Density is mass per unit volume (d = m/V), a measure of how tightly particles are packed in a sample. In AP Chemistry, density differences between solids, liquids, and gases come straight from particle spacing, which you explain using the particulate-level models in Topic 3.3.
Density is the amount of mass packed into a given volume, calculated as d = m/V (usually g/mL or g/cm³ for solids and liquids, g/L for gases). It's an intensive property, meaning it doesn't change with sample size. A drop of water and a swimming pool of water have the same density.
In AP Chem, density is really a story about particle spacing. In solids and liquids, particles are in close contact, so these phases are dense and nearly incompressible. In gases, particles are far apart relative to their size, so gases are much less dense and easily compressed. The CED also points out that a solid's structure depends on interparticle interactions and how well particles can pack together, which is why two solids made of similar atoms can still have different densities. When the exam asks you about density, it's usually testing whether you can connect the macroscopic number to a particulate-level picture.
Density lives in Unit 3: Properties of Substances and Mixtures, specifically Topic 3.3: Solids, Liquids, and Gases, supporting learning objective 3.3.A: represent the differences between solid, liquid, and gas phases using a particulate-level model. The huge density gap between gases and condensed phases (solids and liquids) is one of the clearest pieces of evidence for those models. Gas particles are spread out with mostly empty space between them; liquid and solid particles are touching. Density also threads through the rest of Unit 3, since gas density connects to the ideal gas law and molar mass, and through lab-based questions where d = m/V is your tool for identifying substances or converting between mass and volume.
Keep studying AP Chemistry Unit 3
Particulate-Level Model (Unit 3)
Density is the macroscopic fingerprint of particle spacing. If you're asked to draw or interpret a particle diagram, the denser phase should show particles closer together. A gas diagram with particles crammed edge-to-edge is an instant wrong answer.
Mass and Volume (Unit 3)
Density is the ratio that ties these two together, and it converts between them in stoichiometry problems. If a problem gives you the volume of a liquid and you need moles, density is the bridge from volume to mass before molar mass takes you to moles.
Ideal Gas Law and Gas Density (Unit 3)
Rearranging PV = nRT gives gas density as d = PM/RT. That means heavier gases are denser at the same conditions, and heating a gas at constant pressure makes it less dense. This is the logic behind why hot air rises and why you can find a gas's molar mass from its density.
Interparticle Forces and Solid Packing (Units 2-3)
The CED says solid structure is shaped by interparticle interactions and how well particles pack. Crystalline solids with efficient, orderly packing tend to be denser than amorphous solids of the same substance, because order leaves less wasted space.
Density shows up in two main ways. First, in particulate-model questions like the Topic 3.3 multiple-choice stems asking why gases lack a definite shape or what solids and liquids share. The expected reasoning is about particle spacing and motion, which is density at the particle level. Second, in quantitative problems where d = m/V converts volume to mass (or vice versa) inside a stoichiometry chain, or where d = PM/RT links a gas's density to its molar mass and conditions. Gas-behavior FRQs, like the 2021 question with O₂(g) in a piston-and-cylinder setup, reward you for reasoning about how the amount of gas, volume, and conditions relate. Whatever the format, you have to do more than plug into the formula. You need to explain the answer in terms of what the particles are doing.
Density is an absolute measurement with units (like 1.84 g/mL for concentrated sulfuric acid). Specific gravity is that density divided by the density of water, so it's unitless. They tell you the same physical story, but if a problem hands you a specific gravity, multiply by water's density (about 1.00 g/mL) to get an actual density you can use in calculations.
Density equals mass divided by volume (d = m/V), and it is an intensive property that stays the same no matter how big the sample is.
Solids and liquids are dense because their particles are in close contact, while gases are far less dense because their particles are spread out with mostly empty space between them.
On particulate-model questions, the denser phase must be drawn with particles closer together, and gases must show large gaps between particles.
For gases, density depends on molar mass, pressure, and temperature through d = PM/RT, so heavier gases and colder, higher-pressure gases are denser.
Solid density depends on how well particles pack, which is influenced by interparticle interactions, so crystalline order generally packs tighter than amorphous disorder.
Specific gravity is just density compared to water, so it has no units, while density always carries units like g/mL or g/L.
Density is mass per unit volume, d = m/V, typically in g/mL for liquids and solids or g/L for gases. In AP Chem it's tied to Topic 3.3, where you explain density differences between phases using particle spacing.
Usually, but not always. Water is the famous exception, since ice floats because its crystalline structure holds H₂O molecules in an open arrangement that's less dense than liquid water. The safe AP claim is that solids and liquids are both far denser than gases.
Density has units (g/mL); specific gravity is density divided by the density of water, so it's a unitless ratio. A specific gravity of 1.84 means a density of about 1.84 g/mL, since water is about 1.00 g/mL.
Gas particles are far apart relative to their size, so most of a gas sample is empty space. Solid and liquid particles are in close contact, which packs much more mass into the same volume. This spacing difference is also why gases are compressible and condensed phases aren't.
Rearrange the ideal gas law to d = PM/RT, where M is molar mass. This also works in reverse: measure a gas's density at known pressure and temperature, and you can solve for its molar mass, a classic AP Chem calculation.