What is latent heat? The latent heat definition describes the amount of energy that is released or absorbed during a phase change at a constant temperature. A phase change is the change of a substance’s physical state from solid to liquid, liquid to gas, gas to solid, and so on. Matter can exist in three states including solid, liquid, and gas.
Latent heat is caused by overcoming the intermolecular forces that hold the atoms together and dispersing the molecules. By dispersing the solid molecules and increasing the intermolecular space, the solid turns into liquid. Further dispersing the liquid molecules transitions the liquid to gas.
There are a total of four types of latent heats:
- The latent heat of vaporization, which is the energy required to convert a liquid to a vapor at a specific temperature.
- The latent heat of fusion, which is the energy required to transition between the solid and the liquid phases at a specific temperature.
- The latent heat of sublimation, which is the energy needed to convert a specific amount of solid into vapor at a fixed temperature.
- The latent heat of condensation, which is the energy needed to convert a quantity of vapor to liquid at a fixed temperature.
What is latent heat of vaporization?
The latent heat of vaporization is the thermal energy required for a liquid to vaporize to a gas or the amount that is released when a gas condenses to a liquid. It can be defined as the heat required to change one mole of liquid at its boiling point under standard atmospheric pressure. The latent heat of vaporization is also referred to as the enthalpy of vaporization.
It is a fundamental concept in thermodynamics and is expressed in units of energy per unit mass ( joules per kilogram, J/kg) or per mole (joules per mole, J/mol).
When a substance is in its liquid state and you want to convert it into a gas at the same temperature and pressure, you need to supply a certain amount of heat energy to break the intermolecular forces (such as hydrogen bonds or van der Waals forces) that hold the molecules together in the liquid phase. This energy is used to overcome these forces and convert the substance into a gas, with the molecules moving more freely in the gaseous state.
The latent heat of vaporization is usually associated with processes like boiling or evaporation. For example, when you heat water on a stove, the heat energy you supply is used to increase the temperature of the water until it reaches its boiling point (100 degrees Celsius at standard atmospheric pressure). Once the water reaches this temperature, it doesn’t get any hotter; instead, the added heat energy is used to break the hydrogen bonds between water molecules, allowing them to escape into the vapor phase as steam. The latent heat of vaporization for water is approximately 2,257 J/g (joules per gram) at its boiling point.
Latent heat of vaporization has many industrial applications, such as in the operation of steam turbines and refrigeration systems where the phase transition from liquid to gas (or vice versa) is a critical part of the process.
What is latent heat of Fusion?
The latent heat of fusion, also known simply as “latent heat of melting” or enthalpy of fusion is the amount of heat energy required to change a given amount of a substance from a solid to a liquid state at a constant temperature and pressure.
The latent heat of fusion of a substance also accounts for the energy required to accommodate any increase in the volume of the substance post the change of its physical state. The temperature at which the substance undergoes the phase transition is called the melting point of the substance. This temperature point can also be referred to as the freezing point of the substance when the heat of solidification is being considered.
Like the latent heat of vaporization, it is a fundamental concept in thermodynamics and is expressed in units of energy per unit mass (typically joules per kilogram, J/kg) or per mole (joules per mole, J/mol). Unless specified to be otherwise, the pressure of the environment (when expressing the latent heat of fusion of a substance) is always assumed to be 1 atmosphere of pressure (which is roughly equal to 101.325 kilopascals).
When a substance is in its solid state and you want to convert it into a liquid at the same temperature and pressure, you need to supply a certain amount of heat energy to break the intermolecular forces that hold the solid’s molecular structure together. This energy is used to weaken these forces and allow the molecules or atoms in the substance to move more freely, transitioning from a rigid, ordered arrangement in the solid phase to a more disordered, fluid state in the liquid phase.
The latent heat of fusion is often associated with processes like melting. For example, when you heat a block of ice, the heat energy you supply is used to raise the temperature of the ice until it reaches its melting point (0 degrees Celsius at standard atmospheric pressure). Once the ice reaches this temperature, it doesn’t get any hotter; instead, the added heat energy is used to break the bonds between the water molecules in the ice lattice, allowing them to transition into the liquid phase. The latent heat of fusion for water is approximately 334 J/g (joules per gram) at its melting point.
Applications of the concept of latent heat of fusion include phase change heat storage systems, refrigeration and understanding the behavior of materials during freezing and melting processes.
Latent Heat of Fusion vs Latent Heat of Vaporization: Key Differences
Phase Transition
- Latent Heat of Fusion: It is the heat energy required to change a substance from a solid phase to a liquid phase at its melting point, while keeping the temperature constant.
- Latent Heat of Vaporization: It is the heat energy required to change a substance from a liquid phase to a gaseous phase at its boiling point, while maintaining a constant temperature.
Process Direction
- Fusion involves the transition from a solid to a liquid.
- Vaporization involves the transition from a liquid to a gas.
Temperature Change
- During fusion, the temperature remains constant at the melting point.
- During vaporization, the temperature remains constant at the boiling point.
Magnitude of Energy
- Latent Heat of Fusion: It is generally lower than the latent heat of vaporization for the same substance.
- Latent Heat of Vaporization: It is generally higher than the latent heat of fusion for the same substance.
Molecular Movement
- During fusion, the molecules move from a fixed, ordered arrangement in the solid to a more disordered arrangement in the liquid, with increased molecular motion.
- During vaporization, the molecules go from the relatively ordered liquid state to a highly disordered gas state, with even greater molecular motion.
State of Matter
- Fusion involves a change from a solid to a liquid, both of which are condensed phases of matter.
- Vaporization involves a change from a liquid to a gas, with a significant increase in intermolecular spacing.
Importance
- Fusion is important for processes like melting ice, soldering, and phase changes in materials.
- Vaporization is important in processes such as boiling, evaporation, and distillation.
Applications
- Latent Heat of Fusion is utilized in applications like refrigeration systems (e.g., ice production), the food industry (e.g., freezing), and materials processing.
- Latent Heat of Vaporization is used in applications such as steam power generation, cooking, and in cooling systems (e.g., air conditioning).
Key Takeaways
| Basis of Comparison | Latent Heat of Fusion | Latent Heat of Vaporization |
|---|---|---|
| Phase Transition | Solid to Liquid | Liquid to Gas |
| Temperature Change | Constant (Melting Point) | Constant (Boiling Point) |
| Magnitude of Energy | Lower | Higher |
| Molecular Movement | Increased | Highly Increased |
More takeaways
- The latent heat of sublimation is the heat required at a specific temperature to convert a certain amount of solid to vapor directly without passing through the liquid phase. Example: dry ice, which is solid carbon dioxide, sublimates and enters the gas phase without liquifying midway.
- The latent heat of condensation is the amount of heat released when gas particles condense to form liquid droplets at a fixed temperature. Example: water vapor condenses and turns into liquid droplets. This is observed in morning dew.