1.1.4 · D5Matter, Measurement & the Mole
Question bank — Physical vs chemical change
How to use this bank: each line is a trap laid by a common misconception about physical vs chemical change. Cover the answer, commit to a full-sentence justification (never bare "yes/no"), then reveal.
Prerequisite ideas leaned on here: States of Matter, Chemical Reactions, Energy in Chemistry, Conservation of Mass, Molecular Structure, Identification of Substances.
True or false — justify
A change that gives off heat must be chemical.
False — many physical changes release heat too; water freezing releases about 6 kJ/mol (this is the enthalpy of fusion of ice, the energy freed as liquid molecules settle into fixed hydrogen-bonded lattice positions), yet the molecule stays H₂O, so identity is unchanged.
If a process is reversible, it cannot be chemical.
False — some chemical reactions reverse readily (e.g. hydrated ↔ anhydrous copper sulfate on heating/cooling); reversibility hints at physical but is not a guarantee.
Melting always keeps the same molecular formula.
True — melting only overcomes intermolecular forces between molecules, so the molecules themselves (their covalent bonds) survive intact.
Dissolving salt in water is a chemical change because ions form.
Debatable/mostly physical — the crystal splits into Na⁺ and Cl⁻ ions that already existed in the ionic lattice, and evaporating the water gives back identical salt, so no new substance is created.
A large energy change always signals a chemical reaction.
False — it strongly suggests one (hundreds of kJ/mol usually means covalent bonds broke, per the energy-scale figure), but sublimating a strongly-bonded solid or vaporising a metal can also demand large energy without breaking covalent bonds within molecules.
Burning magnesium and boiling water both produce gas, so both are chemical.
False — boiling water gives H₂O vapour (same molecule, physical), while burning Mg makes new MgO; producing gas alone does not decide the case.
Conservation of mass holds in physical changes but is broken in chemical ones.
False — Conservation of Mass holds in both; atoms are only rearranged in a chemical change, never created or destroyed, so total mass is unchanged either way.
Cutting a magnet in half changes its identity because each piece is now a new smaller magnet.
False — it is physical; each piece is the same material with the same molecular structure, just smaller in form.
Spot the error
"Steam condensing on a mirror is chemical because a new liquid appeared."
Error — no new substance appears; H₂O(g) → H₂O(l) is a state change, the same molecule in a denser phase.
"Rusting can be reversed by drying the nail, so it's physical."
Error — drying removes water but leaves Fe₂O₃; you cannot recover shiny metallic iron by simple physical means, so it is chemical.
"Sugar caramelising and sugar dissolving are the same kind of change since both involve sugar and heat."
Error — dissolving disperses intact sucrose (physical), but caramelising decomposes it into new brown compounds (chemical); heat alone doesn't classify a change.
"CO₂ bubbling out of soda proves a reaction occurred in the glass."
Error — the CO₂ was dissolved gas escaping (physical release), not newly made; contrast this with vinegar + baking soda where CO₂ is genuinely produced.
"An exothermic bond-forming step means energy was destroyed."
Error — energy is conserved; forming bonds releases stored energy to the surroundings, it is not annihilated (see Energy in Chemistry).
"Because rust weighs more than the iron, mass was created."
Error — the extra mass is oxygen (and water) from the air bonding to iron; count the whole system and mass is conserved.
"Ice, water, and steam are three different substances."
Error — they are three States of Matter of one substance, H₂O; only the arrangement and motion of the molecules differ.
Why questions
Why do chemical changes typically involve 10–100× more energy than physical changes?
Because chemical changes break/form covalent bonds (~400–800 kJ/mol) inside molecules, while physical changes only overcome weaker intermolecular forces such as hydrogen bonds (~10–40 kJ/mol) or dispersion (~0.1–10 kJ/mol) between molecules — exactly the gap shown in the energy-scale figure.
Why isn't "color change" a foolproof test for chemical change?
Because color can shift physically — e.g. iodine looks purple as a crystal but brown dissolved in alcohol — without any new molecule forming; only an intrinsic electronic change from a new compound is chemical.
Why does cooling a gas back to its original liquid confirm a physical change?
Because recovering the identical starting substance by a simple physical operation shows the molecular identity never changed (see Identification of Substances).
Why can a phase change like boiling look dramatic yet stay physical?
Because the visible upheaval is molecules separating and speeding up, not their internal covalent bonds breaking; the molecular formula is identical before and after.
Why does baking a cake count as chemical even though you started with edible ingredients?
Because heat drives several irreversible reactions (CO₂ release from baking soda, protein denaturation/cross-linking, Maillard browning) that create new substances you cannot un-mix.
Why is checking the molecular formula more reliable than checking appearance?
Because appearance (color, bubbles, texture) can change physically, but a changed molecular formula directly proves bonds rearranged into new structures — the definition of a chemical reaction.
Edge cases
Water at exactly 0 °C sitting as ice + liquid together — physical or chemical?
Physical — melting/freezing equilibrium is a coexistence of two states of the same molecule H₂O; no bonds within molecules break.
Dissolving CO₂ in water to form a little carbonic acid (H₂CO₃) — which is it?
Partly chemical — most CO₂ just dissolves (physical), but a small equilibrium fraction reacts with water forming a new molecule H₂CO₃, so a real (reversible) chemical change occurs alongside the physical dissolving.
Stretching a rubber band until its molecules align but bonds stay intact — physical or chemical?
Physical — the polymer chains reorient and the shape changes, but no covalent bonds are broken or made, so the molecular identity is unchanged.
Heating sugar so it first melts, then browns — where is the boundary?
The melting portion is physical (intact sucrose changing state), and the boundary to chemical is crossed the instant decomposition/caramelisation begins producing new brown compounds.
An ideal, perfectly reversible reaction that returns to the exact starting molecules — is reversibility now proof of "physical"?
No — reversibility is not the deciding test; if bonds genuinely broke and reformed to new species (even briefly), it was chemical regardless of whether the system can be driven back.
Photosynthesis stores light energy in glucose with no obvious heat or flame — chemical or physical?
Chemical — CO₂ and H₂O are converted into new molecules (glucose and O₂) with new covalent bonds; the absence of a flame doesn't make it physical.
Ionising a gas into a plasma (stripping electrons) — is that a physical or chemical change?
It is beyond the ordinary chemical/physical pair — no molecules are rearranged into new compounds; electrons are torn off atoms (a very high-energy physical state change, the "fourth" state of matter), which is why it doesn't fit the molecular-formula test.
Nuclear fission or fusion (splitting or merging atomic nuclei) — chemical, physical, or neither?
Neither — chemical and physical changes both leave atomic nuclei untouched and rearrange only electrons/molecules; fission and fusion change the identity of the atoms themselves, releasing millions of kJ/mol, so they sit off the entire chemical–physical scale.
Recall One-line discriminator to memorise
Did the molecular identity change (new formula, new structure)? Yes → chemical. No → physical. Nuclear/plasma changes fall outside this test entirely. All other clues (energy, color, gas, reversibility) are only hints.