Latent Heat of Fusion - Definition, Explanation, Formula, Examples, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:45 PM IST

Introduction - Latent Heat of Fusion

In 1761 Joseph Black deduced the application of heat to ice. He found that there is no rise in the temperature of ice/water, but on the other hand, there is an increase in the amount of water in that mixture. Black also observed that there is no increase in heat to boiling water but an increase in the amount of steam can be observed, so he observed that the heat which is supplied, is combined with the particles of ice and to boiling water and become latent in nature, this marks the beginning of thermodynamics related to the theory of latent heat. In this section first, we discuss latent heat than the latent heat of fusion. Without changing the temperature but a change in its physical state by the release or absorption of energy is so called Latent heat.

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Introduction - Latent Heat of Fusion
What is Heat of Fusion?
Define Latent Heat of Fusion.
Explain Latent Heat of Fusion.
Define Specific Heat of Fusion.
Latent Heat of Fusion Examples:
Define Enthalpy of Fusion:
Heat of Solidification:
Latent heat of Sublimation Definition:

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What is Heat of Fusion?

The latent heat of fusion is basically change in enthalpy observed in the melting of solid and freezing of liquid.

Define Latent Heat of Fusion.

Defining the latent heat in scientific terms we can write the definition as the amount of heat released or absorbed per mole in the units of joule or calories while undergoing a change in its state is called the latent heat of fusion. Materialistically we can define it as to overcome the forces that hold up the atoms or molecules together in any materials such required work is termed as Latent heat of fusion. For converting the physical state of any solid material, an amount of energy must be supplied to trigger the change in enthalpy of fusion (the latent heat of fusion). The enthalpy of fusion or latent heat of fusion occurs in constant environmental conditions.

Explain Latent Heat of Fusion.

An increase in the amount of substance which is by volume of the substance is the latent heat of fusion. A phase transition that occurs at this point is due to temperature and is termed the melting point of a substance. Change in phase can occur due to temperature rise or fall, falling of temperature may result in solidification of a substance which is termed as the heat of solidification. The heat of solidification is the opposite of the latent heat of fusion. In the whole process of latent heat of fusion, the atmospheric pressure is considered to be 1.

Generating the vibrations inside the object is the effect of heat. The heat is considered as raised entropy of particles or kinetic energy. Heat always rises but in the case of temperature, high and low can be possible. Latent heat of fusion is different for every state of matter that is solid, liquid and gas. It should be kept in mind the fact that temperature remains constant, additionally, we can add or subtract the heat (latent heat) in an object during the phase change.

Formula for Latent Heat of Fusion will be as Follows: Latent heat of fusion is considered to be written as Q, m will be the mass and latent heat can be denoted as therefore the formula is:

∆Q=mL

Which is the general required form of the equation for the latent heat of fusion. Here the symbol delta Q is used for heat energy in joules, The formula is given accordingly as, when m kg of mass of any solid changes to liquid at its constant temperature and which is at its melting point, then the heat absorbed by the solid is given by the above mentioned equation. In some cases, it can be seen that the temperature varies where we have to take two temperatures i.e. lower temperature and higher temperature for concluding the formula of latent heat of fusion. So heat gain or release, in that case, will be as follows:

∆Q=mC×∆t

∆Q=mC×(t₂-t₁)

Here the two temperatures are higher and lower temperatures, Total of the latent heat of fusion energies gain or release can be represented:

Q=mL+m×C×∆t

The term latent heat of fusion can also be used as specific latent heat of fusion.

Define Specific Heat of Fusion.

Specific latent heat of fusion can be defined as the amount of heat that is being required to change the state of substance by unit mass and it can be represented as

L= Q/m

Which can be written as Q=mL

Latent heat of fusion of water or specific latent heat of water is the same as 334 joules per gram.

Temperature vs Energy

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Latent Heat of Fusion Examples:

If in case 64500 calories of heat are extracted from the given mass of 100g of steam at a temperature of 100℃.Given that the latent heat of fusion of steam and ice are 80 calories per gram and 540 calories per gram respectively. Find the amount of water converted into ice.

The temperature will definitely fall from 100℃ to 10℃, when the full steam is converted into water.

A part of the water is also converting into water at 0℃.

The heat gain by water at 100℃ for conversion of steam at 100℃

C=m×L

C=100×540cal

=54000cal.

Now heat gain by water at 100℃ for conversion into the water at 0℃.

