Foundations — Physical vs chemical change

This page assumes nothing. Before you can read Physical vs Chemical Change you must be able to see every word and symbol it throws at you. We build them one at a time, each one leaning on the one before.

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1. Matter, atoms, molecules — the actors

WHY the topic needs this: The whole question "did the identity change?" means "did the shapes of the little clipped groups change?" You cannot ask that until you can picture the group. See Molecular Structure and States of Matter for where these live.

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2. The chemical formula — reading ${\text{H}}_2\text{O}$, ${\text{Fe}}_2{\text{O}}_3$

Now you can read the parent's symbols:

• ${\text{CO}}_2$ ::: one carbon, two oxygens (carbon dioxide gas)
• $\text{Fe}$ ::: one iron atom, all by itself — a pure metal
• ${\text{Fe}}_2{\text{O}}_3$ ::: two irons, three oxygens (rust)
• ${\text{C}}_{12}{\text{H}}_{22}{\text{O}}_{11}$ ::: sugar — 12 carbons, 22 hydrogens, 11 oxygens

WHY the topic needs this: The single sharpest test for "chemical change?" is did the formula change? Sugar dissolving keeps ${\text{C}}_{12}{\text{H}}_{22}{\text{O}}_{11}$ (physical). Iron rusting turns $\text{Fe}$ into ${\text{Fe}}_2{\text{O}}_3$ (chemical). If you cannot compare formulas, you cannot use the test. See Identification of Substances.

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3. States of matter and the arrow $\to$

WHY the topic needs this: The arrow is where the decision lives. If only the bracket letter changes ($(s)\to (g)$, same formula) → physical. If the formulas on each side differ → chemical. Every worked example in the parent is really "look at the two sides of one arrow."

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4. Bonds vs. intermolecular forces — the two kinds of "stickiness"

This is the heart of the whole topic, so we slow right down. There are two completely different strengths of pull, and confusing them is where every mistake comes from.

The named IMFs the parent lists, weakest to strongest:

• dispersion (dispersion force) ::: faintest tug, present in everything; holds dry ice together
• dipole–dipole ::: pull between molecules that have a slightly + and slightly − end
• hydrogen bond ::: the strongest IMF; the special tug that makes water sticky
• (then far above all of these: the covalent bond itself)

WHY the topic needs this: The famous "10× to 100× energy difference" is only the difference between snapping threads and snapping staples. Without this distinction the energy numbers are just random figures. See Energy in Chemistry.

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5. Energy, $\Delta$, and the units $\text{kJ/mol}$

Recall Why a covalent number dwarfs an IMF number

Snapping a staple (O–H bond) costs about $463\text{kJ/mol}$; snapping the water–water threads costs only about $40.7\text{kJ/mol}$. That ratio $463$-ish to $40$-ish is exactly the "physical is cheap, chemical is expensive" story the parent derives. Roughly how many times bigger is chemical than physical for water? ::: about 20–25 times

WHY the topic needs this: The parent's whole derivation is subtraction and division of these $\Delta H$ values. If $\Delta$, $\text{kJ}$, and $\text{/mol}$ are mysterious, the derivation is unreadable.

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6. Reversibility and "new substance" — the two verdicts

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Prerequisite map

Read it top to bottom: you cannot judge "reversible or new substance" until you can read formulas, see states, and tell a bond from a thread — and only then does the parent topic make sense.

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Equipment checklist

Cover the right side and test yourself. Reveal only when you can say it in your own words.

• What does the small $2$ in ${\text{H}}_2\text{O}$ mean? ::: two hydrogen atoms inside one water molecule
• What does the big $2$ in $2{\text{H}}_2\text{O}$ mean? ::: two whole water molecules (copies)
• What does the arrow $\to$ separate? ::: reactants (left) from products (right)
• What do $(s)$, $(l)$, $(g)$ label? ::: the physical state — solid, liquid, gas — of the same substance
• Difference between a covalent bond and an intermolecular force? ::: bond = strong staple inside a molecule; IMF = weak thread between molecules
• Which one does a physical change break? ::: only the weak intermolecular threads
• Which one does a chemical change break? ::: the strong covalent staples inside molecules
• What does $\Delta$ mean? ::: "the change in" a quantity
• What does $\text{kJ/mol}$ measure? ::: energy in kilojoules per one standard pile (mole) of molecules
• The single strongest test for chemical change? ::: the chemical formula changed (a new substance formed)
• Order IMFs weakest to strongest ::: dispersion, then dipole–dipole, then hydrogen bond
• Is melting ice physical or chemical, and why? ::: physical — same formula ${\text{H}}_2\text{O}$, only threads cut, staples intact