The specific latent heat of vaporization of a substance is the quantity of heat needed to change unit mass from

Question

Prepp Team · Accepted Answer

Understanding Specific Latent Heat of Vaporization

The specific latent heat of vaporization is a fundamental concept in thermodynamics that describes the amount of heat energy required to change the state of a substance from a liquid to a gas (vapour) at a constant temperature and pressure. It's 'specific' because it's defined per unit mass of the substance.

Defining Specific Latent Heat of Vaporization

When a substance absorbs heat energy, its temperature usually rises. However, during a phase transition, such as boiling (liquid to vapour) or melting (solid to liquid), the absorbed heat energy is used to break the bonds between molecules rather than increasing their kinetic energy. This energy absorbed or released during a phase change at a constant temperature is called latent heat.

Specifically, the latent heat of vaporization refers to the heat needed to convert a substance from liquid to vapour. The specific latent heat of vaporization ($L_v$) is the quantity of heat ($\Delta Q$) required per unit mass ($m$) to achieve this phase change.

Mathematically, this is expressed as:

$\qquad L_v = \frac{\Delta Q}{m}$

or

$\qquad \Delta Q = m L_v$

The unit of specific latent heat of vaporization is typically Joules per kilogram (J/kg) or calories per gram (cal/g).

Phase Change: Liquid to Vapour

Vaporization, or boiling, is the process where a liquid turns into a gas. This occurs at a specific temperature for a given pressure, known as the boiling point. When a liquid is heated to its boiling point, adding more heat energy does not increase the temperature of the liquid or the vapour formed. Instead, this energy is used to overcome the intermolecular forces holding the liquid molecules together, allowing them to escape into the gaseous phase.

Therefore, the transition from liquid to vapour at the boiling point happens without a change in temperature. The heat energy added during this process is the latent heat of vaporization.

Analyzing the Options

Let's examine the given options based on the definition of specific latent heat of vaporization:

Option 1: liquid to vapour with a change of temperature
- This is incorrect. The phase change from liquid to vapour (vaporization) at the boiling point occurs at a constant temperature. Heat added before the boiling point increases temperature, and heat added after all liquid is vaporized increases vapour temperature, but the phase change itself is isothermal.
Option 2: liquid to vapour without a change of temperature
- This aligns perfectly with the definition of specific latent heat of vaporization. It is the heat required to convert a unit mass of substance from liquid to vapour state at its boiling point, where the temperature remains constant.
Option 3: vapour to liquid without a change of temperature
- This describes the process of condensation (vapour to liquid). While this phase change also occurs at a constant temperature (the boiling/condensation point), the specific latent heat of vaporization specifically refers to the energy needed for vaporization (liquid to vapour), not condensation (vapour to liquid). The magnitude of heat exchanged is the same, but the direction is opposite (released during condensation, absorbed during vaporization). The question asks about vaporization.
Option 4: vapour to liquid with a change of temperature
- This is incorrect. Condensation (vapour to liquid) at the condensation point occurs at a constant temperature. A change in temperature would happen before the vapour reaches the condensation point or after it has completely turned into liquid and continues to cool.

Based on the analysis, the specific latent heat of vaporization is the quantity of heat needed to change unit mass from liquid to vapour without a change of temperature.

Summary of Phase Transitions and Latent Heat
Phase Change	Process	Temperature Change	Heat Exchange
Solid to Liquid	Melting/Fusion	No change (at melting point)	Heat absorbed (Latent heat of fusion)
Liquid to Solid	Freezing/Solidification	No change (at freezing point)	Heat released
Liquid to Vapour	Vaporization/Boiling	No change (at boiling point)	Heat absorbed (Latent heat of vaporization)
Vapour to Liquid	Condensation	No change (at condensation point)	Heat released
Solid to Vapour	Sublimation	No change (at sublimation temp)	Heat absorbed (Latent heat of sublimation)
Vapour to Solid	Deposition	No change (at deposition temp)	Heat released

Revision Table: Key Definitions

Thermodynamic Definitions
Term	Definition
Specific Heat Capacity (c)	Heat required to raise the temperature of unit mass of a substance by $1^\circ$C (or 1 K) with a change in temperature. Unit: J/kg°C or J/kg K.
Latent Heat	Heat energy absorbed or released during a phase change at constant temperature.
Specific Latent Heat (L)	Heat energy absorbed or released per unit mass during a phase change at constant temperature. Unit: J/kg.
Specific Latent Heat of Fusion ($L_f$)	Heat required per unit mass to change a substance from solid to liquid at its melting point.
Specific Latent Heat of Vaporization ($L_v$)	Heat required per unit mass to change a substance from liquid to vapour at its boiling point.

Additional Information: Related Concepts

Understanding specific latent heat is crucial for studying phase transitions and energy transfer. Here are a few related points:

Energy during phase change: The energy supplied as latent heat is used to change the potential energy of the molecules as they move further apart (in vaporization) or closer together (in condensation), overcoming the intermolecular forces. The kinetic energy of the molecules, which is related to temperature, does not change during the phase transition itself.
Boiling vs. Evaporation: Both are processes of vaporization. Boiling occurs at a specific temperature (boiling point) throughout the bulk of the liquid. Evaporation can occur at any temperature below the boiling point and happens only at the surface of the liquid. Latent heat of vaporization applies primarily to boiling.
Pressure dependence: The boiling point of a substance depends on the external pressure. Consequently, the specific latent heat of vaporization also varies slightly with pressure.
Condensation: The process of condensation (vapour to liquid) releases the same amount of heat energy per unit mass as the specific latent heat of vaporization. This is why steam burns are often more severe than burns from boiling water at the same temperature – the steam releases its large latent heat upon condensing on the skin.

In summary, the specific latent heat of vaporization quantifies the energy needed for a specific mass of a substance to transition from liquid to gas, a process that uniquely occurs without a change in temperature at the boiling point.