Safety: burning magnesium, silver nitrate and flames are for a teacher or a supervised school lab only. Wear eye protection; silver nitrate stains skin and can damage eyes; never look straight at burning magnesium.
These clues suggest that a new substance has been made. One clue on its own can trick you, so look for several, and ask: is there a new substance, and can I easily get the old one back?
A new colour that is not just mixing paints: iron turning orange-brown as it rusts, a cut apple going brown.
Trick: mixing blue and yellow paint makes green without a reaction.
Fizzing when baking soda meets vinegar (carbon dioxide), or bubbles of hydrogen when a metal reacts with acid.
Trick: boiling water also bubbles, but the gas is just steam.
Two clear solutions make a cloudy solid, like silver chloride or the scum from soap in hard water.
Trick: salt crystals appearing as water evaporates is physical.
The mixture gets hot (burning, hand warmers) or cold (baking soda and vinegar, citric acid and baking soda) by itself.
Trick: melting and dissolving can take in heat too.
Flames, fireworks, burning magnesium, glow sticks.
Trick: a light bulb glows without making a new substance.
Milk going sour, food cooking or burning, rotten eggs.
Trick: perfume spreading is just particles moving.
In any change, atoms are not made or destroyed, so the total mass stays the same. But a balance only weighs what is on it. Run each setup and explain the reading.
Masses are worked out from the equations: 2.00 g of baking soda with excess vinegar makes about 1.05 g of carbon dioxide; 0.24 g of magnesium makes about 0.40 g of magnesium oxide.
The diagram you label in a test: a chemical reaction drawn with particles (reactants, products, atoms, molecules and bonds), the signs of a chemical change, and a physical change. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a label worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Ten questions about changes, signs, particles and mass.
Tap a change, then tap the box it belongs in. Check when all twelve are placed.
Melting, freezing, boiling, condensing, dissolving, cutting, bending, crushing, and mixing substances that do not react, are physical changes. The substance keeps its identity: ice, water and steam are all H₂O. Many physical changes are easy to reverse, but not all (a torn page is still paper).
Burning, rusting, cooking, rotting and reactions in a test tube are chemical changes. Bonds between atoms break and new bonds form, so new substances with new properties appear. We show them with word equations and symbol equations: reactants → products.
Atoms are never made or destroyed in a chemical change, only rearranged, so the total mass stays the same (Antoine Lavoisier showed this with careful weighing in the 1770s–80s). If a gas escapes, a balance shows less mass; if a gas from the air joins in, it shows more. That is why symbol equations must balance.
Predict before each experiment and keep your score. Compare boiling water with baking soda and vinegar: both bubble, but only one makes a new gas. Then weigh baking soda and vinegar in an open beaker and in a sealed bottle.
| Physical change | Chemical change | |
|---|---|---|
| New substance? | No: same particles | Yes: atoms rearranged into new particles |
| What changes | shape, size, state, arrangement or mixing | composition: the bonds between atoms |
| Reversing it | often easy (freeze, evaporate, filter, use a magnet) | usually hard; needs another chemical reaction |
| Energy | heat taken in or given out when the state changes; no new substance | often heat or light given out or taken in |
| Mass | conserved | conserved (count any gases that escape or join) |
| Examples | melting ice, boiling water, dissolving salt, tearing paper, crushing a can, separating iron from sand | burning, rusting, cooking an egg, baking a cake, milk going sour, baking soda + vinegar, fireworks |
| Experiment | Change | Equation |
|---|---|---|
| Burning paper | chemical | cellulose + oxygen → carbon dioxide + water (C₆H₁₀O₅ + 6O₂ → 6CO₂ + 5H₂O for each sugar unit; real burning also makes ash and smoke) |
| Baking soda + vinegar | chemical | NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂ |
| Iron rusting | chemical | 4Fe + 3O₂ → 2Fe₂O₃ (rust is hydrated iron(III) oxide; water is needed) |
| Silver nitrate + salt water | chemical | AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) |
| Burning magnesium | chemical | 2Mg + O₂ → 2MgO |
| Cooking an egg | chemical | proteins unfold (denature) and form new links between them |
| Melting ice, boiling water | physical | H₂O(s) → H₂O(l) → H₂O(g) |
| Dissolving salt | physical | NaCl(s) → Na⁺(aq) + Cl⁻(aq); evaporating the water gets the salt back |
A physical change alters the form, size or state of a substance but not what it is made of (ice melting). A chemical change makes one or more new substances with new properties (wood burning).
Key takeaway: same substance = physical; new substance = chemical.A colour change, gas or bubbles, a solid (precipitate) forming, a temperature change, light, or a new smell. One sign alone is not proof: boiling water bubbles but is a physical change.
Key takeaway: colour, gas, precipitate, temperature, light, smell.Physical. The molecules are H₂O before and after; they just move more freely in the liquid. Freezing the water gives ice back.
Key takeaway: melting is physical.At school level it is a physical change: no new substance is made, and you get the salt or sugar back by evaporating the water. (Salt breaks up into ions as it dissolves, but they are the same ions that were in the crystal.)
Key takeaway: dissolving is physical: evaporate to get it back.Both. The wax melts (physical), and the wax vapour burns with oxygen to make carbon dioxide and water (chemical), giving out heat and light.
Key takeaway: melting wax is physical; burning wax is chemical.Yes. Heat makes the egg proteins unfold and join together with new bonds, so the clear egg white turns white and solid, and cooling it does not make it raw again.
Key takeaway: cooking an egg is chemical.The wood reacts with oxygen to make carbon dioxide and water vapour, which escape into the air, leaving light ash. If you could collect all the gases, the total mass of the products would equal the mass of the wood plus the oxygen used.
Key takeaway: mass escapes as gas; it is not destroyed.Usually not by simple means like cooling or filtering. Some can be reversed by another chemical reaction (charging a battery, or turning iron oxide (iron ore) back into iron in a blast furnace), but that is a new chemical change, not undoing the old one by a physical step.
Key takeaway: only by another chemical reaction.Bonds between atoms break and new bonds form, so the atoms join up in new combinations. The number of each kind of atom stays the same, which is why mass is conserved and equations must balance.
Key takeaway: atoms are rearranged, not created or destroyed.Yes. Speed does not decide it: iron reacts with oxygen and water to make hydrated iron(III) oxide, a new substance that is flaky, orange-brown and weaker than iron.
Key takeaway: slow changes can still be chemical.