Latent Heat of Vaporization and Heat of Fusion

The latent heat of vaporization, heat of vaporization, and heat of fusion are important concepts studied in physics and chemistry classes. Understanding why your skin feels cool when water evaporates and how energy is absorbed when ice melts can help you better understand the phenomena around you. In this article, we will explain these types of heat, their definitions, and examples, and provide strategies for passing the hazardous materials handler exam.

What Are the Latent Heat of Vaporization and Heat of Vaporization?

Latent heat of vaporization refers to the amount of heat absorbed when a liquid changes into a gas. Energy is required for molecules to escape from the surface of the liquid and become gas, and this energy is called “latent heat of vaporization.” The term heat of vaporization is also used interchangeably.

Examples of Latent Heat of Vaporization

For example, have you ever felt a cooling sensation when applying water or alcohol to your skin on a hot day? This happens because water or alcohol absorbs heat from the skin’s surface as it evaporates. For a liquid to turn into a gas, it must absorb heat from its surroundings. This is why the liquid feels cool during evaporation.

What Is Heat of Fusion?

Heat of fusion refers to the amount of heat absorbed when a solid changes into a liquid. For example, when ice turns into water, it absorbs energy from its surroundings, and the temperature remains at 0°C until the ice is completely melted. This absorbed energy is called “heat of fusion” or “latent heat of fusion.”

Examples of Heat of Fusion

When ice changes into water, its temperature stays constant at 0°C, but energy is needed for the ice to melt. The heat of fusion for ice is 334 J/g, which is the amount of energy required to melt 1g of ice into water. As this energy is absorbed, the surrounding temperature drops, and the ice melts.

The Role of Latent Heat of Vaporization and Heat of Fusion in Water

The latent heat of vaporization for water at its boiling point of 100°C is 2256.3 J/g, a very large value. This means that a significant amount of energy is required for water to evaporate. On the other hand, the heat of fusion for ice at 0°C is 334 J/g. These values indicate that water can store energy easily and has a high cooling effect. Water utilizes this property to play an important role in temperature regulation and firefighting.

For example, when humans sweat, the body cools itself by evaporating the sweat. As the sweat evaporates from the skin, it removes heat from the body, effectively regulating body temperature. Similarly, when using ice to cool a drink, the ice absorbs the surrounding heat as it melts, lowering the temperature of the beverage.

The Concept of Latent Heat in Vaporization and Fusion

The phenomenon where energy is transferred without a change in temperature during a phase change, such as with latent heat of vaporization or heat of fusion, is called “latent heat.” Latent heat refers to the energy change that does not accompany temperature changes but plays a crucial role in phase transitions (solid → liquid, liquid → gas, etc.).

Latent Heat of Vaporization and Heat of Fusion in the Hazardous Materials Handler Exam

In the hazardous materials handler exam, understanding the latent heat of vaporization and heat of fusion is important. Highly volatile liquids, such as gasoline, evaporate easily, making them prone to fires or explosions, so it is essential to understand their characteristics. Additionally, it is important to know that heat absorption during the transition of solids to liquids can significantly lower the surrounding temperature.

Exam Preparation Tips

• Absorption and Release of Energy: Understand that energy is absorbed during vaporization and released during condensation (the process of a gas turning into a liquid). Similarly, energy is absorbed during fusion (solid to liquid), and energy is released during solidification (liquid to solid).
• Volatility of Substances: Learn that liquids like gasoline and ethanol evaporate easily and pose a high fire risk, so appropriate handling methods should be studied.
• Understanding Practical Examples: Familiarize yourself with practical examples of latent heat of vaporization and heat of fusion, such as the evaporation of sweat or alcohol and the melting of ice, to deepen your understanding.

Sample Exam Question 1

Explanation:
(1) Correct. The heat of fusion and solidification are equal in absolute value under the same pressure.
(2) Correct. The heat required to raise the temperature of 1g of a substance by 1°C is called “specific heat.”
(3) Correct. The heat absorbed during the transition from solid to liquid without temperature change is “heat of fusion.”
(4) Incorrect. The heat required for a liquid to turn into a gas is called “latent heat of vaporization.”

Answer: (4)

Sample Exam Question 2

Explanation:
To turn 20g of ice at 0°C into water at 20°C, we calculate the required heat in two steps.

1. Step 1: Heat to Melt Ice into Water at 0°C

The heat required to melt ice is calculated using the “latent heat of fusion.”

Formula:
Q1 = m × Lf
Where,
Q₁: Heat required for fusion (J)
m: Mass of ice (g)
L_f: Latent heat of fusion (J/g)
Mass of ice m = 20 g
Latent heat of fusion L_f = 334.88 J/g

Calculation:
Q1 = 20 g × 334.88 J/g = 6697.6 J

2. Step 2: Heat to Raise the Temperature of Water from 0°C to 20°C

The heat required to raise the temperature of water from 0°C to 20°C is calculated using the “specific heat.”

Formula:
Q2 = m × c × ΔT
Where,
Q₂: Heat required to heat water (J)
m: Mass of water (g)
c: Specific heat of water (J/(g·K))
ΔT: Temperature change (K)
Mass of water m = 20 g
Specific heat of water c = 4.186 J/(g·K)
Temperature change ΔT = 20 K

Calculation:
Q2 = 20 g × 4.186 J/(g·K) × 20 K = 1674.4 J

3. Total Heat Calculation

Add the heat calculated in Step 1 and Step 2 to find the total amount of heat required to turn ice from 0°C into water at 20°C.

Formula:
Qtotal = Q1 + Q2
Qtotal = 6697.6 J + 1674.4 J = 8372 J

Answer: (4)

Conclusion

The latent heat of vaporization, heat of vaporization, and heat of fusion are important concepts that help understand daily phenomena and basic scientific principles. By understanding energy transfer when a liquid turns into a gas or a solid turns into a liquid, you can gain a deeper understanding of everyday events and hazardous materials handling. For exam preparation, make sure to grasp the roles and examples of latent heat of vaporization and heat of fusion.