Changes of state all year round
Snow melts in spring. On a summer day, a cold can of soda from the fridge is soon covered in droplets. Dew forms on the grass on autumn mornings, and frost patterns appear on car windows in winter. Mothballs in a closet slowly shrink until they disappear.
All of these are substances moving between solid, liquid, and gas. They are called changes of state物态变化, or phase changes.
The six changes of state
| Change | Direction | Heat absorbed or released? | Example |
|---|---|---|---|
| Melting熔化 | Solid → liquid | Absorbed | Snow melting |
| Freezing凝固 | Liquid → solid | Released | A pond freezing over |
| Vaporization汽化 | Liquid → gas | Absorbed | Wet clothes drying, water boiling |
| Condensation液化 | Gas → liquid | Released | Dew, droplets on a cold can |
| Sublimation升华 | Solid → gas | Absorbed | Mothballs shrinking, dry ice turning into gas |
| Deposition凝华 | Gas → solid | Released | Frost on grass or windows |
A handy rule: changes in the direction solid → liquid → gas (particles moving farther apart) absorb heat, and changes in the other direction release heat. Particles need energy to pull away from each other’s attraction, and they give that energy back when they come together again.
Vaporization happens in two ways:
- Evaporation蒸发 happens at any temperature, only at the surface of the liquid, and fairly slowly.
- Boiling沸腾 happens only at a particular temperature (the boiling point), throughout the liquid, and vigorously.
The temperature at which a crystalline solidcrystalline solidA solid whose particles are arranged in a regular, repeating pattern, such as ice, salt, or a metal. Crystalline solids have a fixed melting point. Solids without one, such as glass and wax, are called amorphous: they soften gradually as they warm. (such as ice, salt, or a metal) melts is its melting point熔点, and the temperature at which a liquid boils is its boiling point沸点. At standard atmospheric pressure (1 atm), ice melts at 0 °C and water boils at 100 °C. (Words with a dotted underline have a short explanation: hover over them or tap them.)
Note that a change of state only changes the form of a substance. Water is still water: no new substance is formed. In the next lesson you’ll see that this makes it a physical change.
The heating curve
Heat a block of ice at −20 °C over a steady flame, record the temperature at regular intervals, and plot temperature against time. The result is water’s heating curve加热曲线. Before you read it, here’s a quick refresher on reading graphs:
🔧 Math tool card · Reading GraphsSkip it if you already know this
Start with the axes: what quantity does each axis show, and in what unit? In the graph below, the horizontal axis is time (min) and the vertical axis is temperature (°C).
Check the scale: how much is each gridline worth? The vertical axis runs from −40 °C to 140 °C in 9 equal steps, so each step is 20 °C.
Reading a point: from the point, go straight across to the vertical axis and straight down to the horizontal axis, and read both values.
Reading a line: a line going up means the vertical quantity is increasing; a flat line means it isn’t changing; and the steeper the line, the faster the change.
A graph's vertical axis runs from 0 °C to 100 °C in 5 equal steps. How much is each step worth?
100 °C ÷ 5 steps = 20 °C per step.
Drag the slider or click Play to see what state the water is in and what’s happening at each point. Then switch to Cooling to see the reverse:
The times on this graph are simplified, but the key features match a real experiment:
- Sections AB, CD, and EF: there is only one state. The heat absorbed makes the particles move faster, so the temperature rises.
- Sections BC (melting) and DE (boiling): two states exist together, and the temperature stays constant. The heat absorbed goes into pulling particles away from each other and changing their arrangement, not into making them move faster.
- DE is much longer than BC: separating water molecules completely to form a gas takes far more energy than melting does.
Example 1: Reading a melting curve
Lauric acid is a waxy solid often used in school labs. A student heats some and plots the data: from 0 to 4 min the temperature rises from 20 °C to 44 °C; from 4 to 10 min it stays at 44 °C; after 10 min it rises again. What is the melting point of lauric acid? How long did it take to melt? What state was it in at 6 min?
