Physical and Chemical Change — Science Sage Gr10

What is a Physical Change?

A physical change is a change in which the physical properties of a substance (state, shape, size, or appearance) alter, but no new substance is formed. The particles present before the change are exactly the same particles present after the change — they have simply been rearranged, moved apart, moved closer together, or reshaped.

Key idea: In a physical change, the chemical composition of the substance stays the same. Water is still H₂O whether it is ice, liquid water, or steam. Physical changes are generally easy to reverse.

Common types of physical change:

Change of state — melting, freezing, boiling, evaporating, condensing, subliming (e.g. ice melting into water).
Dissolving — a solute mixes uniformly into a solvent to form a solution (e.g. sugar dissolving in tea). The sugar molecules are still sugar; they are simply spread out among the water molecules.
Change of shape or size — cutting, tearing, crushing, bending, stretching (e.g. tearing a sheet of paper, breaking a stick of chalk).
Mixing (without reacting) — e.g. stirring sand into water; the sand can be filtered out unchanged.

What is a Chemical Change?

A chemical change occurs when a chemical reaction takes place: the bonds between atoms in the reactants break, and new bonds form to create one or more new substances (products) with properties completely different from the original substances.

Key idea: In a chemical change, atoms are rearranged into new molecules. The products are chemically different from the reactants and generally cannot be turned back into the original substances by simple physical means. Chemical changes are generally difficult (or impossible) to reverse.

Common examples of chemical change: burning (combustion), rusting (corrosion), baking a cake, digestion of food, photosynthesis, a battery discharging, and milk turning sour.

Evidence of a Chemical Change

Because you cannot always "see" a new substance directly, chemists look for observable clues (evidence) that a chemical reaction has probably occurred. On its own, no single clue is absolute proof — but several of these together are strong evidence:

Evidence	Description	Example
Colour change	The substance changes colour, indicating a new substance with different light-absorbing properties	Iron turning orange-brown as it rusts
Gas / bubbles produced	Effervescence or fizzing shows a gas is being released	Bubbles when vinegar reacts with baking soda
Precipitate forms	An insoluble solid suddenly appears in a solution that was clear	A white solid forming when two clear solutions are mixed
Temperature change	Energy is released (exothermic) or absorbed (endothermic) during the reaction	A fire feels hot; an instant cold-pack feels cold
Light produced	Energy is released as light	A sparkler burning, a firework exploding
Permanent / difficult to reverse	The change cannot easily be undone by physical means	You cannot "un-bake" a cake or "un-burn" wood

Exothermic = releases energy (feels hot) | Endothermic = absorbs energy (feels cold)

Physical vs Chemical Change — Side by Side

Feature	Physical Change	Chemical Change
New substance formed?	No	Yes
Chemical composition	Unchanged	Changed
Reversibility	Usually easy to reverse	Usually difficult / impossible to reverse
Energy change	Usually small	Often a noticeable temperature/light change
Example	Melting ice, dissolving salt	Burning wood, rusting iron

Worked Classification Examples

Let's classify some everyday changes and explain why:

Melting chocolate — Physical change. The chocolate simply changes from solid to liquid due to heat; it is still the same chocolate and will re-solidify on cooling.
Burning wood — Chemical change. Wood (cellulose) reacts with oxygen to form ash, carbon dioxide, and water vapour — completely new substances with different properties. Light and heat are produced. This cannot be reversed.
Dissolving salt in water — Physical change. The salt (NaCl) is still NaCl, just spread evenly among water molecules. Evaporating the water recovers the original salt.
Iron rusting — Chemical change. Iron reacts with oxygen and water to form iron oxide (rust), a reddish-brown new substance with completely different properties from shiny metallic iron.
Boiling water — Physical change. Liquid water becomes water vapour (steam); it is still H₂O. Condensing the steam recovers liquid water.
A match burning — Chemical change. The match head undergoes combustion, producing light, heat, smoke, and ash — new substances that cannot be turned back into the original match head.

Exam tip: When asked to classify a change, always give a reason linked to the evidence — "no new substance is formed" for physical changes, or name the actual evidence (colour change, gas produced, permanent change, etc.) for chemical changes.

IEB Extension — Energy Diagrams & Everyday Applications

Energy profile of a reaction: Every chemical change involves an energy change. In an exothermic reaction, the products have less stored (chemical potential) energy than the reactants — the difference is released as heat and/or light to the surroundings (e.g. combustion, respiration). In an endothermic reaction, the products have more stored energy than the reactants — energy is absorbed from the surroundings, which cools down (e.g. photosynthesis, the reaction in an instant cold-pack).

Applying the concept: Consider baking a cake. The raw batter (reactants) undergoes several simultaneous chemical changes when heated: proteins denature, sugars caramelise (Maillard/browning reactions), and raising agents (e.g. baking powder) decompose to release CO₂ gas — evidence: colour change (browning), gas produced (rising), and the change is permanent (you cannot "un-bake" a cake back into batter). This makes baking a rich real-world example combining several types of evidence for chemical change in a single process.

Distinguishing tricky cases: Some changes look chemical but are physical, and vice versa. Dry ice (solid CO₂) "smoking" as it sublimes looks dramatic (mist, temperature drop) but is a physical change of state (sublimation) — the CO₂ remains CO₂. Conversely, a battery discharging shows no colour change or gas visible from outside, yet chemical reactions are occurring inside it (chemical energy is being converted to electrical energy) — a chemical change with subtle external evidence.

Classify the Scenario

Scenario 1 of 8

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Ice melting in the sun

Score

0/ 0 answered

Accuracy

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Question 1 (1 mark)

Which of the following is an example of a physical change?

Ice melting is a physical change — the water is still H₂O, just changed from solid to liquid state. No new substance is formed, and freezing it again reverses the change. Rusting, burning, and souring are all chemical changes.

