Melting, also known as fusion, is the physical process where a substance transitions from a solid state to a liquid state. This change occurs when the solid absorbs energy, usually supplied as heat, causing its internal energy to increase. When the solid reaches its melting point, the absorbed energy is sufficient to begin the transformation into a less ordered liquid. The solid must continuously absorb energy from its surroundings for melting to proceed.
The Mechanism of Phase Transition
Melting begins when thermal energy is transferred to the solid, increasing the kinetic energy of the molecules. This causes them to vibrate more rapidly within their fixed positions. Once the melting point is reached, the molecules acquire enough energy to overcome the attractive forces holding the rigid solid structure together. This energy breaks the fixed bonds, allowing the molecules to move freely past one another as a liquid.
The energy required to complete this transformation without raising the temperature is known as the Latent Heat of Fusion. For ice, this value is approximately 334 Joules per gram at \(0^\circ\text{C}\). This energy input is necessary to dismantle the organized crystal structure and convert the solid into liquid water. Once the entire mass has converted, any further absorbed energy will begin to raise the temperature of the water.
Why Ice is Unique
Water exhibits unusual behavior when it freezes and melts, distinguishing it from most other substances. This uniqueness stems from the strong hydrogen bonds that form between water molecules. When liquid water cools to form ice, these bonds force the molecules into a rigid, open, crystalline lattice structure. This arrangement spaces the molecules further apart than they are in the liquid state.
Because of the open lattice structure, a given mass of ice occupies a larger volume than the same mass of liquid water. Consequently, ice is less dense than liquid water, a property known as the density anomaly. This low density allows ice to float, which has implications for aquatic life and climate.
External Factors That Change the Melting Point
The standard melting point of ice at \(0^\circ\text{C}\) can be altered by external conditions, primarily the introduction of solutes or changes in pressure. Adding impurities, such as salt, causes freezing point depression. The dissolved ions interfere with the ability of water molecules to form the ordered hydrogen-bonded lattice required for a solid state. This disruption means a lower temperature is required for the water to solidify, causing existing ice to melt even when the air temperature is below \(0^\circ\text{C}\).
This principle is applied in winter when salts are spread on roads to prevent ice formation or melt existing layers. The effectiveness of a de-icing agent relates directly to the number of particles it releases into the water. An increase in external pressure is the second factor that lowers the melting point of ice. This occurs because liquid water takes up less space than solid ice, and increasing pressure favors the state with the smaller volume. This effect plays a role in natural processes, such as at the base of glaciers where the weight of the ice sheet can cause melting.