C=100×1×(100-0)

=10000cal

Therefore, the extracted heat =54000cal+ 10000cal=64000cal.

The remaining heat from the given heat will be =64500cal-640000=500cal.

Now, as according to the question the amount of water that is converted to ice is

m=Q/L_ice

m=500/80

m=6.25g.

Hence, we concluded the result that the amount of water that is converted into ice is 6.25g.

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Define Enthalpy of Fusion:

As we discussed earlier in the section that fusion generally means melting, changing of solid to liquid requires the energy or heat at its melting point that is called Heat of fusion. But the opposite of this is different as removal or taken away the energy or heat from something results in freezing. The heat of fusion is simply measured in energy terms as in joules per gram or calories per gram. It is to be noted that the heat of fusion is not at all the same for all the substances, but having constant value for each individual substance. The heat of fusion can also be represented in terms of enthalpy of fusion both are the same.

Formula for concluding heat of fusion can be written as follows:

∆H_f=q/m

Here, ∆H_f sows the heat of fusion, q is heat joule and m is mass in kg.

Enthalpy of fusion of ice when heat energy is supplied to one kilogram of water for the conversion of water to one kilogram of ice with no change in the temperature of the environment which means at zero degree Celsius is 333.55kilojoules.

Energy vs Temperature

Heat of Solidification:

Earlier we were discussing latent heat of fusion, but the opposite of latent heat of fusion is the heat of solidification. In order to change the phase from liquid to solid, the amount of energy required is called the heat of solidification. It is important to note that the value of latent heat of fusion and heat of solidification is the same in magnitude but differs in sign.

The temperature point at which the freezing is started is termed the heat of solidification. Example of solidification, the energy required to convert the water into ice is the same as the energy required to convert ice into the water but of different signs. The latent heat of fusion is commonly positive except in the case of Helium which means the heat of solidification generally comes out to be negative.

Solidification is always an exothermic process, the heat released during the process.

NCERT Chemistry Notes:

Latent heat of Sublimation Definition:

This is simply defined as the amount of heat or enthalpy that must be added to a solid of one mole at a constant pressure to convert directly into gas with no change in the phase of liquid. All the forces, ions, or molecules are broken down and it comes in the state of the gas.

The unit of latent heat of sublimation can be given in joules per mole or kilogram of substance and represented as ∆H_s.

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Frequently Asked Questions (FAQs)

1. Write down the examples of solidification?

Hardening of lava into solid rock.
Snow formation
Melted candle into the wax

2. What is fusion in matter?

This is the process where we get the power for the planets like the Sun and the stars. The reaction occurs between two atoms of hydrogen when combined together to form an atom called helium. Here in the process of fusion some of the mass of hydrogen is converted into energy. The process requires higher temperature as when they are heated to the maximum they get ionized and form plasma. The sun and the stars can do this by gravity. It is an inexhaustible source of energy.

3. What are the examples of latent heat?

The examples of latent heat are as follows:

Heat of vaporization
Heat of freezing

4. The process that is opposite to latent heat of fusion is?

The heat of solidification is considered as the opposite of it and also the sign conventions are used differently in each but are the same in magnitude.

5. What is the latent heat of fusion?

The change in enthalpy observed in the melting of solid and freezing of liquid.

6. What is the latent heat of fusion?

The latent heat of fusion is the amount of heat energy required to change a substance from a solid to a liquid state without changing its temperature. It represents the energy needed to overcome the intermolecular forces holding the solid structure together.

7. Why do some substances have higher latent heats of fusion than others?

Substances with higher latent heats of fusion typically have stronger intermolecular forces. More energy is required to overcome these forces and separate the particles from their solid structure, resulting in a higher latent heat of fusion.

8. What is the relationship between latent heat of fusion and melting point?

There is often a correlation between latent heat of fusion and melting point. Substances with higher melting points tend to have higher latent heats of fusion because stronger intermolecular forces typically require more energy to overcome during melting.

9. What is the relationship between entropy and latent heat of fusion?

Latent heat of fusion is directly related to the change in entropy during melting. The energy absorbed as latent heat increases the disorder of the system as it transitions from an ordered solid to a less ordered liquid state, resulting in an increase in entropy.

10. How does pressure affect the latent heat of fusion?

Pressure generally has a small effect on the latent heat of fusion for most substances. However, increased pressure can slightly decrease the latent heat of fusion by bringing molecules closer together, reducing the energy needed to break intermolecular bonds.

