Phase changes involve energy transfer as matter shifts between solid, liquid, and gas states. Melting and vaporization require energy input, while condensation and freezing release energy. The amount needed depends on the substance and its mass.

Latent heat calculations use the formula Q = mL, where Q is energy transferred, m is mass, and L is latent heat. During phase changes, temperature remains constant as energy breaks or forms bonds, affecting particles' kinetic energy.

Phase Change and Latent Heat

Energy transfer in phase changes

• Energy transfer required for matter to change phases
  • Melting (solid to liquid) and vaporization (liquid to gas) require energy input
  • Condensation (gas to liquid) and freezing (liquid to solid) release energy
• Amount of energy required depends on substance and mass undergoing phase change
  • Different substances have different latent heats of fusion (melting/freezing) and vaporization (boiling/condensation)
• During phase change, temperature remains constant as energy used to break or form intermolecular bonds
  • Melting and vaporization: energy breaks bonds, increasing average kinetic energy of particles
  • Freezing and condensation: energy released as bonds form, decreasing average kinetic energy of particles

Latent heat calculations

• Energy required for phase change calculated using formula: $Q = mL$
  • $Q$: energy transferred (joules, J)
  • $m$: mass of substance (kilograms, kg)
  • $L$: latent heat of specific phase change (J/kg)
• Latent heat of fusion ($L_f$) used for melting and freezing
  • To melt 2 kg of ice at 0°C, with $L_f$ of ice = 334 kJ/kg, energy required is: $Q = 2 \text{ kg} \times 334 \text{ kJ/kg} = 668 \text{ kJ}$
• Latent heat of vaporization ($L_v$) used for vaporization and condensation
  • To vaporize 1.5 kg of water at 100°C, with $L_v$ of water = 2,260 kJ/kg, energy required is: $Q = 1.5 \text{ kg} \times 2,260 \text{ kJ/kg} = 3,390 \text{ kJ}$

Temperature effects during phase changes

• During phase change, temperature of substance remains constant despite continuous addition or removal of energy
  • Energy used to break or form intermolecular bonds rather than changing average kinetic energy of particles
• On heating curve, phase changes appear as horizontal lines, indicating constant temperature
  • Length of horizontal line represents energy required for phase change
• On cooling curve, phase changes also appear as horizontal lines, indicating constant temperature
  • Length of horizontal line represents energy released during phase change
• Between phase changes, temperature of substance increases (during heating) or decreases (during cooling) linearly with addition or removal of energy
  • Slope of line depends on specific heat capacity of substance in each phase (solid, liquid, gas)