Latent heat of vaporization is a key concept in thermodynamics and phase changes. It explains how a liquid turns into a gas without a temperature change when heat is applied. This principle is crucial in everyday applications, from boiling water to industrial steam production.
In this topic, we will explore what latent heat of vaporization is, its formula, real-life applications, and important questions that help students understand this essential topic.
What is Latent Heat of Vaporization?
Latent heat of vaporization is the amount of heat energy required to convert 1 kilogram of a liquid into a gas at its boiling point without changing its temperature.
For example, when water boils at 100°C, it absorbs heat but remains at 100°C until all of it has turned into steam.
Formula for Latent Heat of Vaporization
The latent heat of vaporization ( L_v ) is calculated using the equation:
Where:
- Q = Heat energy absorbed or released (Joules)
- m = Mass of the substance (kg)
- L_v = Specific latent heat of vaporization (J/kg)
The SI unit of latent heat of vaporization is Joules per kilogram (J/kg).
Examples of Latent Heat of Vaporization
- Boiling Water into Steam
- The latent heat of vaporization of water is 2260 kJ/kg.
- This means 2,260,000 Joules of heat is required to convert 1 kg of water into steam at 100°C.
- Evaporation of Sweat
- When sweat evaporates, it absorbs latent heat from the skin, causing a cooling effect.
- Steam in Power Plants
- Steam is used in turbines to generate electricity, utilizing the heat energy stored during vaporization.
- Boiling of Liquid Nitrogen
- Liquid nitrogen has a very low boiling point and requires latent heat of vaporization to turn into a gas.
Real-Life Applications of Latent Heat of Vaporization
1. Cooling Mechanisms
When sweat evaporates, it removes heat from the body, helping maintain a stable temperature. This is why sweating is an effective cooling process.
2. Steam Engines and Power Plants
Water is heated to produce steam, which expands and moves turbines, converting thermal energy into mechanical energy.
3. Refrigeration and Air Conditioning
Refrigerants in cooling systems absorb latent heat during vaporization, lowering the temperature inside refrigerators and air conditioners.
4. Cooking and Food Processing
Boiling water ensures that food is cooked at a constant temperature, as the latent heat keeps the boiling water at 100°C.
Multiple Choice Questions (MCQs) on Latent Heat of Vaporization
- What is latent heat of vaporization?
- Heat required to change solid into liquid
- Heat required to convert liquid into gas at constant temperature
- Heat required to raise the temperature of a substance
- Heat lost during freezing
- What is the SI unit of latent heat of vaporization?
- Joules (J)
- Joules per kilogram (J/kg)
- Calories
- Newtons
- Which of the following has a latent heat of vaporization value of 2260 kJ/kg?
- Alcohol
- Water
- Mercury
- Oxygen
- Why does water remain at 100°C while boiling?
- It does not absorb heat
- The heat energy is used for phase change
- The heat energy is used to increase temperature
- Water does not follow the laws of thermodynamics
- Which of the following processes absorbs latent heat?
- Condensation
- Freezing
- Vaporization
- Sublimation
True or False Questions
- Latent heat of vaporization only applies to water.(True / False)
- During boiling, temperature remains constant until all liquid has turned into gas.(True / False)
- Latent heat of vaporization is the energy required to change liquid to gas.(True / False)
- Condensation releases the same amount of latent heat as vaporization absorbs.(True / False)
- The higher the latent heat of vaporization, the more energy is needed to boil a substance.(True / False)
Numerical Problems on Latent Heat of Vaporization
1. Finding Heat Required to Boil Water
Question:
How much heat is required to boil 3 kg of water at 100°C, given that the latent heat of vaporization of water is 2260 kJ/kg?
Solution:
Using the formula:
Thus, 6780 kJ of heat is required to convert 3 kg of water into steam.
2. Finding Mass of Water Vaporized by Heat Energy
Question:
A heater supplies 4520 kJ of heat to a container of boiling water. How much water is vaporized, given the latent heat of vaporization of water is 2260 kJ/kg?
Solution:
Using the formula:
Thus, 2 kg of water is vaporized when 4520 kJ of heat is supplied.
Common Mistakes in Understanding Latent Heat of Vaporization
1. Assuming Temperature Increases During Boiling
Many students think that when heat is added, temperature rises, but during boiling, temperature remains constant until all the liquid turns into gas.
2. Confusing Latent Heat of Fusion and Vaporization
- Latent heat of fusion applies to solid to liquid (e.g., ice to water).
- Latent heat of vaporization applies to liquid to gas (e.g., water to steam).
3. Ignoring Mass in Calculations
Heat required for vaporization depends on mass. The larger the mass, the more heat energy is needed.
4. Forgetting Heat is Released During Condensation
When a gas condenses into a liquid, it releases the same amount of latent heat as it absorbed while vaporizing.
Frequently Asked Questions (FAQs)
1. Why does steam cause more burns than boiling water?
Steam has extra energy from latent heat of vaporization, making it hotter and more damaging than boiling water.
2. Can latent heat of vaporization be negative?
Yes, during condensation, the latent heat is released into the surroundings, making it negative.
3. Why does water take longer to evaporate compared to alcohol?
Water has a higher latent heat of vaporization, meaning it requires more heat to turn into vapor.
4. Does latent heat of vaporization apply to all substances?
Yes, but different substances have different latent heat values depending on their molecular structure.
Latent heat of vaporization is crucial for understanding boiling, evaporation, and condensation. It plays a vital role in science, engineering, and daily life. By mastering the concepts, formulas, and solving numerical problems, students can better grasp phase changes and their applications.