11. What is the significance of latent heat of fusion in the food industry?

In the food industry, latent heat of fusion is important for freezing and thawing processes. It affects freezing times, energy requirements for freezing and thawing, and can impact food texture and quality. Understanding this concept is crucial for designing efficient freezing systems and maintaining food quality.

12. What is the relationship between latent heat of fusion and the crystal structure of a solid?

The crystal structure of a solid is closely related to its latent heat of fusion. More tightly packed and ordered crystal structures generally require more energy to break apart during melting, resulting in higher latent heats of fusion. This relationship helps explain differences in melting behavior among various materials.

13. Why is the latent heat of fusion of helium important in cryogenics?

Helium's extremely low latent heat of fusion is crucial in cryogenics. It allows helium to remain liquid at very low temperatures, making it valuable as a coolant in superconducting magnets and other low-temperature applications.

14. How does latent heat of fusion impact the formation of icebergs?

The high latent heat of fusion of water affects iceberg formation by slowing the freezing process of seawater. As water freezes, it releases latent heat, warming the surrounding water and slowing further ice formation. This process influences the rate of iceberg growth and the dynamics of polar ice sheets.

15. How does latent heat of fusion contribute to the formation of hail in thunderstorms?

In hail formation, latent heat of fusion plays a crucial role. As water freezes on the surface of a hailstone, it releases latent heat, which can keep the hailstone's surface near 0°C even in the cold upper atmosphere. This process allows the hailstone to accumulate layers of ice and grow larger.

16. How is latent heat of fusion different from specific heat capacity?

Latent heat of fusion involves a phase change at constant temperature, while specific heat capacity relates to temperature change within a single phase. Latent heat is the energy required to change state, whereas specific heat is the energy needed to raise the temperature of a substance by one degree.

17. What is the formula for calculating latent heat of fusion?

The formula for latent heat of fusion is Q = mL, where Q is the heat energy absorbed or released, m is the mass of the substance, and L is the specific latent heat of fusion for that substance.

18. How does latent heat of fusion contribute to the effectiveness of salt in deicing roads?

Salt lowers the freezing point of water, creating a brine solution. As this solution melts ice, it absorbs latent heat of fusion from its surroundings, including the remaining ice and road surface. This process accelerates ice melting and helps keep roads clear in winter.

19. What is the significance of latent heat of fusion in the production of artificial snow?

In artificial snow production, understanding latent heat of fusion is crucial. As water droplets freeze to form snow particles, they release latent heat. This heat must be removed quickly to ensure efficient snow formation, influencing the design and operation of snow-making equipment.

20. How does latent heat of fusion affect the process of soldering in electronics?

In soldering, the latent heat of fusion of the solder material determines the energy required to melt it. This affects the temperature and duration needed for effective soldering, influencing the design of soldering equipment and processes to ensure proper joint formation without damaging sensitive electronic components.

21. Why does temperature remain constant during melting despite heat being added?

Temperature remains constant during melting because the added heat energy is used to break the intermolecular bonds in the solid, rather than increasing the kinetic energy of the particles. This energy is absorbed as latent heat, maintaining a constant temperature until the phase change is complete.

22. How does the molecular structure of a substance influence its latent heat of fusion?

The molecular structure affects latent heat of fusion through the strength and type of intermolecular forces. Substances with stronger or more numerous intermolecular bonds (like hydrogen bonding or metallic bonding) generally have higher latent heats of fusion.

23. Why is the latent heat of fusion of water higher than that of most other substances?

Water's latent heat of fusion is unusually high due to its strong hydrogen bonding. These bonds require significant energy to break during melting, resulting in a higher latent heat compared to substances of similar molecular weight that lack hydrogen bonding.

24. How does the latent heat of fusion affect the formation of frost?

When water vapor in the air deposits directly as solid frost on surfaces, it releases latent heat of fusion. This process can slightly warm the surrounding air and surface, affecting local temperature and humidity conditions.

25. What is the role of latent heat of fusion in the formation of magma in the Earth's crust?

Latent heat of fusion plays a significant role in magma formation. As rocks melt, they absorb latent heat, which affects the temperature and rate of melting. This process influences the composition and behavior of magma, impacting volcanic activity and the formation of igneous rocks.

26. Can latent heat of fusion be negative?

No, latent heat of fusion cannot be negative. It always represents the energy absorbed by a substance to change from solid to liquid. The reverse process (freezing) releases the same amount of energy, but it's referred to as the latent heat of solidification.

