One substance, three forms
Ice cubes from the freezer, water in a glass, and the invisible water vapor rising from a pot are all the same substance: water. An ice cube keeps its own shape. Water takes the shape of whatever you pour it into. Water vapor spreads out until it fills the whole room.
These are the three most common states of matter: solid固态, liquid液态, and gas气态. Why does one substance behave so differently in each? This lesson answers that with the particle model.
What you can see: the three states
| State | Shape | Volume | Can it be compressed? | Examples |
|---|---|---|---|---|
| Solid | Fixed | Fixed | Hardly at all | Ice, iron, table salt |
| Liquid | Takes the shape of its container | Fixed | Hardly at all | Water, rubbing alcohol, cooking oil |
| Gas | Not fixed | Not fixed: fills the whole container | Easily | Air, water vapor, carbon dioxide |
A propane tank for a backyard grill holds a lot of gas squeezed into a small space, and a bike pump can force plenty of air into a tire. Both work because gases are easy to compress.
The particle model
All matter is made of huge numbers of tiny particles (molecules, atoms, and so on, covered in Module 3). Three statements describe them:
- Particles are tiny. A single drop of water contains about water molecules. ( means 1.7 times 10 to the 21st power: 17 followed by 20 zeros. This way of writing numbers is called scientific notation; the tool card in lesson 1.3.2 covers it.)
- Particles are always moving. The higher the temperature, the faster they move.
- There are spaces between particles, and particles attract each other. The closer they are, the stronger the attraction.
Together these statements make up the particle model粒子模型 of matter. With it, the differences between the states make sense:
- Solids: the particles are packed closely in a regular pattern and held firmly by their attractions. They can only vibrate in place, so a solid keeps a fixed shape and volume.
- Liquids: the particles are still close together, but not in a regular pattern, and they can slide past each other. That’s why liquids flow and take the shape of their container. There’s almost no empty space between the particles, so the volume is fixed and hard to compress.
- Gases: the particles are far apart, attract each other only weakly, and move quickly and randomly in all directions. That’s why a gas fills its whole container and is easy to compress.
The container below always holds the same 30 particles. Click Solid, Liquid, and Gas to see how the particles are arranged and how they move, then push the piston to see which state can be compressed:
| Spacing | Very close, in a regular pattern |
|---|---|
| Motion | Vibrate in fixed positions |
| Shape | Fixed |
| Volume | Fixed |
| Compressible? | Hardly at all |
Notice that the number and size of the particles never change from one state to another. Only their arrangement, spacing, and speed change.
Example 1: The syringe test
Fill one syringe with air and another with water. Block the tip of each with your finger and push the plunger. Which one moves easily, and why?
- Observe. The plunger of the air-filled syringe pushes in a long way. The water-filled one barely moves.
- Think about particles. Gas particles have large spaces between them. Pushing the plunger shrinks those spaces, so the volume goes down.
- Compare with the liquid. Liquid particles have almost no space between them, so there’s nowhere to squeeze them into. The liquid is very hard to compress.
- Conclusion. A gas is easy to compress and a liquid isn’t, because of the difference in particle spacing.
Example 2: Why does steam take up so much more room?
At 100 °C, water that turns into steam at the same temperature takes up about 1600 times as much volume. Did the water molecules get bigger?
- Number. No new water molecules appear when water turns into steam.
- Size. Each water molecule stays the same size.
- Spacing. In liquid water the molecules are close together. In steam, the spaces between them become huge.
- Conclusion. The volume grows because the spaces between molecules grow, not because the molecules get bigger or more numerous.
- You open a bottle of perfume, and soon you can smell it across the room.
Perfume particles keep moving and spread throughout the room.
- Propane gas can be squeezed into a small tank.
Gas particles are far apart. Under pressure, the spaces between them shrink.
- Mixing 50 mL of water with 50 mL of alcohol gives less than 100 mL.
The two kinds of particles differ in size, so some particles fit into the spaces between the others.
- Wet clothes dry faster in the sun than in the shade.
At a higher temperature, water particles move faster and escape into the air more easily.
- Two very smooth, flat blocks of lead pressed firmly together stick to each other.
When the surfaces get close enough, attractions between the particles hold the two blocks together.
- The liquid in a thermometer rises when it's heated.
Heating makes the spaces between particles bigger, so the liquid expands. The particles themselves don't grow.
- Food coloring spreads faster in hot water than in cold water.
The higher the temperature, the faster the particles move.
- A solid iron bar is very hard to pull apart.
The particles in iron attract each other strongly.
Common mistakes
Mistake 1: “Things expand when heated because their particles get bigger.” The particles stay the same size. What grows is the space between them.
Mistake 2: “The particles in a solid don’t move.” Particles in a solid are always moving too. They just vibrate in place instead of moving around.
Mistake 3: “Gases are invisible, so they have no mass.” Gases are matter, and they have mass. At room temperature, 1 L of air has a mass of about 1.2 g.
Quick check
Which observation best shows that gas particles are far apart?
A gas can only be squeezed because there's empty space between its particles. B shows that particles move, C is a change of state, and D shows strong attraction between particles in a solid.
When water freezes into ice, which statement is correct?
The water molecules themselves don't change when water freezes. Only their arrangement and spacing do, and the molecules in ice are still vibrating.
Summary
- A solid has a fixed shape and volume. A liquid has a fixed volume but takes the shape of its container. A gas has no fixed shape or volume and is easy to compress.
- The particle model: particles are tiny; they are always moving, faster at higher temperatures; and there are spaces between them, along with attractions.
- The states differ in how their particles are arranged, spaced, and moving. The size and number of the particles stay the same.
On to AP (for later)
AP Chemistry topic 3.3 expects you to draw and interpret particle diagrams of solids, liquids, and gases. Topic 3.1 explains what the attractions between particles actually are (intermolecular forces), and topic 3.5, kinetic molecular theory, shows quantitatively that a higher temperature means a higher average kinetic energy of the particles.