L5. Measurement and Units

Matter and Measurement

R-report

L5. Measurement and Units

How can we tell if a change is only physical (shape/state) or if a new substance formed?

Central Idea:

Physical changes alter the form or state of a substance without creating new substances (for example, H2O(s) → H2O(l) when ice melts). Chemical changes produce one or more new substances with different formulas or composition (for example, iron rusting to form iron oxide, represented by a balanced equation).

How To Observe and Record Changes

Set up a clear observation plan: measure mass (include units, e.g., g), note color, temperature (°C), and whether bubbles, a solid precipitate, or a smell appear. During a melting test, weigh the closed container with ice, let it melt, then re-weigh the closed container and record the mass (example: 50.0 g → 50.0 g). For rusting, record the initial mass of the iron sample and the mass after exposure in a closed system if possible, and note color and texture changes. Separate observations (what you see) from explanations (why it might have happened). When possible, run a simple control: identical sample kept without the suspected cause (e.g., nail kept dry beside a wet nail).

Apply To New Situations:

Compare what stays the same and what changes by asking focused questions: does the chemical formula change? (If not, likely physical.) Is mass conserved in a closed system? (Yes for both, but watch for escaping gas.) Do you see gas bubbles, a new solid that forms from two liquids (precipitate), a persistent color shift, or a temperature change that continues after mixing? Use these simple tests to falsify a physical-change hypothesis: try to reverse the change (can you refreeze the water?), look for gas or precipitate that you would not get by just changing shape, and re-measure mass with units. If a change is reversible by a pure physical process (freezing, dissolving and filtering back, cutting) it is probably physical; if new, separable substances appear, it is chemical.

Synthesize Ideas:

Keep useful vocabulary at hand: physical change, chemical change, precipitate, reactant, product, reversible, closed system, and units (g, °C, s). A worked example: measure 50.0 g of ice in a closed cup (50.0 g H2O(s)); after melting you record 50.0 g H2O(l). The formula H2O stays the same, so the molecules are unchanged. For rusting, write and check a balanced symbolic equation to follow atoms: 4 Fe + 3 O2 → 2 Fe2O3 (atoms conserved: 4 Fe, 6 O). Always include units in measured values (for mass use grams, for temperature use °C) and check that your chemical equations conserve atoms and charge.

Final checks before you conclude: 1) Confirm units are present for every measured number (e.g., 12.0 g, 25 °C). 2) Tell the micro-level story: what happens to molecules or atoms? 3) Match evidence to explanation: gas/precipitate/color/temperature changes point to chemical change; reversible state or shape change points to physical. Repeatable sequence: observe → measure with units → test reversibility → look for new substance evidence → write balanced equation or molecular formula check.