27. How does the latent heat of fusion affect the freezing of lakes in winter?

The high latent heat of fusion of water slows down the freezing process of lakes. As the surface water freezes, it releases heat, warming the water below. This process, combined with water's maximum density at 4°C, helps prevent lakes from freezing solid and protects aquatic life.

28. How does the concept of latent heat of fusion apply to the formation of snowflakes?

When water vapor in clouds condenses and freezes to form snowflakes, it releases latent heat of fusion. This release of energy can contribute to atmospheric dynamics and cloud formation processes, influencing weather patterns.

29. What role does latent heat of fusion play in the process of zone refining?

In zone refining, latent heat of fusion is crucial for purifying materials. As a narrow molten zone moves through a solid rod, impurities tend to concentrate in the liquid phase due to differences in solubility. The release and absorption of latent heat during melting and solidification drive this purification process.

30. Why is understanding latent heat of fusion important in metallurgy?

In metallurgy, understanding latent heat of fusion is crucial for processes like casting and welding. It determines the energy required to melt metals, affects cooling rates, and influences the final structure and properties of metal products.

31. Why is the concept of latent heat of fusion important in understanding glacial movements?

Latent heat of fusion plays a role in glacial movements through the process of regelation. As ice melts under pressure at the base of a glacier, it absorbs latent heat. When the meltwater moves to areas of lower pressure and refreezes, it releases this heat, contributing to the plastic flow of glaciers.

32. How does latent heat of fusion affect the process of metal casting?

In metal casting, latent heat of fusion determines the amount of heat that must be removed for the molten metal to solidify. This affects cooling rates, solidification patterns, and the final microstructure of the cast product, influencing its mechanical properties and quality.

33. What is the significance of latent heat of fusion in the process of zone melting for purifying semiconductors?

In zone melting, latent heat of fusion is crucial for the purification process. As a narrow molten zone moves through a solid rod, impurities tend to concentrate in the liquid phase. The absorption and release of latent heat during melting and solidification drive this process, allowing for effective purification of semiconductor materials.

34. How does latent heat of fusion affect the formation of sea ice in polar regions?

The high latent heat of fusion of water slows the formation of sea ice. As seawater begins to freeze, it releases latent heat, warming the surrounding water and slowing further ice formation. This process influences the rate and extent of sea ice formation, affecting polar climates and ocean circulation patterns.

35. Why is understanding latent heat of fusion important in the design of thermal energy storage systems for solar power plants?

In solar power plants, thermal energy storage systems often use materials with high latent heats of fusion. These materials can store large amounts of energy as they melt during the day and release it as they solidify at night, allowing for continuous power generation. Understanding latent heat is crucial for optimizing these systems.

36. Why is the latent heat of fusion of water important for life?

Water's high latent heat of fusion is vital for life because it helps stabilize temperatures in aquatic environments and on Earth's surface. It prevents rapid freezing or melting, protecting organisms from sudden temperature changes and maintaining liquid water availability in various ecosystems.

37. How does latent heat of fusion affect the process of making ice cream?

In ice cream making, the latent heat of fusion of water plays a crucial role. As the ice cream mixture freezes, it absorbs heat from its surroundings (including the other ingredients). This process helps create small ice crystals, contributing to the smooth texture of ice cream.

38. How does latent heat of fusion contribute to climate regulation?

Latent heat of fusion plays a crucial role in climate regulation through the melting and freezing of water. Large amounts of energy are absorbed when ice melts (cooling the environment) and released when water freezes (warming the environment), helping to moderate temperature changes in regions with significant ice cover.

39. How does hydrogen bonding affect the latent heat of fusion of water?

Hydrogen bonding significantly increases water's latent heat of fusion. These strong intermolecular forces require more energy to break during melting, resulting in a higher latent heat of fusion compared to substances of similar molecular weight without hydrogen bonding.

40. Why is latent heat of fusion important in the design of thermal energy storage systems?

Latent heat of fusion is crucial in thermal energy storage systems because it allows for storing large amounts of energy in a relatively small volume without significant temperature change. This property is utilized in phase change materials for efficient energy storage and release.

41. How does latent heat of fusion affect the process of freeze-drying?

In freeze-drying, the latent heat of fusion is crucial. The process involves freezing the material and then sublimating the ice directly to vapor under low pressure. The latent heat of fusion determines the energy required for the initial freezing step, affecting the overall energy requirements of the process.