Question 2 (1 mark)

Which of these is NOT good evidence that a chemical change has occurred?

A change in shape only (e.g. cutting, tearing, crushing) is evidence of a PHYSICAL change, not a chemical one — no new substance forms. Colour change, gas production, and temperature change are all classic evidence of a chemical reaction.

Question 3 (1 mark)

Dissolving sugar in a cup of tea is classified as a physical change because:

Dissolving does not create a new substance — the sugar molecules remain chemically unchanged, just dispersed among the water molecules. Boiling off the water would recover the original sugar, showing the change is reversible.

Question 4 (1 mark)

Which pair correctly matches a change with its type?

Tearing paper only changes its shape/size — no new substance is formed, so it is a physical change. Boiling water is also physical (still H₂O); fireworks and matches burning are chemical changes (light, heat, new substances, permanent).

Question 5 (2 marks)

A learner adds vinegar to baking soda and observes vigorous fizzing and bubbles, and the container feels cooler afterwards. This is evidence of:

Two separate pieces of evidence point to a chemical reaction: gas produced (bubbles/fizzing) and a temperature change (cooling = endothermic). A colour change is not required for a change to be chemical — multiple types of evidence can occur together or separately.

Question 6 (1 mark)

Which statement best explains why chemical changes are generally difficult to reverse?

Chemical changes rearrange atoms into new bonds, forming genuinely new substances. Simply reversing the physical conditions (e.g. cooling) cannot "unmake" these new substances, which is why chemical changes are generally difficult or impossible to reverse.

Question 7 (2 marks, Analysis)

A learner burns a strip of magnesium ribbon in a Bunsen flame. It burns with a brilliant white light and forms a fine white ash (magnesium oxide). The learner then tries melting the white ash, hoping to re-form the shiny metal ribbon, but it stays a white powder no matter how much it is heated. Which combination of observations gives the STRONGEST evidence that this is a chemical change?

Speed and temperature are not reliable evidence of a chemical change (physical changes can also be fast or hot). The strongest evidence combines two things: (1) the product (white magnesium oxide powder) has completely different properties from the reactant (shiny grey magnesium metal), and (2) attempting to reverse the change by simple physical means (melting) does not recover the original substance — because the atoms have formed new chemical bonds with oxygen, not just changed state.

IEB Question 1 IEB (2 marks)

Dry ice (solid CO₂) placed on a table appears to "smoke" and slowly disappears, with a visible temperature drop in the surrounding air. Explain why this is classified as a physical, not chemical, change — despite the dramatic temperature change and mist.

Sublimation is a change of state (solid → gas), so it is physical — the substance is CO₂ before and after. The visible "smoke" is actually water vapour from the air condensing in the cold zone around the dry ice, not a new substance produced by the CO₂ itself. A temperature drop alone does not prove a chemical change; it must be linked to a new substance forming.

IEB Question 2 IEB (2 marks)

A battery being discharged in a torch shows no visible colour change, gas, or precipitate — yet a chemical change is definitely occurring inside it. What is the best justification for classifying this as a chemical change?

Evidence of a chemical change is not always visible externally. Inside the battery, electrochemical reactions occur continuously at the electrodes, producing new substances and converting stored chemical energy into electrical energy. The absence of external evidence (colour, gas, precipitate) does not mean no chemical change is happening — the reaction is simply enclosed.

Show clear reasoning for every classification. Refer to the specific evidence you observe or would expect to observe.

Question 1

Classify each of the following as a PHYSICAL change or a CHEMICAL change. Give a brief reason for each answer.

(a) Butter melting in a hot pan
(b) A firework exploding
(c) Folding a piece of paper into an origami shape
(d) Silver jewellery tarnishing (turning black) over time
(e) Freezing water into ice cubes
(f) A candle wick burning with a flame
(g) Whisking egg whites into a foam
(h) Bread dough rising due to yeast fermentation

Question 2

A learner mixes two colourless solutions, Solution A and Solution B, in a test tube. Immediately, a yellow precipitate forms and the test tube becomes noticeably warmer.
(a) Is this a physical or a chemical change? (1)
(b) Identify TWO pieces of evidence from the description that support your answer. (2)
(c) Explain, in terms of particles/atoms, what has happened to the substances in Solutions A and B. (2)

Question 3

Explain, using the term "reversible", why melting chocolate is a physical change but burning wood is a chemical change. Your answer should refer to whether the original substance can be recovered. (4)

Question 4

A metal spoon is left outside and slowly develops a reddish-brown coating.
(a) Name the process taking place. (1)
(b) State the type of change (physical or chemical) and justify your answer with TWO reasons. (3)
(c) Suggest ONE way this change could have been prevented. (1)

Question 5

For each scenario below, state whether it is exothermic, endothermic, or if there is not enough information to tell — and explain your choice.

(a) A hand warmer packet becomes hot when activated.
(b) An instant cold-pack becomes very cold when squeezed and activated.
(c) Photosynthesis in a green leaf.
(d) A candle flame burning steadily.

Question 6 — Mass and Density Data Through a Physical Change

A learner measures a 90.0 g block of ice, allows it to melt completely into liquid water at 0°C, and measures the resulting volume:

State	Mass (g)	Volume (cm³)
Ice (solid)	90.0	98.0
Water (liquid, after melting)	90.0	90.0

(a) Calculate the density of the ice and the density of the water, showing your working.
(b) The mass stayed exactly the same (90.0 g) before and after melting. What law does this illustrate, and why does it confirm that melting is a physical change?
(c) Explain why the volume decreased even though no mass was lost — refer to the arrangement of water molecules in ice compared to liquid water.
(d) Use your answer to (a) to explain why ice floats on liquid water.