L6. Significant Figures and Scientific Notation

Atoms and the Periodic Table

R-report

L6. Physical vs Chemical Change

How can you tell if matter changed its form or became a new substance?

Central Idea:

A physical change alters the form or state of a substance without making a new substance, while a chemical change produces one or more new substances with new formulas (for example, melting ice: H2O(s) → H2O(l) is physical; rusting iron: 4Fe + 3O2 → 2Fe2O3 is chemical).

How to Observe Changes:

Gather the sample and list materials and initial conditions. Watch carefully and note visible changes: shape, state, color, bubbles, cloudiness, or temperature. Try to reverse the change: apply cooling, evaporation, or simple separation to see if the original material returns. Test for new substances: smell safely (if allowed), check for gas release, measure mass, or look for a solid forming in a solution (precipitate). Record temperatures with units (°C) and masses with units (g) before and after if possible. Separate your observations (what you saw) from your explanations (why it happened). If you suspect a chemical change, compare the atoms or formulas before and after to see if new substances appear.

Practice Tasks:

Ask whether the name or atomic composition stays the same: does the formula match before and after? Try to reverse the change (physical changes are usually reversible by simple means). Check for clear signs of chemical change: gas bubbles that are not just boiling, a solid forming from two clear solutions (precipitate), a permanent color shift, or a temperature change without added heat. Measure mass with units (g) before and after: for isolated systems mass should be conserved; large unexplained mass change can signal gas loss or experimental error. Finally, consider a simple micro-level story: are atoms merely rearranged in position, or did they form new bonds and new formulas?

Synthesis:

Rule of thumb: if the substance keeps the same formula and you can undo the change with simple physical methods, it is a physical change; if new formulas, gases, precipitates, permanent color shifts, or heat appear without added energy, it is likely chemical. Use vocabulary like 'precipitate', 'product', 'reactant', and 'state change'. Worked check: 50.0 g ice → 50.0 g water shows a physical change (mass 50.0 g conserved); rust: 4Fe + 3O2 → 2Fe2O3 is chemical and conserves atoms.

Check units on measurements (°C, g). Tell the micro-level story: are atoms rearranged or unchanged? Look for consistent evidence (gas, precipitate, color, temperature). Ask whether the change is easily reversed.