Exercises — Physical vs chemical change

Left column (physical) ::: same coloured molecule, just closer/farther apart or in a new phase — no line inside a molecule is cut. Right column (chemical) ::: the coloured shapes are cut apart and re-assembled into new shapes — new substances.

Decide: Solid butter → liquid butter. The fat molecules are identical; only their arrangement loosened (solid packing → liquid flow). Test: Cool it → it re-solidifies into the same butter. Reversible by a physical means (temperature). Answer: Physical change (a melting = solid → liquid phase change, States of Matter).

Decide: The brown colour is a new compound (melanin-like pigments) made when enzymes let oxygen react with molecules in the apple flesh — new O-containing bonds form. Test: You cannot un-brown it by cooling or wiping. Irreversible by simple means. Answer: Chemical change. (Contrast with a dye simply spreading, which would be physical.)

Decide: The Na⁺ and Cl⁻ ions separate and get surrounded by water, but no new substance is created — evaporate the water and identical NaCl crystals return. Test: Reversible by evaporation; formula unchanged. Answer: Physical change (dissolving = a physical dispersal). Bonds broken here are ion–ion attractions to the lattice, replaced by ion–water attractions — no covalent bonds inside a molecule are cut.

Reaction: ${\text{CH}}_3\text{COOH}+{\text{NaHCO}}_3\to {\text{CH}}_3\text{COONa}+{\text{H}}_2\text{O}+{\text{CO}}_2(g)$. The gas: carbon dioxide, ${\text{CO}}_2$ — a molecule that did not exist before. It is a new substance, not water vapour from boiling. Recovery test: Cool the gas and you get dry ice (solid ${\text{CO}}_2$), not vinegar. So a chemical event happened earlier. Answer: Chemical change. The fizz is genuine new-substance gas production, not a phase change.

Decide: ${\text{H}}_2\text{O}(l)\to {\text{H}}_2\text{O}(g)$ — same molecule, new phase. Physical. Energy meaning: Boiling only breaks hydrogen bonds between molecules ($40.7\text{kJ/mol}$). It never touches the O–H bonds inside each water molecule ($463\text{kJ/mol}$ each). Ratio, to feel the gap: $\frac{2\times 463}{40.7}=\frac{926}{40.7}\approx 22.8$. Breaking the molecule apart would cost ~23× more energy than merely boiling it. Answer: Physical change. The small energy is the fingerprint: intermolecular forces broke, covalent bonds survived. (Energy in Chemistry)

Tarnish: $2\text{Ag}+{\text{H}}_2\text{S}\to {\text{Ag}}_2\text{S}+{\text{H}}_2$. Silver atoms form new Ag–S bonds → a new compound, silver sulfide (black). Chemical change. Dust: grey dust particles just land on the surface — no bond to the silver, wipe it off and shiny silver returns. Physical (a mixture, not a reaction). Answer: Not the same kind. Tarnish is a genuine chemical transformation; dust is a physical covering. Recovery test separates them: polishing removes tarnish only by chemically reducing ${\text{Ag}}_2\text{S}$ back to Ag, whereas dust just wipes away.

(a) ${\text{CO}}_2(s)\to {\text{CO}}_2(g)$: same molecule, so physical change (sublimation = solid → gas, States of Matter). Only weak dispersion forces between ${\text{CO}}_2$ molecules break. (b) One ${\text{CO}}_2$ has two C=O bonds: $2\times 799=1598\text{kJ/mol}$. Ratio: $\frac{1598}{25}=63.9$. Interpretation: Actually destroying the molecule would cost ~64× more energy than sublimation. The tiny 25 kJ/mol is exactly why we call it physical.

Conclusion: No chemical change occurred at the stage that produced this gas — it was a phase change (liquid → gas → liquid). Why decisive: Conservation of Mass plus identity: if condensing the gas restores the identical liquid (same formula, same properties), the molecules were never rebuilt. A true chemical product would condense into something different (e.g. ${\text{CO}}_2$ → dry ice, not the vinegar you began with). Edge case to note: This test can be fooled if a reversible reaction happens to run backward on cooling — so always confirm the recovered liquid's properties match the original, not just its appearance. (Identification of Substances)

Left: Fe: 4; O: $3\times 2+6\times 1=6+6=12$; H: $6\times 2=12$. Right: each ${\text{Fe(OH)}}_3$ has 1 Fe, 3 O, 3 H, times 4 → Fe: 4; O: 12; H: 12. Match: Fe 4=4, O 12=12, H 12=12. ✓ Balanced. Meaning: No atom is created or destroyed (Conservation of Mass) — bonds merely rearranged, which is the signature of a chemical change.

Steps i–iii each make new substances (CO₂, cross-linked protein networks, brown Maillard compounds) and are irreversible — you cannot un-bake a cake. Each is a chemical change (Chemical Reactions). Water evaporation (${\text{H}}_2\text{O}(l)\to {\text{H}}_2\text{O}(g)$) is physical, but it is a side process — it changes the batter's texture, not the chemistry. Overall verdict: Chemical change, because any genuine new-substance formation makes the net process chemical. Heat supplied the activation energy (Energy in Chemistry) for the bond rearrangements.

Part A — dissolving: Stir sugar into warm water, then gently evaporate all the water in the pan.

• Confirming result: solid crystals return that taste identically sweet → same molecule recovered → physical. Part B — burning: Heat sugar strongly until it blackens and gives off smoke.
• Confirming result: a black carbon residue and gases appear; cooling does not give sugar back; the residue is not sweet → new substances → chemical. Key contrast: Part A passes the recovery test; Part B fails it. Same starting substance, two different fates decided by whether identity is preserved. (Identification of Substances)

Bonds broken per H₂O: two O–H bonds → $2\times 463=926\text{kJ}$ per mole of water. Ratio to boiling: $\frac{926}{40.7}\approx 22.8$. Proof: Chemically pulling water apart costs ~23× more than physically boiling it. That order-of-magnitude gap is the quantitative reason physical changes are "small energy" and chemical changes are "large energy," exactly as the parent note's energy hierarchy predicts.

What happens: ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}\rightleftharpoons {\text{CuSO}}_4+5{\text{H}}_2\text{O}$. Water molecules bonded within the crystal structure (coordinated to Cu²⁺) are driven off, changing the compound's colour and composition. Why "reversible = physical" is too quick: Reversibility is a hint, not a proof. Here the substance's formula changes ($\cdot 5{\text{H}}_2\text{O}$ present vs. absent), colour changes because Cu²⁺'s bonding environment (its coordination) changes, and bonds to water are broken/reformed — hallmarks of a chemical event. Verdict: This is best classified as a chemical change (dehydration/hydration is a reaction), even though it is reversible. Lesson: No single indicator is decisive; here new formula + bonding change outweighs the reversible hint. (Molecular Structure)

Order (smallest → largest): $25<40.7<926<1598$, i.e. (a) < (b) < (c) < (d). Labels:

• (a) 25 kJ/mol — physical (sublimation).
• (b) 40.7 kJ/mol — physical (boiling).
• (c) 926 kJ/mol — chemical (splitting water molecules).
• (d) 1598 kJ/mol — chemical (splitting CO₂ molecules). Pattern: The two physical processes sit far below the two chemical ones — the energy scale itself cleanly separates the two categories, matching the hierarchy from the parent note.

The single deciding question ::: Did molecular identity change (bonds broken/formed → chemical) or only form/state (→ physical)? Why energy size helps ::: intermolecular forces (physical) are ~5–40 kJ/mol; covalent bonds (chemical) are ~400–1600 kJ/mol — roughly a 10–100× gap.