Two kinds of attraction, two kinds of change
In Topic 4.1 you used what you can see to spot a change. This topic asks a deeper question: what happens to the particles? The answer depends on which attractions change.
| Attraction | Where it acts | Examples | Typical strength |
|---|---|---|---|
| Chemical bonds | Inside a substance’s particles, or holding an ionic or metallic lattice together | Covalent O–H in water; ionic Na⁺ Cl⁻ in salt; metallic bonding in magnesium | Strong |
| Intermolecular (interparticle) attractions | Between separate molecules, or between ions and solvent molecules | London dispersion, dipole–dipole, hydrogen bonding, ion–dipole | Much weaker |
From this comes the working rule:
- A process that breaks and/or forms chemical bonds is usually classified as a chemical process. Atoms end up in new combinations, so new substances form.
- A process that changes only intermolecular attractions is usually classified as a physical process. Phase changes (melting, boiling, freezing, condensing, subliming) are the main examples. The particles are the same before and after.
Figure 1 shows the difference for water.
Why the energy scale is so different
Intermolecular attractions are much weaker than chemical bonds. Water shows this clearly:
- Boiling 1 mol of water at 100 °C needs about 41 kJ (its enthalpy of vaporization). This only pulls the molecules apart.
- Breaking both O–H bonds in 1 mol of gaseous water molecules needs about 2 × 460 ≈ 920 kJ (using the average O–H bond enthalpy).
That is more than 20 times as much energy. This is why heating water on a stove makes steam, not hydrogen and oxygen. You will calculate energies like these in Unit 6 (Topics 6.5 and 6.7).
Linking what you see to what the particles do
Topic 4.1 listed signs of a chemical change: heat or light given out, a gas forming, a precipitate forming, a colour change. These are evidence, but every one of them can also come from a physical change:
| Observation | Possible chemical cause | Possible physical cause |
|---|---|---|
| A gas forms | Marble chips (calcium carbonate) reacting with acid, giving CO₂ | Water boiling; dry ice subliming |
| A colour change | A new coloured compound forms | Solid iodine turning into a violet vapour |
| The temperature changes | A reaction releasing or absorbing energy | Some salts dissolving and making the water colder or warmer |
| A solid appears | A precipitate from two solutions | Crystals forming as a saturated solution cools |
So an observation alone rarely settles the question. You need the particle-level explanation: did chemical bonds break and form, giving new substances? The strongest experimental evidence is that the product has different properties (melting point, boiling point, colour, reactivity) that do not return when you simply reverse the temperature change.
A decision routine
Use these steps for any process:
- Write what is present before and after, as formulas with state symbols.
- Compare the formulas. If a new formula appears (H₂O becomes H₂ and O₂), atoms have been rearranged, so chemical bonds broke and formed: a chemical process.
- If the formulas are the same and only the state or the spacing changes (H₂O(l) becomes H₂O(g)), only intermolecular attractions changed: a physical process.
- If an ionic solid separates into the ions it already contained as it dissolves in water, you are in the borderline case discussed below. Choose a classification and justify it. (A molecular substance that makes new ions as it dissolves, such as HCl, has a new formula afterwards, so step 2 applies.)
- Back the answer with evidence: a measured property, a test for a new substance, or a particle diagram.
Separating a mixture (filtering sand from water, distilling ethanol from water, evaporating seawater) is also physical. Each substance keeps its own particles; only the mixture is pulled apart.
Worked example 1: classifying by bonds
Question. Classify each process as physical or chemical, and explain at the particle level.
(a) Solid iodine, I₂, is warmed gently and becomes a violet gas. (b) A shiny copper wire is held in a hot flame. Its surface turns black and stays black after cooling. (c) A spoon of sugar (sucrose) dissolves in hot tea.
- (a) Iodine. Before and after, the particles are I₂ molecules. The I–I covalent bond inside each molecule does not break. Warming overcomes the London dispersion forces between molecules, so they separate into a gas. The colour change and the gas could suggest a reaction, but the substance is still I₂. Physical.
- (b) Copper in a flame. The black coating is copper(II) oxide, a new substance: 2 Cu(s) + O₂(g) → 2 CuO(s). Metallic bonds between copper atoms at the surface break, the O=O bond breaks, and copper and oxide ions form ionic bonds in the new solid. Here the colour change really does come from a new substance. Chemical.
- (c) Sugar in tea. Sucrose molecules separate from the crystal and spread through the water. The covalent bonds inside each sucrose molecule are not broken. Hydrogen bonds between sucrose molecules are replaced by hydrogen bonds between sucrose and water. Physical.
