Evaporation and Latent Heat — Study Notes

Overview

Evaporation and latent heat are critical sub-topics under "Matter in Our Surroundings" that appear frequently in SOF NSO Class 9–10. Understanding evaporation helps explain everyday phenomena like cooling of water in earthen pots, drying of clothes, and formation of clouds. Latent heat explains why ice at 0°C feels less cold than water at 0°C, and why steam causes more severe burns than boiling water at the same temperature.

Students must master the factors that speed up or slow down evaporation — surface area, temperature, humidity, and wind speed — and distinguish clearly between the two types of latent heat: fusion (solid↔liquid) and vaporization (liquid↔gas). Numerical problems often ask for heat calculations during state changes, requiring correct use of latent heat values. Conceptual questions test understanding of cooling effects and energy absorption without temperature change.

This topic bridges physical states of matter with thermodynamics, making it a favorite for multi-step problems in the Achievers Section and HOTS questions linking real-world applications.

Key Concepts

• **Evaporation** is the process where liquid particles at the surface gain enough kinetic energy to escape into vapor phase, occurring at all temperatures below boiling point. Unlike boiling (which happens throughout the liquid at a fixed temperature), evaporation is a surface phenomenon.

• **Latent heat** is the energy absorbed or released during a state change at constant temperature. The temperature remains fixed because energy is used to break intermolecular bonds (not to increase kinetic energy of particles).

• **Latent heat of fusion** is the energy required to convert 1 kg of solid to liquid at its melting point without temperature change. For ice, this value is 3.36 × 10⁵ J/kg or 336 kJ/kg.

• **Latent heat of vaporization** is the energy required to convert 1 kg of liquid to gas at its boiling point without temperature change. For water, this value is 2.26 × 10⁶ J/kg or 2260 kJ/kg — much higher than fusion because all intermolecular forces must be overcome.

• **Evaporation causes cooling** because high-energy particles escape the liquid surface, leaving behind particles with lower average kinetic energy, thus reducing the liquid's temperature.

• Evaporation rate increases with: increased surface area, increased temperature, decreased humidity, and increased wind speed. All four factors relate to how easily fast-moving particles can escape and how quickly vapor is removed from above the liquid surface.

• The boiling point and melting point are characteristic temperatures for pure substances, but evaporation occurs at any temperature — even ice can sublimate (solid to gas) at temperatures below 0°C.

Formulas / Key Facts

**Q = mL** — Heat energy absorbed or released during state change, where Q is heat (joules), m is mass (kg), and L is latent heat (J/kg).

**Latent heat of fusion of ice** = 3.36 × 10⁵ J/kg = 336 J/g = 80 cal/g.

**Latent heat of vaporization of water** = 2.26 × 10⁶ J/kg = 2260 J/g = 540 cal/g.

Temperature remains constant during melting, freezing, boiling and condensation — all the absorbed/released energy goes into changing state, not temperature.

**1 calorie** = 4.18 joules (unit conversion often needed in numerical problems).

During evaporation, no fixed temperature is required — it can occur at any temperature, unlike boiling which occurs at a specific temperature (100°C for water at 1 atm).

Humidity is the amount of water vapor present in air. High humidity slows evaporation because air is already saturated with vapor.

Wind speed affects evaporation by removing vapor particles from above the liquid surface, maintaining a concentration gradient that favors more evaporation.

Worked Examples

**Example 1:** Calculate the heat required to melt 500 g of ice at 0°C to water at 0°C.

*Solution:* Given: m = 500 g = 0.5 kg, L (fusion) = 3.36 × 10⁵ J/kg Using Q = mL Q = 0.5 × 3.36 × 10⁵ = 1.68 × 10⁵ J = 168 kJ **Answer: 168 kJ of heat is required.**

---

**Example 2:** How much energy is released when 2 kg of steam at 100°C condenses to water at 100°C?

*Solution:* Condensation releases the same amount of energy that vaporization absorbs. Given: m = 2 kg, L (vaporization) = 2.26 × 10⁶ J/kg Q = mL = 2 × 2.26 × 10⁶ = 4.52 × 10⁶ J = 4520 kJ **Answer: 4520 kJ of energy is released.**

---

**Example 3:** Why does a desert cooler cool better on a hot, dry day than on a humid day?

*Solution:* Evaporation rate depends on humidity. On a hot, dry day, air has low humidity, so water from the cooler evaporates rapidly, absorbing more heat from surroundings and causing greater cooling. On a humid day, air is already saturated with water vapor, so evaporation slows down, reducing the cooling effect. **Answer: Low humidity increases evaporation rate, enhancing cooling.**

Common Mistakes

**Mistake:** Assuming temperature increases during melting or boiling. **Fix:** During any state change, temperature stays constant. All absorbed energy breaks intermolecular bonds, not increasing particle kinetic energy (which determines temperature).

**Mistake:** Using latent heat of fusion value when the problem asks about boiling/condensation. **Fix:** Fusion relates to solid↔liquid changes (melting/freezing). Vaporization relates to liquid↔gas changes (boiling/condensation). Always check which state change is occurring.

**Mistake:** Confusing evaporation with boiling — thinking evaporation only happens at 100°C. **Fix:** Evaporation is a surface phenomenon occurring at any temperature. Boiling is a bulk phenomenon at a fixed temperature (100°C for water at standard pressure).

**Mistake:** Believing wind causes cooling by lowering temperature of the liquid directly. **Fix:** Wind increases evaporation rate by removing vapor particles above the liquid surface. The increased evaporation then causes cooling, not the wind itself.

**Mistake:** Ignoring unit conversions — mixing grams with kg or calories with joules. **Fix:** Always convert to SI units: mass in kg, latent heat in J/kg. Remember 1 cal = 4.18 J if problem uses calories.

Quick Reference

• Evaporation = surface phenomenon; occurs at all temperatures; causes cooling.
• Latent heat = energy for state change at constant temperature.
• L (fusion, ice) = 336 kJ/kg; L (vaporization, water) = 2260 kJ/kg.
• Formula: Q = mL (m in kg, L in J/kg, Q in joules).
• Evaporation increases with: ↑surface area, ↑temperature, ↓humidity, ↑wind speed.
• Steam burns more than boiling water because steam releases 2260 kJ/kg extra energy when condensing.