How is the melting point of a substance defined?
The correct answer and explanation is:
Correct Answer:
The melting point of a substance is defined as the temperature at which the solid and liquid phases of that substance coexist in equilibrium under a given pressure, usually standard atmospheric pressure.
Explanation:
The melting point is an important physical property used to characterize and identify a substance. It marks the specific temperature at which a solid becomes a liquid. At this temperature, the forces holding the particles in a fixed, orderly structure weaken enough to allow the particles to move freely. This process requires energy to overcome the intermolecular forces that keep the particles locked in place. For pure crystalline solids, the melting point is sharp and occurs at a definite temperature. This means the entire sample melts at the same temperature under constant pressure. In contrast, mixtures and impure substances often melt over a range of temperatures because different components have different melting characteristics and impurities disrupt the orderly crystal lattice.
The melting point can also indicate the purity of a substance. A pure substance has a narrow and well-defined melting point range, while the presence of impurities usually lowers the melting point and broadens the temperature range over which melting occurs. For this reason, melting point determination is widely used in laboratories to check purity and confirm the identity of chemical compounds.
Pressure also affects the melting point. Under higher pressures, the melting point of most substances increases because the particles are forced closer together, making it harder for the solid structure to break apart. However, some substances like ice behave differently because their solid phase is less dense than the liquid phase, so increasing the pressure can lower the melting point.
Overall, the melting point provides valuable information for chemists and material scientists, helping them understand a substance’s thermal stability, purity, and physical characteristics.