Phase Change Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Phase Change.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A transition from one state of matter to another, caused by adding or removing energy.

Add enough heat and a solid melts to liquid, then boils to gas. Remove heat and the reverse happens.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: During a phase change, temperature stays constant even though energy is being added — the energy breaks intermolecular bonds instead of raising temperature.

Common stuck point: Temperature does NOT change during a phase change — all the energy goes into breaking or forming bonds between particles.

Worked Examples

Example 1

easy
Name the six phase changes and indicate whether each absorbs or releases energy.

Solution

  1. 1
    Group the phase changes into those that absorb energy and those that release it.
  2. 2
    Endothermic changes absorb energy: melting (solid→liquid), vaporization (liquid→gas), and sublimation (solid→gas).
  3. 3
    Exothermic changes release energy: freezing (liquid→solid), condensation (gas→liquid), and deposition (gas→solid).

Answer

\text{Endothermic: melt, vaporize, sublime. Exothermic: freeze, condense, deposit.}
Phase changes involve energy transfer but not temperature change (at the phase transition point). The energy goes into breaking or forming intermolecular bonds rather than increasing kinetic energy.

Example 2

medium
Why does the temperature remain constant during a phase change, even though heat is being added?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
What phase change occurs when dry ice (\text{CO}_2) goes directly from solid to gas?

Example 2

hard
A heating curve shows a substance warming from -20°\text{C} to 120°\text{C}. It has a melting point of 0°\text{C} and a boiling point of 100°\text{C}. Explain why there are two flat (horizontal) sections on the curve, even though heat is continuously added.
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Background Knowledge

These ideas may be useful before you work through the harder examples.

state of matter