- Find the flat section. The part where the temperature stays constant is the melting, and it’s at 44 °C. So the melting point is 44 °C.
- Read the times. Melting starts at 4 min and ends at 10 min, so it takes 10 − 4 = 6 min.
- Identify the state. At 6 min the lauric acid is melting, so solid and liquid are both present.
- Think about it. The burner was on the whole time, yet the temperature didn’t change from 4 to 10 min. Melting keeps absorbing heat.
Example 2: Why are steam burns worse?
A burn from 100 °C steam is usually worse than a burn from 100 °C boiling water. Why?
- What happens first? When steam touches your cooler skin, it first condenses into water at 100 °C.
- Condensation releases heat. Condensing gives off a large amount of heat, and all of it goes into your skin.
- Then it cools. The 100 °C water formed this way cools down and releases more heat, just like boiling water would.
- Conclusion. Steam delivers the extra heat of condensation on top of what boiling water delivers, so the burn is worse.
- A popsicle turns to liquid in your hand.
Solid to liquid: melting.
- A layer of ice forms on a lake in winter.
Liquid to solid: freezing.
- Wet clothes on a line dry out.
Water goes from liquid to gas and escapes into the air: vaporization (evaporation).
- A cold can of soda “sweats” on a summer day.
Water vapor in the air touches the cold can and turns into droplets: condensation.
- Mothballs in a closet get smaller over time.
The solid turns directly into a gas: sublimation.
- Frost forms on the grass on a cold, clear night.
Water vapor in the air turns directly into tiny ice crystals: deposition.
- The white cloud you see just above a boiling kettle
Water vapor itself is invisible. The white cloud is tiny droplets formed when the vapor condenses in the cooler air.
- A chunk of dry ice (solid carbon dioxide) slowly disappears.
Dry ice turns straight into carbon dioxide gas without becoming a liquid: sublimation.
- Molten steel is poured into a mold to make a steel bar.
Liquid steel turns into solid steel: freezing (solidification).
Common mistakes
Mistake 1: “The white cloud from a kettle is water vapor.” Water vapor is a colorless, invisible gas. The white cloud you see is countless tiny droplets of liquid water formed by condensation.
Mistake 2: “Turning up the burner makes boiling water hotter.” At 1 atm, boiling water stays at 100 °C. A bigger flame only makes it boil away faster.
Mistake 3: “Changes of state are chemical changes.” Ice, liquid water, and steam are all the same substance. A change of state forms no new substance, so it is a physical change.
Quick check
Which of these changes releases heat?
In deposition a gas turns directly into a solid, so the particles come together and release heat. Melting, vaporization, and sublimation all go in the solid → liquid → gas direction and absorb heat.
At 1 atm, what is the temperature of a mixture of ice and water while the ice is melting?
Ice is a crystalline solid, so while it melts the temperature stays at its melting point, 0 °C, no matter how much ice and water there are.
Summary
- Six changes of state: melting, vaporization, and sublimation absorb heat; freezing, condensation, and deposition release heat.
- Vaporization happens by evaporation or boiling. At 1 atm, ice melts at 0 °C and water boils at 100 °C.
- On a heating curve, the sloped sections show rising temperature, while the flat sections (melting, boiling) stay at constant temperature as the heat changes how the particles are arranged and spaced.
- Changes of state form no new substance, so they are physical changes.
On to AP (for later)
In topic 6.5 you’ll calculate the energy of phase changes. Water’s heat of fusion is about 6.01 kJ/mol and its heat of vaporization about 40.7 kJ/mol, which is exactly why the boiling plateau is so much longer than the melting plateau. Heat on the sloped sections is calculated with (topic 6.4). AP questions also ask you to explain that the energy absorbed on a plateau goes into overcoming intermolecular forces (topic 3.1), so the average kinetic energy of the particles doesn’t increase.