What is Specific Latent Heat?

S4-SA1-0598

Grade Level:

Class 6

Space Technology, EVs, Climate Change, Biotechnology, HealthTech, Robotics, Chemistry, Physics

Definition

What is it?

Specific latent heat is the amount of heat energy needed to change the state of 1 kilogram of a substance (like ice to water or water to steam) without changing its temperature. It's the 'hidden' heat that helps a substance melt or boil.

Simple Example

Quick Example

Imagine you are boiling water for chai. Even after the water reaches 100 degrees Celsius, you still need to keep giving it heat for it to turn into steam. This extra heat, which doesn't make the water hotter but changes it to steam, is the specific latent heat.

Worked Example

Step-by-Step

PROBLEM: You have 2 kg of ice at 0 degrees Celsius and want to melt it into water at 0 degrees Celsius. The specific latent heat of fusion for ice is 334,000 Joules per kilogram (J/kg).

STEP 1: Identify the given values.
Mass of ice (m) = 2 kg
Specific latent heat of fusion (L) = 334,000 J/kg

---STEP 2: Recall the formula for calculating heat energy for state change.
Heat energy (Q) = mass (m) × specific latent heat (L)

---STEP 3: Substitute the values into the formula.
Q = 2 kg × 334,000 J/kg

---STEP 4: Calculate the total heat energy required.
Q = 668,000 J

ANSWER: You need 668,000 Joules of heat energy to melt 2 kg of ice into water at 0 degrees Celsius.

Why It Matters

Understanding specific latent heat helps engineers design better cooling systems for EVs and refrigerators, and even predict climate change effects. It's crucial for careers in refrigeration, food processing, and even space technology where temperature control is vital.

Common Mistakes

MISTAKE: Thinking that temperature changes when specific latent heat is added. | CORRECTION: Specific latent heat causes a change in state (solid to liquid, liquid to gas) but the temperature remains constant during this process.

MISTAKE: Confusing specific latent heat with specific heat capacity. | CORRECTION: Specific latent heat is for changing state, while specific heat capacity is for changing temperature of a substance without changing its state.

MISTAKE: Forgetting to include the mass of the substance in calculations. | CORRECTION: The formula Q = mL clearly shows that the amount of heat depends on both the specific latent heat and the mass of the substance.

Practice Questions

Try It Yourself

QUESTION: If the specific latent heat of vaporization of water is 2,260,000 J/kg, how much heat is needed to turn 1 kg of water at 100 degrees Celsius into steam at 100 degrees Celsius? | ANSWER: 2,260,000 J

QUESTION: A substance has a specific latent heat of fusion of 100,000 J/kg. If 50,000 J of heat is supplied, what mass of the substance will melt? | ANSWER: 0.5 kg

QUESTION: You want to melt 0.5 kg of a metal. The specific latent heat of fusion for this metal is 200,000 J/kg. If you have a heater that supplies heat at a rate of 10,000 J per second, how long will it take to melt all the metal? | ANSWER: 10 seconds

MCQ

Quick Quiz

What happens to the temperature of water when it is boiling and turning into steam, assuming heat is continuously supplied?

It increases rapidly

It decreases slightly

It remains constant

It fluctuates up and down

The Correct Answer Is:

During a phase change like boiling, the heat energy supplied is used to break the bonds between molecules, not to increase their kinetic energy, so the temperature remains constant.

Real World Connection

In the Real World

In India, the 'matka' (earthen pot) keeps water cool due to evaporation. The water inside slowly evaporates from the porous surface, absorbing specific latent heat from the remaining water, thus cooling it down. This is why a matka feels cool to touch on the outside.

Key Vocabulary

Key Terms

LATENT HEAT: Hidden heat absorbed or released during a change of state without temperature change | FUSION: The process of a substance changing from a solid to a liquid | VAPORIZATION: The process of a substance changing from a liquid to a gas | KILOGRAM: A unit of mass (kg) | JOULE: A unit of energy (J)

What's Next

What to Learn Next

Next, explore 'Specific Heat Capacity' to understand how much heat is needed to change a substance's temperature. You'll see how these two concepts work together to explain all heat changes!