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What is the Latent Heat of Fusion?
Grade Level:
Class 8
Space Technology, EVs, Climate Change, Biotechnology, HealthTech, Robotics, Chemistry, Physics
Definition
What is it?
The Latent Heat of Fusion is the amount of heat energy required to change 1 kilogram of a substance from its solid state to its liquid state at its melting point, without any change in temperature. This 'hidden' heat energy breaks the bonds holding the solid structure together.
Simple Example
Quick Example
Imagine you have a block of ice at 0°C. You start heating it. Even though you are adding heat, the temperature of the ice will stay at 0°C until all of it has melted into water. The extra heat you added during this melting phase, which didn't raise the temperature, is the Latent Heat of Fusion.
Worked Example
Step-by-Step
PROBLEM: How much heat energy is needed to melt 2 kg of ice at 0°C into water at 0°C? (Given: Latent Heat of Fusion of ice = 334,000 J/kg)
STEP 1: Identify the given values.
Mass of ice (m) = 2 kg
Latent Heat of Fusion of ice (L_f) = 334,000 J/kg
---STEP 2: Recall the formula for heat absorbed during fusion.
Heat energy (Q) = mass (m) × Latent Heat of Fusion (L_f)
---STEP 3: Substitute the values into the formula.
Q = 2 kg × 334,000 J/kg
---STEP 4: Calculate the total heat energy.
Q = 668,000 J
ANSWER: 668,000 Joules of heat energy are needed.
Why It Matters
Understanding latent heat is crucial for designing efficient refrigeration systems in your home or in cold storage for food. It's also vital in space technology for managing extreme temperatures in spacecraft and in biotechnology for preserving sensitive materials. Engineers and scientists use this concept daily.
Common Mistakes
MISTAKE: Thinking that temperature changes during melting. | CORRECTION: During the phase change from solid to liquid, the temperature remains constant at the melting point. The added heat energy is used to change the state, not to increase temperature.
MISTAKE: Confusing Latent Heat of Fusion with Specific Heat Capacity. | CORRECTION: Latent Heat of Fusion is about changing state (solid to liquid) at constant temperature. Specific Heat Capacity is about changing temperature of a substance without changing its state.
MISTAKE: Forgetting the units or using incorrect units. | CORRECTION: Latent Heat of Fusion is typically measured in Joules per kilogram (J/kg) or calories per gram (cal/g). Always ensure your units are consistent in calculations.
Practice Questions
Try It Yourself
QUESTION: If the Latent Heat of Fusion of a metal is 200,000 J/kg, how much heat is needed to melt 0.5 kg of it at its melting point? | ANSWER: 100,000 J
QUESTION: 167,000 J of heat energy is supplied to completely melt a block of ice at 0°C. If the Latent Heat of Fusion of ice is 334,000 J/kg, what was the mass of the ice block? | ANSWER: 0.5 kg
QUESTION: You have 1 kg of ice at -5°C. The specific heat capacity of ice is 2100 J/kg°C and the Latent Heat of Fusion of ice is 334,000 J/kg. How much total heat is required to convert this ice to water at 0°C? (Hint: First raise the temperature of ice to 0°C, then melt it.) | ANSWER: 344,500 J
MCQ
Quick Quiz
What happens to the temperature of a substance while it is undergoing fusion (melting) at its melting point?
It increases rapidly
It decreases slightly
It remains constant
It fluctuates unpredictably
The Correct Answer Is:
C
During fusion, the heat energy absorbed is used to break the bonds between particles to change the state from solid to liquid, not to increase the kinetic energy of particles, so the temperature remains constant.
Real World Connection
In the Real World
In India, during summer, street vendors often keep large blocks of ice in their carts to keep cold drinks and fruits chilled. The ice slowly melts, absorbing a large amount of heat from the surroundings (the latent heat of fusion), effectively keeping the items cool for a long time without needing electricity. This is a common, practical application of latent heat.
Key Vocabulary
Key Terms
FUSION: The process of changing from a solid to a liquid state | MELTING POINT: The specific temperature at which a solid turns into a liquid | LATENT HEAT: 'Hidden' heat energy absorbed or released during a phase change without temperature change | JOULE: The SI unit of energy | KILOGRAM: The SI unit of mass
What's Next
What to Learn Next
Great job understanding Latent Heat of Fusion! Next, you should explore 'Latent Heat of Vaporization'. This concept is similar but deals with the energy needed to change a liquid into a gas, which is equally important in understanding how pressure cookers work or how clouds form.
