Phase changes are crucial in understanding matter's behavior. They involve energy transfer without temperature change, known as latent heat. This concept is key to grasping how substances transition between solid, liquid, and gas states.

Latent heat calculations, temperature stability during transitions, and phase diagrams are essential tools. These help us predict and analyze how materials behave under different conditions, from everyday occurrences like ice melting to complex industrial processes.

Phase Change and Latent Heat

Energy calculations for phase changes

• Latent heat represents energy required to change a substance's phase without altering its temperature
  • Latent heat of fusion ($L_f$) is energy needed to convert a substance between solid and liquid states (melting or freezing)
  • Latent heat of vaporization ($L_v$) is energy needed to convert a substance between liquid and gas states (evaporation or condensation)
• Calculate the energy required for a phase change using the formula $Q = mL$
  • $Q$ represents the energy required in joules (J)
  • $m$ represents the substance's mass in kilograms (kg)
  • $L$ represents the specific latent heat for the phase change in joules per kilogram (J/kg)
• Examples:
  • Energy required to melt 2 kg of ice at 0°C ($L_f$ of water is 334 kJ/kg): $Q = 2 \text{ kg} \times 334,000 \text{ J/kg} = 668,000 \text{ J}$
  • Energy required to vaporize 0.5 kg of water at 100°C ($L_v$ of water is 2,260 kJ/kg): $Q = 0.5 \text{ kg} \times 2,260,000 \text{ J/kg} = 1,130,000 \text{ J}$

Temperature and energy in transitions

• During a phase transition, a substance's temperature remains constant while energy is added or removed
  • The energy is used to break or form intermolecular bonds instead of increasing the particles' kinetic energy
• When a substance changes from solid to liquid or liquid to gas, energy is added to the system
  • This energy overcomes the attractive forces between particles, allowing them to move more freely
  • Examples: melting ice, boiling water
• When a substance changes from gas to liquid or liquid to solid, energy is removed from the system
  • This allows the attractive forces between particles to dominate, causing them to move closer together and form a more ordered structure
  • Examples: condensing steam, freezing water
• The energy involved in phase changes is also known as the heat of transformation

Analysis of phase diagrams

• A phase diagram graphically represents a substance's states of matter at different temperatures and pressures
• The phase diagram has three main regions: solid, liquid, and gas
  • Lines separating these regions represent conditions where two phases can coexist in equilibrium (solid-liquid, liquid-gas, solid-gas)
• The triple point is where all three phases (solid, liquid, gas) can coexist in equilibrium
• The critical point is where the distinction between liquid and gas phases disappears
• To determine a substance's state of matter at a given temperature and pressure:
  1. Locate the point on the phase diagram corresponding to the given conditions
  2. If the point lies within one of the three main regions, the substance exists in that state of matter
  3. If the point lies on a line separating two regions, the substance exists in both states of matter simultaneously
• Examples:
  • Water at 1 atm and 25°C: liquid state
  • Carbon dioxide at 1 atm and -80°C: solid state (dry ice)
  • Water at 0.006 atm and 0.01°C: triple point (solid, liquid, and gas coexist)
• The direct transition from solid to gas state is called sublimation

Thermodynamic concepts in phase changes

• Enthalpy is a measure of the total heat content of a system, which changes during phase transitions
• Specific heat capacity is the amount of heat required to raise the temperature of a unit mass of a substance by one degree
• Latent energy is stored or released during phase changes without changing the temperature
• Phase equilibrium occurs when two phases of a substance coexist at a specific temperature and pressure