Understanding Heat Transfer: Phase Change vs. Temperature Change

The question asks for the specific quantity of heat required to convert a unit mass of a substance from a liquid state to a vapour state, crucially without altering its temperature. This process involves a change of state, also known as a phase transition.

Defining Key Thermodynamic Terms

Let's clarify the definitions of the terms provided in the options to understand why the correct answer applies:

• Specific Heat: This is the amount of heat energy required to raise the temperature of a unit mass (e.g., 1 kg) of a substance by one degree Celsius (or one Kelvin). It relates to temperature change, not state change. The formula often involves \( Q = mc\Delta T \), where \( Q \) is heat, \( m \) is mass, \( c \) is specific heat capacity, and \( \Delta T \) is the temperature change.
• Specific Latent Heat: This is defined as the quantity of heat energy absorbed or released by a unit mass of a substance during a change of state (phase transition) that occurs at a constant temperature. This perfectly matches the description given in the question – heat for changing from liquid to vapour at a constant temperature. It is often denoted by \( L \), and the heat involved is Q = mL .
• Thermal Capacity: This refers to the total amount of heat energy required to raise the temperature of an entire object or system by one degree Celsius (or one Kelvin). It depends on both the mass and the specific heat capacity of the substance ( C = mc ). It deals with temperature change, not state change.
• Heat Energy: This is a general term for the energy transferred from one system to another due to a temperature difference. It's the overall concept of thermal energy in transit, not a specific measure for phase change per unit mass.

Identifying the Correct Term

Based on the definitions:

• The question explicitly mentions changing from liquid to vapour, which is a phase change.
• It also specifies without changing the temperature, a key characteristic of phase transitions.
• Furthermore, it refers to the heat needed for a unit mass.

Therefore, the term that accurately describes the quantity of heat needed to change the state of unit mass from liquid to vapour at a constant temperature is specific latent heat. This represents the energy required to overcome the intermolecular forces during vaporization (or released during condensation).