What Are Phase Transitions?
Phase transitions are processes where matter changes between solid, liquid, and gas states without changing chemical identity. Energy input or removal drives these changes, crossing clear boundaries defined by temperature and pressure.
Phase transitions occur when thermal energy overcomes or is overcome by intermolecular forces. Melting, freezing, evaporation, condensation, sublimation, and deposition are the six main transitions between three states.
- 1.Melting — Solid → Liquid (heat input)
- 2.Evaporation — Liquid → Gas (heat input)
- 3.Sublimation — Solid → Gas (heat input, skip liquid)
- 4.Condensation — Gas → Liquid (heat removal)
- 5.Freezing — Liquid → Solid (heat removal)
- 6.Deposition — Gas → Solid (heat removal, skip liquid)
Step-by-step worked examples
Ice at 0 °C absorbs 334 kJ/kg heat. Does temperature change during melting?
No. During a phase transition, temperature remains constant. All absorbed energy goes to breaking molecular bonds, not kinetic energy. Temperature rises again only after all ice melts.
Water boils at 100 °C at sea level, 95 °C at 2000 m altitude. Why?
Boiling point depends on atmospheric pressure. At altitude, lower pressure → less force pushing down on liquid. Molecules escape to gas more easily → lower boiling point.
Dry ice (solid CO₂) disappears at room temperature without becoming liquid. What's this process?
This is sublimation — solid transforms directly to gas. CO₂ phase diagram shows: solid → gas without liquid phase at atmospheric pressure.
Flashcards
Quick quiz
Q1.Ice melting at 0 °C is which type of phase transition?
Q2.Which transition requires heat removal?
Q3.Dry ice subliming means…
Q4.Boiling point increases with…
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Common mistakes
Temperature rises during melting. — Correct: Temperature stays constant; energy breaks bonds, not accelerating molecules.
Boiling and evaporation are the same. — Correct: Boiling is vigorous, at the boiling point; evaporation is gradual from the surface.
All solids sublime. — Correct: Only certain substances sublime under normal conditions (dry ice, naphthalene).
Phase transitions change the chemical formula. — Correct: Only the physical state changes; the substance remains chemically identical.
FAQ
What are the six main phase transitions?
Melting, evaporation, sublimation (heat in); freezing, condensation, deposition (heat out).
Why does ice melt at 0 °C?
At 0 °C, thermal energy equals the lattice energy holding the solid together.
What is latent heat?
Energy absorbed or released during a phase transition without temperature change.
Can water evaporate below 100 °C?
Yes — evaporation occurs at any temperature from the liquid surface; boiling is rapid evaporation at 100 °C.