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 physical transition from one state of matter to another caused by adding or removing thermal energy, during which the temperature remains constant while the energy goes into rearranging the particles (breaking or forming intermolecular attractions) rather than into raising or lowering temperature.

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: Phase Change asks what the sample is, what property is being used, and whether a new substance is formed.

Common stuck point: Students often know a formula related to phase change but skip the recognition step: Am I classifying matter or using properties, state, particle behavior, or mixture evidence to describe a sample? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I classifying matter or using properties, state, particle behavior, or mixture evidence to describe a sample?

Worked Examples

Example 1

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

Answer

Endothermic: melt, vaporize, sublime. Exothermic: freeze, condense, deposit.\text{Endothermic: melt, vaporize, sublime. Exothermic: freeze, condense, deposit.}

First step

1
Group the phase changes into those that absorb energy and those that release it.

Full solution

  1. 2
    Endothermic changes absorb energy: melting (solid→liquid), vaporization (liquid→gas), and sublimation (solid→gas).
  2. 3
    Exothermic changes release energy: freezing (liquid→solid), condensation (gas→liquid), and deposition (gas→solid).
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?

Example 3

medium
During boiling, why does the temperature of water stay at 100∘C100^\circ\text{C} even though heat keeps being added?

Example 4

medium
At −10∘C-10^\circ\text{C} snow can disappear without melting. Explain.

Example 5

medium
A heating curve has a flat segment at 0 °C and another flat segment at 100 °C for pure water. What is happening at each flat segment?

Example 6

hard
Why does sweating cool the human body?

Example 7

hard
Calculate the heat needed to vaporize 25.0 g of water at 100 °C. Use Lv=2260 J/gL_v = 2260\ \text{J/g}.

Example 8

hard
A 200 g sample of ice at 0∘C0^\circ\text{C} is heated until it becomes water at 25∘C25^\circ\text{C}. Approximate the total heat absorbed. Use Lf=334 J/gL_f = 334\ \text{J/g} and cwater=4.18 J/(g\cdotp°C)c_\text{water} = 4.18\ \text{J/(g·°C)}.

Example 9

challenge
On a phase diagram for water, what does the triple point represent, and approximately what temperature is it for water?

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 (CO2\text{CO}_2) goes directly from solid to gas?

Example 2

hard
A heating curve shows a substance warming from −20°C-20°\text{C} to 120°C120°\text{C}. It has a melting point of 0°C0°\text{C} and a boiling point of 100°C100°\text{C}. Explain why there are two flat (horizontal) sections on the curve, even though heat is continuously added.

Example 3

easy
What is the name of the phase change from solid to liquid?

Example 4

easy
What is the phase change from liquid to gas called?

Example 5

easy
What is the phase change from gas to liquid called?

Example 6

easy
What is the phase change from liquid to solid called?

Example 7

easy
Dry ice turns directly from solid to gas. Name this phase change.

Example 8

easy
Frost forming directly from water vapor onto a cold surface is which phase change?

Example 9

easy
Is a phase change a physical change or a chemical change?

Example 10

easy
During boiling, the temperature of pure water stays at 100 °C. Why does the temperature not rise?

Example 11

medium
Identify the phase change: water vapor in warm air touches a cold glass and forms droplets.

Example 12

medium
A mothball slowly disappears in a closet without leaving any liquid. Which phase change is this and what does it tell you?

Example 13

medium
Explain why melting ice is a physical change even though the ice 'disappears' into liquid.

Example 14

medium
Order the energy requirement: which absorbs energy and which releases it: (a) freezing, (b) melting, (c) condensation?

Example 15

medium
Name the phase change for each: (a) candle wax hardening, (b) puddle drying up, (c) ice cube shrinking in a freezer over weeks.

Example 16

medium
Why does steam cause more severe burns than the same mass of boiling water at 100 °C?

Example 17

medium
During a phase change at constant temperature, where does the added or removed energy go if not into temperature?

Example 18

medium
A student says 'boiling water turns into hydrogen and oxygen gas.' Correct this statement.

Example 19

medium
Name the phase change in each: (a) dew forming on grass at dawn, (b) a snowbank shrinking on a sunny but below-freezing day.

Example 20

challenge
A heating curve for water shows two flat plateaus as you heat ice from -20 °C to steam. Explain what each plateau represents and why temperature is constant there.

Example 21

challenge
Explain why sublimation and deposition are 'opposite' phase changes, give a real example of each, and state which absorbs and which releases energy.

Example 22

challenge
Water boils at 100 °C at sea level but at about 70 °C atop a tall mountain. Explain why boiling temperature changes with altitude, framing boiling as a phase change.

Example 23

easy
Water turns to ice. Which phase change is this?

Example 24

easy
A puddle dries up in the sun. What phase change happened?

Example 25

easy
Dew forms on grass overnight. Name the phase change.

Example 26

easy
Mothballs slowly shrink in a closet. What phase change is happening?

Example 27

easy
What is the boiling point of water in °C at sea level?

Example 28

medium
Does condensation absorb or release energy?

Example 29

medium
List the three endothermic phase changes.

Example 30

medium
During freezing, do water molecules gain or lose energy?

Example 31

medium
Why does an ice cube cool a drink?

Example 32

hard
Why does it take more energy to vaporize water than to melt the same mass of ice?

Example 33

hard
How does increasing pressure affect the boiling point of water?

Example 34

hard
At high altitude, water boils below 100 °C. Why?
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Background Knowledge

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

state of matter