What Are Phase Changes?
Phase changes are transformations between solid, liquid, and gas states. Each change requires or releases energy (heat), and the substance's chemical identity remains the same — only its physical structure changes.
A phase change is when a substance shifts between solid, liquid, and gas. Energy (heat) is absorbed during melting, boiling, and sublimation; energy is released during freezing, condensation, and deposition.
- 1.Melting — Solid → Liquid (absorb heat)
- 2.Boiling — Liquid → Gas (absorb heat)
- 3.Condensation — Gas → Liquid (release heat)
- 4.Freezing — Liquid → Solid (release heat)
- 5.Sublimation — Solid → Gas (absorb heat)
- 6.Deposition — Gas → Solid (release heat)
Step-by-step worked examples
Ice melts at 0°C. Is this melting or freezing?
Ice (solid) → Water (liquid) at 0°C. This is MELTING — the solid phase changes to liquid. Energy (heat) is ABSORBED.
Water boils at 100°C at 1 atm. Describe this phase change.
Liquid water → water vapor (gas) at 100°C. This is BOILING — liquid becomes gas. A large amount of energy (latent heat of vaporization) is ABSORBED.
Dry ice (solid CO₂) disappears at room temperature without melting. What is happening?
Solid CO₂ → CO₂ gas directly (no liquid stage). This is SUBLIMATION — solid bypasses liquid and becomes gas. Energy is ABSORBED.
Flashcards
Quick quiz
Q1.Water freezing at 0°C is an example of…
Q2.Which process absorbs heat?
Q3.Dry ice subliming is…
Q4.When liquid water becomes water vapor, is there a chemical change?
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Common mistakes
Thinking phase changes cause chemical changes. — Correct: Phase changes are physical only — the substance's composition stays the same.
Confusing evaporation with boiling. — Correct: Boiling occurs at a specific temperature throughout the liquid; evaporation is slower and occurs at the surface at any temperature.
Assuming all phase changes require the same amount of energy. — Correct: Different phase changes have different latent heats (e.g., vaporization >> melting).
Thinking sublimation only happens with dry ice. — Correct: Sublimation is common — ice, snow, and naphthalene all sublime.
FAQ
What is the difference between boiling and evaporation?
Boiling occurs at a fixed temperature (100°C for water at 1 atm) throughout the liquid. Evaporation occurs at the surface at any temperature below the boiling point.
Why do phase changes require or release energy?
Particles must overcome or release attractive forces. Melting/boiling/sublimation break bonds (absorb heat); freezing/condensation/deposition form bonds (release heat).
Can a solid become a gas directly?
Yes — this is sublimation. Dry ice (solid CO₂) and frost are common examples.
Are phase changes reversible?
Yes — ice melts to water, water boils to steam, and steam condenses back to water.