Physical and Chemical Changes

In P and R the molecules after are different from the molecules before: bonds broke and new ones formed. In Q every Br₂ molecule is intact; only the arrangement of the molecules changed.

• Correct: P and R: Right: new substances form in P and R.
• Q alone: Q is the one change in which every molecule stays the same, so it is the physical change.
• P, Q and R: Q keeps the same Br₂ molecules, so it is not a chemical change.
• R alone: P also makes new substances (H₂ and O₂ from water).

The molecules end up far apart but each is still Br₂. Only the forces between molecules were overcome: a phase change, which is physical.

• Correct: Bromine evaporating: Br₂ molecules move apart, each staying whole: Right: the molecules separate, but each molecule stays whole.
• Bromine splitting into atoms as Br–Br bonds break on heating: The second box still shows Br₂ pairs, so no Br–Br bond broke.
• Bromine reacting to form a new compound with a higher boiling point: No new compound appears; every particle is still Br₂.
• Bromine dissolving as Br₂ molecules break into Br⁺ and Br⁻ ions: No ions appear in the diagram, and there is no water to dissolve in.

6 NO hold 6 O atoms and 3 O₂ hold 6 O atoms: 12 in all, matching the 6 NO₂ × 2 = 12 O atoms before.

• Answer: 12 atoms

Every reaction keeps the same atoms, so "same atoms" cannot tell physical from chemical. What matters is whether bonds within particles break and new ones form. In P they do.

• Correct: P is chemical: O–H bonds break, and new H–H and O=O bonds form: Right: bonds inside the particles change, so new substances form.
• P is physical, since it involves just the atoms already present: Atoms are present before and after any change; that alone does not make a change physical.
• P is physical, because burning the two gases would undo it again: Whether a change can be undone does not decide it; many chemical changes can be undone by another reaction.
• P is chemical, because water becomes a gas, which is a new state: A change of state alone is physical; P is chemical because new substances form, not because a gas appears.

Boiling needs about 41 kJ/mol, while breaking the O–H bonds needs about 927 kJ/mol, over 20 times more. Boiling separates whole molecules; it does not break covalent bonds.

• Correct: Boiling overcomes forces between molecules, far weaker than O–H bonds: Right: the intermolecular attractions overcome in boiling are much weaker than covalent bonds.
• Boiling at 100 °C breaks about 4% of the O–H bonds in the water: Boiling separates whole molecules; no O–H bond breaks. 41 ÷ 927 is a ratio of energies, not a share of bonds broken.
• Splitting water into atoms is a physical change, since it needs energy: Both processes need energy; that does not decide physical or chemical. Making H and O atoms from water is chemical, since new substances form.
• Hydrogen bonds between molecules are stronger than O–H bonds within them: The data show the opposite: 41 kJ/mol to separate molecules versus 927 kJ/mol to break bonds.

Condensing is the reverse of boiling: the same attractions between molecules form again, so the same 41 kJ/mol is released. No O–H bonds broke on boiling, so none re-form.

• Correct: About 41 kJ/mol is released as attractions between molecules re-form: Right: forming intermolecular attractions releases the energy boiling took in.
• About 927 kJ/mol is released as the broken O–H bonds re-form: No O–H bonds broke when the water boiled, so none re-form on condensing.
• No energy change occurs, because condensing is a physical change: Physical changes involve energy too; boiling and condensing are examples.
• Energy is absorbed, because the molecules move closer together: Particles moving closer as attractions form release energy; separating them is what takes energy.