Track matter through melting, freezing and evaporation

Learning goal: Investigate conservation of matter during state changes by keeping track of the entire system. Water can be solid ice, liquid water or invisible water vapor. Its appearance and occupied space can change. Those changes do not make matter appear from nothing or disappear into nothing. A measurement must include every place the water can go. Scope: 4.PS.1 names three cases (breaking into pieces, dissolving and changing state). This lesson investigates state change; breaking and dissolving are investigated in the companion lesson Measure matter before and after breaking and dissolving. The same rule links all three: when every piece stays inside the measured system, a 10 g clay lump cut into pieces, salt dissolved in water and ice melted in a closed container each keep the same total mass.

Place a small ice sample in a dry, securely closed clear plastic container prepared by an adult. Include the container and lid in the initial mass. Let the ice melt at room temperature while the container remains closed, then measure the same complete system. The ice changes from solid to liquid while remaining inside. The expected total mass is unchanged within the instrument's precision.

A dry exterior matters because water droplets on the outside would add material to the measured system. An adult can keep the outside dry consistently and document any handling. Do not remove liquid from inside to make the reading match. Record the real values, including small differences, and consider the balance resolution and possible leaks before interpreting them.

Freezing also changes state without creating matter. An adult can freeze a small amount of water in a freezer-safe flexible container with ample expansion space, following the container's instructions. Compare the entire closed system before and after. Water expands as it freezes, so never fill a rigid sealed container completely. A larger occupied volume does not mean a larger mass when the same water remains in the system.

Evaporation can make a visible puddle shrink as liquid becomes water vapor and mixes with the air. In an open cup, some water leaves the measured cup system. The cup-plus-water mass can therefore decrease. That result does not contradict conservation of matter: the water is now elsewhere. The missing amount is not included when the balance measures only the remaining cup contents.

To observe another part of the process, place a small amount of water in a securely closed transparent classroom container and watch over time without strong heating. Some liquid may become vapor; cooler interior surfaces may later show droplets when vapor condenses. The water can change location and state within the closed boundary. The whole container system retains its matter if it does not leak.

Visible droplets are liquid water, while the water vapor between them can be invisible. Condensation is the change from vapor to liquid. Droplets forming on the outside of a cold container generally come from water vapor in the surrounding air, not water passing through an intact container wall. An outside droplet belongs to a different path of matter than the water sealed inside.

Compare the questions asked by an open and a closed investigation. The open-cup question can ask how much water leaves the cup over a stated interval. The closed-container question asks whether the mass of the complete retained system changes as water changes state. Both are useful, but they measure different boundaries. An explanation should name the boundary before concluding that an amount was gained or lost.

Worked fictional comparison: A sealed container and ice have mass 120 grams before melting and 120 grams afterward. The evidence supports unchanged total mass in that system. In a separate open-cup example, the mass falls from 120 grams to 116 grams over time. Four grams left that measured cup system; the numbers alone do not identify every destination or exclude a spill.

Worked critique: A learner says a growing ice volume proves new water was created. The claim confuses occupied volume with total mass. Measure the complete retained sample before and after rather than using its height or shape as a substitute.

Guided investigation: Plan and conduct the sealed melting comparison, draw the boundary and record actual mass, appearance and time. Observe a separate covered-versus-open water setup over several days only with adult supervision and clearly labeled containers. Keep all other conditions as similar as practical and record evidence of droplets or leaks.

Independent evidence: Explain a closed-system melting result and an open-cup decrease without claiming matter was destroyed. Identify the source of outside condensation and describe how the measurement boundary affects the conclusion. Keep experiment records and equipment observations; a correct conservation sentence alone does not demonstrate a controlled measurement.