Measure matter before and after breaking and dissolving

Learning goal: Use measurements to investigate conservation of matter when materials are divided, rearranged or dissolved. Matter has mass and takes up space. A change in appearance does not automatically mean a change in total amount. To test whether matter is conserved, define the system being measured and account for every material that can enter or leave it. Scope: 4.PS.1 names three cases (breaking into pieces, dissolving and changing state). This lesson investigates the first two; the companion lesson Track matter through melting, freezing and evaporation investigates the third. Assess all three cases after both lessons.

A system is the collection included in the comparison. For a clay investigation, it might include the clay and its tray. Measure the complete system, divide the clay into several pieces on the tray and measure again. Keep every crumb on the tray. The number and shape of the pieces can change while their combined mass remains the same. Rearranging the pieces into a new shape does not add matter.

A fictional ten-gram clay piece can become pieces of four, three, two and one gram. Their masses sum to ten grams. This is an example calculation, not a report of a real measurement. A real balance may have limited precision and repeated readings may differ slightly. Record the displayed values and the instrument's smallest unit rather than changing results to fit an expected equality.

Dissolving presents a less obvious case. When a small amount of salt dissolves in water, the separate grains may no longer be visible. The salt has not stopped being matter. The resulting solution contains water and dissolved salt. Its mass equals the combined mass of the materials when nothing has entered or escaped. Visibility is not a measure of whether material exists.

An adult can prepare a wide, transparent plastic container with a small amount of water around a stable inner cup of dry salt. Keep the water below the inner cup's rim so the salt remains separate initially. Close the outer container securely and measure the entire setup, including the inner cup and outer lid. Gently tip the closed setup to release the salt into the water, mix without opening it and measure again. Every component stays inside the same closed boundary throughout the change.

Keep the water amount small enough for easy handling and use ordinary classroom-approved materials without tasting. If salt spills or sticks to an excluded spoon, the measured system has changed. If a lid is present before but missing afterward, the comparison cannot isolate dissolving. Draw the system boundary and list everything on the balance at each measurement.

Conservation concerns the total mass, not necessarily the height of liquid or the apparent size of each visible component. Water in a tall narrow container reaches a higher level than the same water in a wide container. Mixing different materials also does not guarantee that their separate volumes add exactly. Do not use liquid height alone to judge the amount of matter.

A good conclusion uses the measured evidence and its limits. If repeated complete-system masses agree within the balance's resolution, the observations support conservation during the tested change. If the reading differs substantially, first check spills, omitted pieces, evaporation, wet container exteriors and balance setup. A mismatch is a reason to investigate the procedure, not to erase data or claim that dissolving destroys matter.

Worked system accounting: In fictional measurements, a container and water together have mass 150 grams. Ten grams of salt are added and retained. The expected total is 160 grams when all other included components are unchanged. If the final system is reported as 158 grams, investigate whether salt or water was lost and whether the same components were weighed.

Worked reasoning: Four clay pieces occupy more spread-out space on a tray than one compact lump. That arrangement does not establish more matter. Measure all pieces together and compare their combined mass with the original lump.

Guided investigation: Conduct the clay comparison first, then the dissolving comparison with adult preparation. Write a system inventory, record actual before-and-after masses, repeat readings and note the balance resolution. Keep sketches showing which properties changed and which measured property stayed approximately constant.

Independent evidence: Explain why the number of pieces and disappearance of visible grains do not establish matter loss. Analyze a trial with an omitted lid or a spill and redesign it. Use actual records to support a conservation claim, including uncertainty. Complete the companion state-change lesson before generalizing this evidence to melting or evaporation.