Check. In (a) and (c) the formula of the substance is the same before and after. In (b) it is not. That quick test agrees with the bond-level reasoning.
Worked example 2: supporting a claim with data
Question. A student heats two white solids, P and Q, in separate open test tubes, then lets each cool and weighs it again. Use the data to decide which solid underwent a chemical change.
| Measurement | Solid P | Solid Q |
|---|---|---|
| Mass before heating | 2.50 g | 4.20 g |
| What happens on heating | Melts to a clear liquid at 146 °C | Stays solid; a colourless gas is given off |
| Gas bubbled through limewater | No gas | Limewater turns cloudy |
| Mass after cooling | 2.50 g | 2.65 g |
| Melting point of cooled solid | 146 °C | Does not melt in the same test; no gas on reheating |
- Claim. Solid Q underwent a chemical change. Solid P underwent only a physical change (melting, then freezing).
- Evidence for P. The mass is unchanged (2.50 g), and the cooled solid has the same melting point (146 °C). The same substance is present before and after.
- Reasoning for P. Melting and freezing change only the attractions between particles. Nothing is lost and no new substance forms.
- Evidence for Q. The mass fell by 1.55 g. A gas formed that turned limewater cloudy (a test for carbon dioxide). The residue behaves differently when reheated.
- Reasoning for Q. Carbon dioxide was not present as a separate substance in the solid. To release it, chemical bonds in Q must have broken and new bonds formed, leaving a different solid behind.
Answer. Q underwent a chemical change; P underwent a physical change.
What makes the argument strong. Each claim is backed by more than one piece of data, and each piece of data is linked to what the particles do. “It gave off a gas” on its own would not be enough, because a melting or boiling solid can also release vapour.
When the line is blurred: dissolving a salt
The rule “bonds broken means chemical” does not always give a clear answer. When an ionic solid such as sodium chloride dissolves:
- The ionic bonds holding Na⁺ and Cl⁻ in the lattice are broken.
- New ion–dipole attractions form between each ion and the surrounding water molecules.
You can make a sensible argument either way.
| Argument for physical | Argument for chemical |
|---|---|
| The same ions are present before and after; no new substances form | Ionic bonds in the lattice are broken |
| Evaporating the water gives back solid NaCl | New ion–dipole interactions form, and the dissolved ions behave differently from the solid (for example, the solution conducts electricity) |
| The equation NaCl(s) → Na⁺(aq) + Cl⁻(aq) only separates ions; it does not rearrange atoms | Each dissolved ion is a new particle arrangement: an ion held by ion–dipole attractions inside a shell of water molecules |
In an exam, either classification can earn credit if it is supported by a particle-level reason. A bare label (“physical”) without reasoning cannot.
Compare a molecular gas such as hydrogen chloride dissolving in water. Here a covalent H–Cl bond breaks and a new O–H bond forms: HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq). A new ion, H₃O⁺, exists afterwards. Most chemists treat this as a chemical change (an acid–base reaction, which you meet in Topic 4.8).
Common misconceptions
- “Bubbles in boiling water are hydrogen and oxygen.” They are water vapour: whole H₂O molecules.
- “A gas or a colour change always means a reaction.” Both can come from phase changes (boiling, iodine subliming).
- “Physical changes are reversible; chemical changes are not.” Reversibility is not the test. Ask whether chemical bonds break or form.
- “Melting breaks the bonds in a molecule.” Melting a molecular solid overcomes attractions between molecules. The covalent bonds inside stay intact.
- “Dissolving is always physical.” Dissolving sugar is physical, but dissolving HCl gas forms new ions, and dissolving salts can be argued either way.
- Giving a label without a reason. Marks come from explaining what happens to bonds and attractions, linked to the evidence.
Where this leads
This topic builds on the particle diagrams of Topic 4.3, Representations of Reactions and the forces of Unit 3. Next, Topic 4.5, Stoichiometry, uses balanced equations for chemical changes to calculate amounts that react and form. The energy comparison returns in Unit 6, where you calculate the energy of phase changes and use bond enthalpies. Try the practice questions now, then use the revision notes and the checklist to consolidate.
Quick questions
Is dissolving salt a physical or a chemical change?
Both arguments are acceptable if you support them. Physical: the same ions are present and evaporating the water gives back the salt. Chemical: ionic bonds in the lattice break and new ion–dipole interactions form. State which view you take and why.
Does a reversible change have to be physical?
No. Many chemical reactions can be reversed, and some physical changes are hard to reverse. Decide by asking whether chemical bonds break or form, not by asking whether the change can be undone.