Ice melts, but a cold cup gets wetter — Physical Science: Matter, Forces & Energy

A cup of ice water sits in a warm room. Inside, ice becomes liquid. Outside, new droplets appear on a surface that began dry. A thermometer can remain near the same temperature for part of the melting interval. These three observations require an account of matter and energy, rather than the statement “the cold escapes.” Water vapor is gaseous water already present in the air and is ordinarily invisible; the visible outside droplets are liquid.

Use a boundary to identify the source of water and the direction of energy transfer. A boundary is the chosen separation between the system being studied and its surroundings. Temperature relates to average particle motion, while total internal energy also depends on the amount and interactions of particles. Energy input can change particle arrangements during a phase change without the same temperature rise seen in single-phase warming.

The chapter moves from states and particle motion to temperature, gas pressure, evaporation and heating curves. Ask why air resists squeezing, why water dries below boiling and why a heating graph can flatten while energy continues to enter. Predict first, then use supplied records or low-risk observations. Revise a particle drawing by changing spacing and motion, never by turning water molecules into larger molecules simply because the sample expands.