Worked example
A capped flask and its contents have mass 220 g. Water evaporates into the headspace. Because the flask stays closed, the vapor is included in the same 220 g total within measurement uncertainty. Vapor has mass even when it cannot be seen.
Contrast and common error
If a flask is open and loses water vapor, the weighed contents can become lighter. If a solid combines with a gas supplied by the air, the weighed solid can become heavier. In each case draw the measurement boundary first and explain what passes through.
Transfer check
A sealed bag containing an ice cube and trapped air has mass 48 g. The ice melts without leaking. What mass is expected after it melts? Check: 48 g within measurement uncertainty. The phase changes while all material remains inside.
Try it, then explain it.
Complete a short set without looking at the example. After each answer, explain which evidence or rule supports your decision and why the alternatives do not fit.
Sources & authorship
Original OpenExam explanations and examples. References support the concepts; they are not the source of copied questions. Independent educator review is pending.
OpenStax Chemistry 2e 1.2 Phases and Classification of Matter