Intermolecular Forces: In solids, these forces are strong enough to lock particles into a fixed lattice. In liquids, they are strong enough to keep particles close but allow them to slide past one another. In gases, these forces are negligible due to high speeds and large distances.
Thermal Energy: Adding heat increases the kinetic energy of particles. When kinetic energy exceeds the energy of the attractive forces, a phase change occurs (e.g., melting or boiling).
Mean Free Path: This is the average distance a particle travels before colliding with another. In gases, the mean free path is very large compared to the particle size, explaining their low density and high compressibility.
Determining State by Properties: To identify a state, observe if the substance has a fixed shape (solid), takes the shape of the container bottom (liquid), or fills the entire container volume (gas).
Calculating Density: Measure the mass using a balance and the volume using geometric formulas (for regular solids) or water displacement (for irregular solids). Use the formula .
Predicting Phase Changes: Use heating curves to identify the temperature points where a substance transitions between states. During a phase change, the temperature remains constant even as energy is added.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Particle Arrangement | Regular, repeating lattice | Random, close together | Random, far apart |
| Particle Motion | Vibration around fixed points | Flowing/sliding past each other | High-speed, random motion |
| Shape | Fixed | Variable (takes shape of container) | Variable (fills container) |
| Volume | Fixed | Fixed | Variable |
| Compressibility | Negligible | Very low | High |
Solids vs. Liquids: While both have high densities and fixed volumes, only liquids can flow because their particles are not locked in a rigid structure.
Liquids vs. Gases: Gases are roughly 1000 times less dense than liquids because gas particles are separated by vast amounts of empty space.
Unit Consistency: Always ensure mass is in kilograms () and volume is in cubic meters () when calculating standard density (). If using , be consistent across all variables.
Interpreting Diagrams: If an exam shows a box with a few dots at the bottom, it represents a liquid. If the dots are spread throughout the box, it represents a gas.
Conservation of Mass: Remember that mass does not change during a phase change. If of ice melts, it becomes exactly of water, even though the volume may change slightly.
Sanity Check: If your calculated density for a gas is higher than that of a solid, re-check your volume calculations; gas density should be significantly lower.
