AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/3/3-3)
Unit 3 · Topic 3.3
3.3 Solids, Liquids, and Gases
Solids, liquids and gases differ in how close their particles are and how freely they move. Solid particles vibrate in place, liquid particles touch but slide past each other, and gas particles are far apart and move freely, which is why a gas fills its container.
Key terms
- crystalline solid
- amorphous solid
- particulate model
- molar volume
- phase
Solids
In a solid, particles stay in fixed positions. They vibrate, but they don't move past one another. A crystalline solid has its particles in a regular, repeating 3-D pattern; table salt and ice are crystalline. An amorphous solid has no long-range order, so its particles are arranged irregularly; glass and many plastics are amorphous.
How a solid's particles pack depends on the attractions between them and on their shapes. Water molecules in ice form hydrogen bonds in an open hexagonal arrangement, which is why ice is less dense than liquid water.
Liquids
In a liquid, particles are still in contact but move and slide past one another. That's why liquids flow and take the shape of their container, while keeping a definite volume.
How the particles move and arrange themselves depends on the IMFs (polarity, hydrogen bonding) and on temperature. Strong attractions keep liquid particles more tightly associated.
Because particles touch in both phases, a substance's solid and liquid have similar molar volumes (volume per mole). For water, a mole of liquid takes up about 18.0 mL and a mole of ice about 19.7 mL.
Gases
Gas particles are in constant, random motion and are, on average, far apart compared with their own size. Attractions between them have little effect. A gas has no definite shape or volume: it spreads out to fill any container.
How often gas particles collide, and how far apart they are on average, depends on temperature, pressure and volume. Squeeze the gas into a smaller volume, and the particles are closer together and collide more often.
Changing phase changes how the particles are arranged, not what they are. Melting and boiling take in energy to overcome attractions between particles; freezing and condensing release energy as attractions re-form. Topic 6.5 puts numbers on those energy changes.
Drawing the three phases
- Solid: particles touching, in neat rows for a crystalline solid or jumbled but touching for an amorphous one.
- Liquid: particles touching but disordered, with a few small gaps. About the same number of particles in about the same space as the solid.
- Gas: a few particles spread widely apart, with lots of empty space between them.
- In every phase, molecules stay intact. Water vapor is still H₂O molecules, not separate H and O atoms.
You won't be tested on
Phase diagrams, the graphs of pressure versus temperature that show where each phase is stable, aren't on the AP exam.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Comparing molar volumes
At 0 °C, ice has a density of 0.917 g/mL; liquid water near room temperature has a density of about 1.00 g/mL. Water vapor at 100 °C and 1.00 atm behaves nearly ideally. Compare the volume of 1 mole of water in each phase.
Show the solutionHide the solution
- Step 1: Molar mass of water = 18.02 g/mol.
- Step 2: Ice: 18.02 g ÷ 0.917 g/mL = 19.7 mL.
- Step 3: Liquid: 18.02 g ÷ 1.00 g/mL = 18.0 mL.
- Step 4: Gas: V = nRT/P = (1.00)(0.08206)(373.15) ÷ 1.00 = 30.6 L = 30,600 mL.
- Step 5: The gas takes up about 1700 times the volume of the liquid, because gas particles are far apart while solid and liquid particles touch.
Answer: About 19.7 mL (ice), 18.0 mL (liquid) and 30.6 L (vapor at 100 °C, 1.00 atm)
- Example 2
Checking a particle diagram (classic trap)
A student draws boiling water as a box at the bottom containing tightly packed H₂O molecules, and above it, separate H atoms and O atoms floating far apart. What's wrong?
Show the solutionHide the solution
- Step 1: Boiling is a physical change. It overcomes the hydrogen bonds between molecules, not the covalent O–H bonds inside them.
- Step 2: The gas phase should show intact H₂O molecules, far apart.
- Step 3: The liquid part should show molecules touching but disordered, not neatly packed like a crystal.
Answer: The vapor should contain intact H₂O molecules spread far apart, and the liquid should be disordered; boiling doesn't break molecules into atoms.
Common mistakes
- Drawing liquids with particles spread far apart, like a gas. Liquid particles are in contact.
- Thinking solid particles don't move at all. They vibrate in place.
- Breaking molecules apart in drawings of phase changes.
- Assuming all solids are crystalline. Glass is an amorphous solid.
On the exam
- Expect to choose or draw particle diagrams for different phases. Get spacing, order and the number of particles right, and keep molecules intact.
- Questions may ask you to explain why compressing a gas is easy but compressing a liquid isn't. The answer is the empty space between gas particles.
Connected topics
Videos
Check yourself
4 questions on 3.3 Solids, Liquids, and Gases. Pick an answer to see if you got it, and why.
One mole of ice occupies about 19.6 mL and one mole of liquid water about 18.0 mL, but one mole of water vapor at 100 °C and 1.00 atm occupies about 30,600 mL. Which of the following best explains these data?
Quartz is crystalline SiO₂, while ordinary window glass is mostly amorphous SiO₂. When heated, quartz melts at a definite temperature, but glass softens gradually over a range of temperatures. Which of the following best explains this?
A syringe of air can easily be pushed to half its volume, but a syringe full of water hardly compresses at all. Which of the following best explains this difference at the particle level?
A closed flask holds liquid water and water vapor at a constant 25 °C. Which of the following correctly compares the molecules in the vapor with the molecules in the liquid?
0 of 4 answered