Understanding Latent Heat of Vaporization of Steam

The question asks about the latent heat of vaporization of steam. Let's break down what this term means.

What is Latent Heat?

Latent heat is the heat energy absorbed or released by a substance during a change in its physical state (phase change) that occurs at a constant temperature and pressure. There are different types of latent heat, depending on the phase change: latent heat of fusion (for melting or freezing) and latent heat of vaporization (for boiling or condensation).

Latent Heat of Vaporization

The latent heat of vaporization is the amount of heat energy required to change a unit mass of a substance from a liquid to a gas at its boiling point and standard atmospheric pressure. For water, this process occurs at 100°C (or 212°F) at standard pressure.

When water turns into steam (vaporizes), it absorbs a significant amount of energy. This energy is used to overcome the intermolecular forces holding the water molecules together in the liquid state and move them further apart into the gaseous state. This happens without a change in temperature until all the water has turned into steam.

Value of Latent Heat of Vaporization of Steam

The standard value for the latent heat of vaporization of water to steam at 100°C and standard atmospheric pressure is approximately 2260 kilojoules per kilogram (kJ/kg). This value can also be expressed in other units, such as calories per gram (cal/g).

The conversion factor between joules and calories is approximately 1 calorie = 4.184 joules. So, to convert kJ/kg to cal/g:

$$ 2260 \frac{\text{kJ}}{\text{kg}} = 2260 \times \frac{1000 \text{ J}}{1000 \text{ g}} = 2260 \frac{\text{J}}{\text{g}} $$

Now, convert Joules to calories:

$$ 2260 \frac{\text{J}}{\text{g}} \times \frac{1 \text{ cal}}{4.184 \text{ J}} \approx 540.15 \frac{\text{cal}}{\text{g}} $$

A commonly used historical value, often rounded for convenience or based on slightly different conditions, is 540 cal/g. However, precise measurements show values closer to 539, or even slightly higher depending on the source and exact conditions (like 539.4 cal/g). The value presented in the options often reflects a standard or historically accepted value used in educational contexts.

Analyzing the Options

Let's compare the given options with the standard value of latent heat of vaporization:

• Option 1: 423 cal/g
• Option 2: 267 cal/g
• Option 3: 324 cal/g
• Option 4: 536 cal/g

Among the given options, 536 cal/g is the closest and most widely accepted value for the latent heat of vaporization of steam at 100°C, often used in textbooks and examinations as a standard value.

Revision Table: Key Thermal Properties

Additional Information on Phase Changes

During a phase change, the temperature of the substance remains constant. The added or removed heat energy is used entirely to change the state of matter. For example, when water boils at 100°C, adding more heat does not increase the temperature above 100°C; instead, it converts more water into steam. Similarly, when steam condenses into water at 100°C, it releases energy (the latent heat of vaporization) without its temperature dropping below 100°C until all the steam has condensed.

Understanding latent heat is crucial in various applications, including refrigeration, air conditioning, and power generation.