42. Why is understanding latent heat of fusion important in the design of heat exchangers?

In heat exchanger design, especially for systems involving phase changes, latent heat of fusion is crucial. It affects the energy transfer rates, the size and efficiency of the heat exchanger, and the overall performance of systems like refrigeration units or industrial cooling processes.

43. What is the difference between latent heat of fusion and latent heat of vaporization?

Latent heat of fusion is the energy required to change a substance from solid to liquid at its melting point, while latent heat of vaporization is the energy needed to change from liquid to gas at its boiling point. Typically, the latent heat of vaporization is higher due to the greater separation of particles in the gas phase.

44. How does latent heat of fusion contribute to the energy balance of the Earth's climate system?

Latent heat of fusion is a significant component of Earth's energy balance, particularly in polar regions. The melting and freezing of ice and snow involve large transfers of latent heat, affecting atmospheric and oceanic circulation patterns and playing a crucial role in global climate regulation.

45. Why is understanding latent heat of fusion important in the design of phase change materials for thermal management?

Latent heat of fusion is a key property in designing phase change materials (PCMs) for thermal management. PCMs with high latent heats can store or release large amounts of thermal energy at a constant temperature, making them effective for applications like temperature regulation in buildings or electronic devices.

46. How can latent heat of fusion be used to verify the purity of a substance?

The latent heat of fusion of a pure substance is constant at a given pressure. By measuring the heat absorbed during melting and comparing it to the known value for a pure sample, one can assess the purity of a substance. Impurities typically lower the latent heat of fusion.

47. How does latent heat of fusion relate to the concept of supercooling?

Supercooling occurs when a liquid is cooled below its freezing point without solidifying. When crystallization finally occurs in a supercooled liquid, the latent heat of fusion is released suddenly, causing a rapid temperature increase to the normal freezing point.

48. How does latent heat of fusion contribute to the effectiveness of ice packs in first aid?

Ice packs utilize the high latent heat of fusion of water. As the ice melts, it absorbs a large amount of heat from the surrounding area (like an injured body part) without changing temperature. This property makes ice packs effective for reducing swelling and providing consistent cooling in first aid applications.

49. What role does latent heat of fusion play in the formation of stalactites and stalagmites in caves?

Latent heat of fusion affects the formation of stalactites and stalagmites through the process of freezing and thawing of water in cave systems. The release of latent heat during freezing can influence local temperature conditions, affecting the rate of mineral deposition and the growth patterns of these formations.

50. How does the concept of latent heat of fusion apply to the design of heat pipes for electronic cooling?

In heat pipes, the latent heat of fusion of the working fluid is crucial. The fluid absorbs heat and vaporizes at one end of the pipe, then condenses and releases latent heat at the other end. This process allows for efficient heat transfer in electronic cooling applications without the need for external power.

51. Why is latent heat of fusion important in understanding the behavior of clathrate hydrates in deep-sea environments?

Latent heat of fusion is significant in the formation and dissociation of clathrate hydrates, ice-like structures that trap gas molecules. The energy absorbed or released during phase changes affects the stability of these hydrates, which is important for understanding their role in marine ecosystems and potential as energy sources.

52. How does latent heat of fusion affect the process of freeze concentration in the food industry?

In freeze concentration, latent heat of fusion is key to the separation process. As a solution is cooled, pure water freezes first, releasing latent heat. This heat must be removed efficiently to continue the concentration process. Understanding this helps in designing effective freeze concentration systems for food processing.

53. What is the significance of latent heat of fusion in the study of planetary ice moons?

Latent heat of fusion is crucial in understanding the internal structure and dynamics of icy moons. It affects processes like subsurface ocean formation, cryovolcanism, and tectonic activity on these bodies. The energy involved in melting or freezing large volumes of ice influences the thermal evolution and potential habitability of these moons.

54. How does latent heat of fusion contribute to the effectiveness of PCR (Polymerase Chain Reaction) in molecular biology?

In PCR, precise temperature control is crucial. The latent heat of fusion of water in the reaction mixture helps maintain stable temperatures during the melting and annealing steps of DNA amplification. This property assists in achieving the rapid and accurate temperature changes required for successful PCR reactions.

55. Why is understanding latent heat of fusion important in the development of new refrigerants?

Latent heat of fusion is a key consideration in developing new refrigerants.