What remains constant during the melting process of a pure substance?
Show answer & explanation
Correct answer: A
Explanation
Correct Answer: Temperature
Explanation
During the phase transition of a pure substance from a solid to a liquid state—commonly known as the melting process or fusion—the Temperature remains completely constant. This fixed thermal point is recognized as the melting point of that particular substance.
Even though thermal energy is continually supplied to the substance throughout the melting process, the temperature does not rise because the incoming heat is absorbed to change the physical state rather than increasing the kinetic energy of the particles. This absorbed heat energy is called the Latent Heat of Fusion. The energy is entirely consumed to break down the rigid intermolecular forces and attractive bonds holding the atoms or molecules in their fixed solid lattice positions. Only after the entire mass of the pure substance has completely transformed into a liquid will the temperature begin to rise again.
Key Points
Mass remains constant during melting because of the fundamental law of conservation of mass, but temperature is the specific thermodynamic property that uniquely characterizes the constant phase change platform of a pure substance under steady conditions.
Volume generally changes when a solid melts into a liquid; most substances expand upon melting, though special substances like ice contract, meaning volume cannot be considered constant.
Pressure is an external environmental condition rather than an intrinsic property of the substance during its physical transition, and the melting point itself shifts if the surrounding pressure is altered.
Important Questions and Answers
Question: What is the specific term used for the thermal energy absorbed by a substance during its phase change that does not cause a rise in temperature?
Answer: It is called latent heat, and specifically 'latent heat of fusion' in the case of melting.
Question: Why does the temperature of an ice-water mixture remain at 0°C even when kept at room temperature until all the ice melts?
Answer: All the heat absorbed from the surroundings is actively utilized to overcome the intermolecular forces holding the ice molecules together rather than raising the temperature of the mixture.