define and use specific latent heat and distinguish between specific latent heat of fusion and specific latent heat of vaporisation

Temperature Scales

Temperature tells us how hot or cold something is. In physics we use three main scales: Celsius (°C), Kelvin (K) and Fahrenheit (°F).

🔹 Celsius is based on the freezing and boiling points of water: 0 °C = water freezes, 100 °C = water boils.

🔹 Kelvin starts at absolute zero, the coldest possible temperature. The scale is the same size as Celsius but shifted: \$T(K) = T(°C) + 273.15\$.

🔹 Fahrenheit uses 32 °F for freezing and 212 °F for boiling. Conversion: \$T(°F) = T(°C) \times 9/5 + 32\$.

💡 Analogy: Think of Celsius as a “water scale” (freezing/boiling), Kelvin as a “physics scale” that starts from nothing, and Fahrenheit as a “household scale” used in the US.

Specific Latent Heat

When a substance changes phase (solid ↔ liquid ↔ gas) at a constant temperature, it absorbs or releases a fixed amount of energy per unit mass. This energy is called the specific latent heat (\$L\$).

Formula: \$Q = mL\$ where \$Q\$ is the heat added (J), \$m\$ is the mass (kg) and \$L\$ is the specific latent heat (J kg⁻¹).

💡 Example: Melting 2 kg of ice at 0 °C requires \$Q = 2 \times 334\,000 = 668\,000\$ J.

Specific Latent Heat of Fusion vs. Vaporisation

Fusion (solid → liquid):

  • Energy is used to break the crystal lattice.

  • Typical value for water: \$L_f = 334\,\text{kJ kg}^{-1}\$.

  • 🔄 Analogy: Melting ice is like a snowball turning into a puddle.

Vaporisation (liquid → gas):

  • Energy is used to overcome intermolecular forces in the liquid.

  • Typical value for water: \$L_v = 2260\,\text{kJ kg}^{-1}\$.

  • 🔥 Analogy: Boiling water is like a pot of soup turning into steam.

💡 Key point: The latent heat of vaporisation is usually much larger than that of fusion because more energy is needed to separate molecules completely.

Phase ChangeSpecific Latent Heat (kJ kg⁻¹)
Fusion (solid → liquid)334
Vaporisation (liquid → gas)2260

🔍 Exam Tip: When you see a question about melting or boiling, check which latent heat value to use. Remember: fusion for solid–liquid, vaporisation for liquid–gas.

🧮 Calculation Steps:

  1. Identify the phase change.

  2. Write the formula \$Q = mL\$.

  3. Insert the correct \$L\$ value.

  4. Compute \$Q\$ or solve for the unknown.