Physical and Chemical Changes: Same Stuff or New Stuff?
Ice melting in your lemon tea, bread browning in a toaster, a bicycle rusting in the rain – things change all the time, but not all changes are the same! In this chapter you will learn to tell a physical change from a chemical change, sort everyday processes into the right group, compare a compound with its elements, and explain why some things get lighter and others heavier when they burn.
You met a quick comparison in "Mixtures and compounds". Here we go deeper. For every change, ask one big question: Has a new substance been formed?
1. Physical Changes: Same Substance, New Look
What is a physical change?
A physical change is a change in which no new substance is formed. The chemical composition – what the substance is made of – stays the same. Only things like its state (solid, liquid or gas), shape or size change.
Example: when the ice in your lemon tea melts, solid water becomes liquid water. It is water (H2O) the whole time; the particles just move more freely.
Many physical changes are easy to reverse
Ice melts into water, and water freezes back into ice. Salt dissolves in water, and you get the salt back by evaporating the water.
But not every physical change is easy to reverse. Crushed stones or torn paper cannot easily be put back together – yet no new substance is formed, so these are still physical changes.
Key takeaway: Physical change = no new substance, no change in chemical composition. Many (not all) are easy to reverse.
2. Chemical Changes: Something New Is Made
What is a chemical change?
A chemical change (or chemical reaction) is a process in which new substances are formed. The atoms are rearranged and joined in new ways, so the new substances have different properties.
Example: at a Country Park barbecue, charcoal (mostly carbon) burns in air. Carbon joins with oxygen to form carbon dioxide, a colourless gas – a brand-new substance:
\(\text{carbon} + \text{oxygen} \rightarrow \text{carbon dioxide}\)
In this word equation, the reactants are on the left and the products (new substances) are on the right.
Chemical changes are usually hard to reverse
You cannot turn a fried egg back into a raw egg, or rebuild paper from its ash and smoke. Most chemical changes are difficult to reverse.
Signs that a chemical change may have happened
1. A gas is given off – bubbles or fizzing (effervescence), e.g. a vitamin C effervescent tablet in water.
2. A colour change – e.g. bread turns brown in a toaster; a cut apple turns brown in air.
3. An energy change – heat and light are given out (e.g. a gas stove flame), or heat is taken in so things feel colder.
4. A precipitate forms – an insoluble solid appears in a liquid, making it cloudy, e.g. breathing out through a straw into clear limewater turns it milky.
Memory aid: "Good Cooks Enjoy Pastry" – Gas, Colour, Energy, Precipitate.
Signs are clues, not proof
Some physical changes show similar signs. Boiling water bubbles, but the bubbles are just water vapour. A heater coil glows red, but it is the same metal when it cools. So always check: is there a new substance?
Did you know? In a "Tiangong classroom" lesson from China's space station, astronauts put an effervescent tablet into a floating ball of water. It fizzed – a chemical change – but in space the gas bubbles stayed inside the water ball instead of rising out.
Key takeaway: Chemical change = new substance formed. Gas, colour change, energy change or a precipitate are useful clues.
3. Physical or Chemical? Sorting Everyday Processes
Physical changes (no new substance)
Melting – a solid turns into a liquid, e.g. butter melting in a hot pan. Same substance, different state.
Evaporation – a liquid turns into vapour, e.g. wet clothes drying. The water is still water, now in the air.
Distillation – a liquid is boiled and its vapour is cooled and condensed into another container, e.g. getting distilled water from sea water. Only changes of state happen.
Filtration – an insoluble solid is separated from a liquid using a filter, e.g. sand from water, or a tea strainer holding back tea leaves.
Crushing of stones – rocks are broken into smaller pieces. They are smaller, but still the same rock.
Dissolving – e.g. sugar in milk tea. The sugar is still there (the tea tastes sweet!). See the chapter "Dissolving".
Distillation, filtration and evaporation are physical methods for separating mixtures – no new substance is made.
Chemical changes (new substances formed)
Burning (combustion) – a substance reacts with oxygen, giving out heat and light, e.g. charcoal or paper burning. New substances such as carbon dioxide form.
Oxidation – a reaction in which a substance combines with oxygen. Burning and rusting are both kinds of oxidation. Another example: shiny copper heated in a Bunsen flame turns black as copper(II) oxide forms on its surface.
Rusting – iron reacts slowly with oxygen and water to form reddish-brown, flaky rust, which is weak, unlike strong iron. Think of railings near the sea in humid Hong Kong.
\(\text{iron} + \text{oxygen} + \text{water} \rightarrow \text{rust}\)
Cooking – e.g. frying an egg (clear egg white turns white and solid) or baking a cake. New substances form and you cannot "uncook" food.
