100 topics · Read, check sources and follow your curiosity.

A little room to wonder

States & Changes of Matter

Solids, liquids and gases differ in their collective particle arrangements and motion. A change of state does not require a change of chemical identity.

What changes when ice melts, and what remains the same?

Ice pieces sit in a glass of water with a lemon wedge; droplets appear on the outside.
Jon Sullivan’s photograph shows solid ice and liquid water together. The photograph alone does not give temperature, purity or a measured rate of phase change.Jon Sullivan · Source · Public domain

Describe behaviour at two scales

A solid maintains its shape under ordinary conditions, a liquid flows while keeping an approximately fixed volume, and a gas spreads to occupy its available container. These large-scale behaviours can be related to how particles move and interact.

Particles in a solid are not motionless: they move about positions in the structure. In a liquid they can move past one another while remaining close. In a gas they are much farther apart on average. The dots in a diagram stand for particles; changing a state does not mean the individual dots must swell into larger atoms.

A phase change transfers energy

Melting changes a solid to a liquid; freezing reverses it. Vaporisation forms a gas from a liquid, while condensation forms a liquid from a gas. For a pure substance at a fixed pressure, energy supplied during melting or boiling can change the phase while temperature remains approximately constant.

Evaporation can occur at a liquid’s surface below its boiling point. Boiling involves vapour formation within the liquid as well. A visible white cloud near water is made of tiny liquid droplets, not the invisible water-vapour gas itself.

Worked example: interpret a fictional heating record

A simplified record for pure water near ordinary atmospheric pressure gives these times and temperatures: minute 0, −10°C; minute 1, −5°C; minute 2, 0°C; minute 3, 0°C; minute 4, +5°C. The equal readings at minutes 2 and 3 are compatible with melting while energy continues to enter.

They do not prove that the heater stopped or that no energy was transferred. To support the melting explanation, you would also record whether solid and liquid were present. After the solid has melted, further heating can raise the liquid temperature. This fictional record illustrates interpretation; it is not an instruction to heat water or handle ice with laboratory equipment.

Keep substance and state separate

An H₂O molecule remains H₂O when water changes between ice, liquid water and water vapour. A chemical reaction, by contrast, changes the substances present. “It looks different” is insufficient on its own to classify the change.

Pressure, purity and conditions matter to transition temperatures. The statement “water boils at 100°C” needs its usual pressure condition; it is not true at every altitude or pressure. Also, not every solid is denser than its liquid: water is a familiar exception. Models should explain their intended case instead of turning a pattern into an absolute rule.

Only if you feel like trying

Read the plateau

  1. Plot the fictional time–temperature pairs on paper, or describe where the temperature changes and where it stays level.
  2. Annotate the 0°C interval with a possible explanation and the additional observation needed to support it.
  3. Draw particle sketches for solid and liquid water using the same number and kind of particles.

Optional. Work on paper, point to the example or talk it through. No account, purchase or public sharing is needed.