Answer Key: Physical vs. Chemical Changes

Full credit for any reasonable notice + wonder. Ideal: Notice — "the melted ice can turn back into ice, but burned toast can't turn back into bread." Wonder — "why can one change be undone but not the other?" Accept any on-topic pair. Science note (not required from students): melting is a physical change (same substance), while burning is a chemical change (new substances — smoke, gases, char).

Accept any reasoned prior idea. Target: the burning toast made new substances (black char, smoke, gases); the melted ice is still water. Credit any answer that points to "can't turn it back" or "it looks/smells totally different" as the reason. Do not penalize incomplete prior knowledge — this is a warm-up.

If the same particles appear before and after (only spread apart or rearranged), it is a physical change and no new substance forms. If new particles appear (the starting particles are rebuilt into different combinations), a new substance formed and it is a chemical change. Accept equivalent wording that contrasts "same particles" with "new particles."

A) Ice melting — P. Evidence: only a change of state (solid→liquid), no bubbles/color change, and it can be refrozen — same substance.
B) Iron rusting — C. Evidence: a new flaky orange-brown coating forms with different properties (brittle, doesn't shine/conduct) — a new substance.
C) Salt dissolving — P. Evidence: liquid stays clear and evaporating leaves the salt behind unchanged — same substance, just mixed.
D) Two liquids poured — C. Evidence: a precipitate (cloudy white solid) forms; also a slight temperature change — a new solid substance appeared.
E) Candle burning — C. Evidence: light and heat given off, new odor, and new gases/water vapor produced — new substances form.
F) Paper torn — P. Evidence: pieces are smaller but still white paper; nothing bubbles, heats, or changes color — same substance, size only.
Scoring: give credit for the correct label and a cited observation. A correct label with no evidence earns partial credit.

The precipitate (a new solid forming from two clear liquids) is the strongest evidence, because a brand-new substance with a new state clearly appeared — that is hard to explain any other way. The slight warmth is weaker by itself because temperature can change for physical reasons too (for example, dissolving or simple mixing can warm or cool a liquid without making a new substance). Defensible alternate: a student may argue both signs together make the case strongest; accept if they still identify the precipitate as the more decisive single sign.

Something "disappearing" only means it changed state or spread out too finely to see — not that new matter formed. For ice (A), the water is still there as liquid and can be refrozen; for salt (C), evaporating the water leaves the salt behind unchanged. Both observations show the same substance is still present, so both are physical changes. Full credit for citing the "refrozen" and/or "left behind unchanged" observations.

a) O — recorded directly in Table 1 (cloudy white solid formed).
b) I — "a new substance was created" is a conclusion reasoned from the sign, not a recorded observation.
c) O — recorded directly (flaky orange-brown layer built up).
d) I — "the rust is a different substance" is a reasoned conclusion, not a direct reading.
Observation = recorded/read directly; inference = a conclusion reasoned from evidence.

Full credit if the student uses "new substance." Model: Boiling water's bubbles are water vapor — still water, just in the gas state — so no new substance forms and it is a physical change. The candle's gas, light, and odor are new substances (gases and smoke) made from the wax reacting with air, so it is a chemical change. The key idea: bubbles alone don't prove a reaction; ask whether the gas is a new substance. Accept equivalent wording.

Model answer. Claim: This is a chemical change. Evidence: (1) A dark coating with a different color and dullness appears on the surface — different properties from the shiny silver. (2) The coating is a distinct layer sitting on top of the metal (shiny metal is still underneath). Reasoning: New properties (color, dullness) mean a new substance formed on the surface where silver reacted with something in the air (tarnish). Because a new substance with different properties is present, it is a chemical change, even though only a thin surface layer changed.
Defensible alternate: A thoughtful student may argue it is borderline / partly physical because the coating "rubs off" and the silver underneath is unchanged, which sounds reversible. Accept this as a 2-point answer if it is reasoned — but note the coating itself is a new substance (tarnish), so the best answer is chemical. This ambiguity is the point of the item; reward the quality of the evidence and reasoning, not just the label.

Full credit for connecting both results to atoms only being rearranged. Model: In tarnishing, atoms from the air join the silver, so the coated spoon gains a little mass. In a burning candle, atoms leave as gas, so the leftover candle loses mass. In both, the total mass of everything (solid + gases + air involved) is unchanged — no atoms were made or destroyed. You would have to trap the gases (a sealed container) to weigh them along with the solid and show the total is constant. Accept equivalent reasoning.

Articulate: accept any clear statement of the core question — "Did a new substance form?" (If yes → chemical; if no → physical.)
Connect: the clearest chemical rows are B (rust), D (precipitate), or E (burning); the named sign should match (new coating/different properties; precipitate; light/heat/odor/new gas). Accept any of these.
Extend: any correct non-table chemical change with a matching sign — e.g., bread baking (browning + gas), a cut apple browning (color change), fireworks (light), baking soda + vinegar (gas/bubbles), a battery corroding. More than one answer is defensible.

Reward a two-column chart with at least four real changes correctly sorted, each with a stated sign or same-substance reason (e.g., chopping = physical/shape only; rusting = chemical/new coating; sugar dissolving = physical/can recover it; wood burning = chemical/light + ash + gas). The circled "least sure" item plus a sentence about why it was tricky earns full credit for reflection.

Reward a short numbered plan that: (1) weighs the sealed bag before mixing; (2) breaks the tube to mix without opening; (3) watches for signs — bubbles/gas, color change, temperature change, a precipitate, light, or odor; (4) weighs the bag after to check conservation of mass (mass should be unchanged because the bag is sealed). Best single convincer of a new substance: a change that clearly makes new matter (precipitate or gas) that cannot be undone. Accept any coherent, observation-only procedure.