Grade 7 Chemistry • Official Chapter II: Substances and their structure
Solids, Liquids, and Gases
Connect what matter does at human scale with how its particles are arranged and move.
Big idea and learning goals
Matter has mass and occupies space. A state of matter describes a recognizable set of physical properties caused by the arrangement, spacing, motion, and attractions of particles. In this lesson we focus on solid, liquid, and gas.
By the end, you should be able to:
- compare shape, volume, flow, density, and compressibility in the three states;
- draw and interpret particle diagrams without changing particle identity;
- use the particle model to explain rigidity, flowing, diffusion, and gas pressure;
- distinguish an observation from a model-based explanation;
- apply the model to safe, worldwide examples from homes, nature, medicine, and engineering.
1. Three scales of description
Chemists move between three connected levels. At the macroscopic level we observe a block keeping its shape, water flowing, or air expanding into a balloon. At the particle level we explain those observations using atoms, molecules, or ions too small to see directly. At the symbolic level we may write a formula and a state symbol, such as H2O(s), H2O(l), or H2O(g).