Every fact in the three states-of-matter lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications to be aware of: the particle pictures (particles packed in a solid, sliding in a liquid, far apart in a gas) are a model. Some materials are hard to sort (sand is a solid made of grains that can be poured; jelly and toothpaste are in between). The white cloud above a kettle is tiny drops of liquid water, not the invisible gas. Boiling points depend on pressure: water boils at 100 °C at normal air pressure (1 atm).
All sources
[1] Chemistry: Atoms First, 1.2 Phases and Classification of Matter
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
A solid is rigid with a definite shape; a liquid takes the shape of its container but keeps a fixed volume; a gas expands to fill its container. Plasma is a gas-like state with many charged particles, found in stars and lightning.
OpenStax Chemistry, hosted by Lumen Learning (CC BY 4.0) · Reliability: High
Type: Peer-reviewed open textbook (adapted copy)
Particles in a solid are tightly packed, often in a regular pattern, and vibrate about fixed positions; in a liquid they are close together but move past each other; in a gas they are far apart and move independently except when they collide. The state depends on the balance between attractive forces and the particles' kinetic energy.
Names the changes (melting, freezing, vaporisation, condensation, sublimation, deposition); temperature stays constant during a change of state; water's normal boiling point is 100 °C at 1 atm; heat of fusion of water 6.01 kJ/mol at 0 °C and heat of vaporisation 40.67 kJ/mol at the boiling point; dry ice (solid CO₂) sublimes without forming a liquid.
[4] Chemistry 2e, 9.5 The Kinetic-Molecular Theory
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Gas molecules are in continuous motion in straight lines, are tiny compared with the distances between them, collide elastically, and their average kinetic energy is proportional to the kelvin temperature.
U.S. Geological Survey, Water Science School · Reliability: High
Type: Government science agency
Water's density is about 1 g/cm³; water is densest near 4 °C; on freezing its density drops by about 9 percent, so ice floats and water expands when it freezes.