Did you know? Rust takes up more space than the iron it forms from. When steel bars inside concrete rust, they push outwards and can crack the concrete – one reason old buildings need regular repairs.
A tricky one: a burning candle
A burning candle shows both changes. The wax near the flame melts (physical – it turns solid again when cool). The wax vapour burns (chemical – carbon dioxide and water vapour form, with heat and light). That is why the candle gets shorter.
Key takeaway: Changing state, separating, dissolving and crushing are physical changes. Burning, oxidation, rusting and cooking are chemical changes.
4. A Compound vs Its Elements: Iron, Sulphur and Iron(II) Sulphide
Key practical task: test the physical properties of a compound and its constituent elements. Physical properties can be observed without making a new substance, e.g. colour, appearance and whether a magnet attracts it.
Step 1: Examine the two elements
Iron filings – grey metal powder, attracted by a magnet.
Sulphur powder – yellow non-metal powder, not attracted by a magnet.
Step 2: Mix them – a mixture is formed
Stir iron filings and sulphur together. With a hand lens you still see separate grey and yellow bits. A magnet (wrapped in paper) pulls out the iron and leaves the sulphur behind. Mixing is a physical change: each element keeps its own properties.
Step 3: Heat the mixture – a compound is formed
Your teacher heats a little of the mixture in a test tube. It glows red, and the glow keeps spreading even after the flame is removed – the reaction gives out heat. A black solid is left.
\(\text{iron} + \text{sulphur} \rightarrow \text{iron(II) sulphide}\)
The black solid is not attracted by a magnet (if all the iron has reacted), and no grey or yellow bits can be seen. A new substance (FeS) has formed, so this is a chemical change.
Comparing the results
Iron: grey powder; attracted by a magnet
Sulphur: yellow powder; not attracted
Mixture: grey and yellow bits; magnet pulls out the iron
Iron(II) sulphide: black solid; not attracted
Conclusion: a compound has different properties from its elements. The iron is chemically joined to the sulphur, so a magnet cannot separate them.
Safety first
Heating sulphur can produce toxic sulphur dioxide. Use small amounts, wear safety goggles and heat in a fume cupboard.
Key takeaway: In a mixture, elements keep their properties and can be separated physically. In a compound, they are chemically joined and have new properties.
5. Lighter or Heavier? Burning Paper and Iron Wool
Key practical task: compare the change in mass of different substances before and after burning.
How to do it
Weigh the paper (or some loosely pulled-apart iron wool) in a heat-proof container on an electronic balance. Burn it or heat it strongly, let it cool, then weigh again.
Paper: the mass goes down
When paper burns, it reacts with oxygen. Most products are gases (carbon dioxide and water vapour) that escape into the air. Only a little ash is left, so the mass decreases.
\(\text{paper} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}\)
Iron wool: the mass goes up
Iron wool is thin strands of iron. When heated, it glows and sparks, turning from shiny grey to a black, brittle solid. The iron has combined with oxygen from the air to form iron oxide, which stays on the balance, so the mass increases.
\(\text{iron} + \text{oxygen} \rightarrow \text{iron oxide}\)
Making sense of the results
Both are chemical changes. The difference is where the matter goes: gases leave the paper, but oxygen joins the iron.
Mass is not created or destroyed. In a sealed container, the total mass would stay the same – see "Chemical reactions" (Extension).
Key takeaway: Burning paper gets lighter (gases escape); burning iron wool gets heavier (oxygen joins).
Common Mistakes
Common mistake 1: "Dissolving is chemical because the sugar disappears." – No! The sugar is still there, spread out as tiny particles. Dissolving is physical.
Common mistake 2: "Bubbles always mean a chemical change." – No! Boiling water bubbles too, but that is just water vapour.
Common mistake 3: "Burning paper loses mass, so matter is destroyed." – No! The mass leaves as gases.
Common mistake 4: "An iron and sulphur mixture is the same as iron(II) sulphide." – No! A magnet can pull iron out of the mixture, but not out of the compound.
Common mistake 5: "If a change cannot be reversed, it must be chemical." – No! Crushing stones cannot be undone, but no new substance forms, so it is physical.
Quick Review
1. Physical change: no new substance; chemical composition unchanged.
2. Chemical change: new substances formed; usually hard to reverse.
3. Clues: gas, colour, energy, precipitate ("Good Cooks Enjoy Pastry").
4. Physical: melting, evaporation, distillation, filtration, crushing of stones, dissolving. Chemical: burning, oxidation, rusting, cooking.
5. Iron(II) sulphide is black and not attracted by a magnet – a compound has different properties from its elements.
6. Burning paper: mass decreases. Burning iron wool: mass increases.
Great work! Next time you watch ice melt in your drink, ask: same stuff or new stuff?