces. While some high-energy molecules escape the surface through evaporation, boiling is the phase transition occurring throughout the entire volume of the liquid. Continued heat addition increases the kinetic energy of the liquid molecules, causing them to move faster.
The boiling point is reached when the molecules possess enough kinetic energy to overcome all attractive forces. At this temperature, the internal vapor pressure of the liquid equals the external atmospheric pressure pushing down on the surface. The Latent Heat of Vaporization supplies the energy to completely separate the molecules, allowing them to break free and move independently. This separation results in a gas, which has no fixed volume or shape and expands to fill its container.