What is the difference between physical and chemical changes in the context of IB MYP Science?

In IB MYP Science, a physical change involves a change in the physical properties of a substance, such as its shape, state, or appearance, without altering its chemical composition. Examples include melting ice or tearing paper. A chemical change, on the other hand, results in the formation of one or more new substances with different chemical properties, such as rusting iron or burning wood.

How can you identify a chemical change during an experiment?

A chemical change can be identified by several indicators: a change in color, the formation of a gas (bubbling or fizzing), the release or absorption of heat (temperature change), the formation of a precipitate (solid), or a change in odor. These signs suggest that a new substance has been formed, which is a key aspect of chemical changes studied in IB MYP Science.

Why are physical and chemical changes important in understanding chemical kinetics?

Understanding physical and chemical changes is crucial in chemical kinetics because it helps students grasp how substances interact and transform over time. Chemical kinetics focuses on the rate of chemical reactions and the factors affecting them. Recognizing whether a change is physical or chemical aids in predicting reaction behavior and understanding the energy changes involved, which are essential concepts in the IB MYP Science curriculum.

Can a physical change be reversed, and how does this compare to chemical changes?

Physical changes are often reversible, such as freezing and melting water. This is because the substance's chemical identity remains unchanged. In contrast, chemical changes usually result in new substances that cannot be easily reverted to their original form, like burning paper turning into ash and gases. This distinction is important in IB MYP Science for understanding the permanence of chemical transformations.

How do temperature and pressure affect physical and chemical changes?

Temperature and pressure can significantly influence both physical and chemical changes. In physical changes, increasing temperature can cause substances to change state, such as ice melting into water. In chemical changes, temperature can affect the rate of reaction, with higher temperatures generally increasing reaction rates. Pressure can also impact reactions, especially those involving gases, by altering the concentration of reactants. These concepts are integral to the study of chemical kinetics in the IB MYP Science curriculum.

Why is "Calling 'dissolving sugar in water' a chemical change." a common mistake in Physical and Chemical Changes?

Why it happens: The sugar 'disappears'. How to avoid it: Sugar molecules are still there — they have just spread out among the water molecules. Evaporate the water and the sugar reappears. PHYSICAL.

Why is "Claiming a chemical reaction can break conservation of mass." a common mistake in Physical and Chemical Changes?

Why it happens: Reactions sometimes seem to gain or lose mass. How to avoid it: Mass changes are always due to a gas entering or leaving the system. Use a SEALED container and the total mass stays the same.

Why is "Balancing equations by changing subscripts (e.g. H₂O → H₃O₂)." a common mistake in Physical and Chemical Changes?

Why it happens: It would balance — but changes the substance. How to avoid it: You can only change COEFFICIENTS (numbers in front of formulae). Subscripts inside a formula are fixed by the substance itself.

Chemical kineticsIB MYP Sciences (Years 1-5)

Physical and Chemical Changes

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Physical and Chemical Changes — IB MYP Sciences: tell them apart by what happens to the substance

Melt ice, dissolve sugar, snap a twig — those are physical changes. Burn wood, rust iron, digest food — those are chemical changes. This MYP Sciences note covers the key differences, signs of a chemical change, and the law of conservation of mass.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Define and distinguish physical and chemical changes.
MYP Sciences A — Identify common signs of a chemical change.
MYP Sciences A — Apply the law of conservation of mass.
MYP Sciences C — Balance simple chemical equations.

Physical changes

Change the shape, state or location — but the SUBSTANCE stays the same.

A physical change doesn't create new substances. The molecules are still the same; they're just rearranged. Examples:

Melting ice → water (still H₂O).
Cutting paper (same paper, just smaller).
Dissolving salt in water (salt is still NaCl; you can recover it by evaporating).
Stretching a rubber band.

Most physical changes are reversible: refreeze water, evaporate salt water, let the rubber band relax. The substance is unchanged at the end.

No new substance.
Usually reversible.
Examples: melting, dissolving, cutting, stretching.

Chemical changes

At least one new substance is formed.

A chemical change (or chemical reaction) produces at least one substance with NEW properties. Reactants are rearranged into products with different molecules.

Common signs that a chemical change has happened:

Colour change (paper turns black when burnt; copper turns green when corroded).
Gas produced (fizz in vinegar + baking soda).
Temperature change (combustion releases heat; some dissolving reactions absorb heat).
Light or sound (burning magnesium; explosions).
Precipitate (an insoluble solid forms when two clear solutions are mixed).
Difficult to reverse (you can't easily unburn the toast).

Examples:

Iron + oxygen + water → rust (Fe₂O₃·nH₂O).
Methane + oxygen → carbon dioxide + water + heat (combustion).
Mg + 2HCl → MgCl₂ + H₂ (acid + metal).
Photosynthesis: CO₂ + H₂O → glucose + O₂.

Physical change: substance unchanged (just rearranged). Chemical change: new substances formed.

At least one new substance.
Usually hard to reverse.
Signs: colour, gas, T change, light, precipitate.

Conservation of mass and balancing equations

Atoms are rearranged but not created or destroyed.

In any chemical reaction:

Total mass of reactants = Total mass of products.

Atoms are rearranged but not made or destroyed. This is the law of conservation of mass.

Implication: in a balanced equation, the number of each type of atom must be the same on both sides. Take this unbalanced equation for hydrogen burning:

$\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$

Atoms: 2H, 2O on the left; 2H, 1O on the right — not balanced. Multiply H₂O by 2:

$\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$

Now: 2H, 2O on left; 4H, 2O on right — still not balanced. Multiply H₂ by 2:

$2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$

Now: 4H, 2O on both sides. ✓

Apparent mass change? If a reaction LOSES mass it's because a gas escapes (e.g. CaCO₃ → CaO + CO₂). If it GAINS mass it's because the substance has absorbed something from outside (e.g. magnesium burning gains the mass of O₂ from the air).

Conservation of mass: atoms aren't created or destroyed.
Balanced equation = same number of each atom both sides.
Apparent mass change = a gas was lost or gained.

How it’s examined

Criterion A tests classification and identifying signs. Criterion C asks you to balance simple equations and explain apparent mass changes when a gas is involved.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — Physical and Chemical Changes

Step-by-step solutions to past-paper-style questions on physical and chemical changes, written exactly the way a tutor would explain them at the board.

1Classifying changes
Getting started• classification
Question
Classify each as physical or chemical: (a) ice melting, (b) wood burning, (c) sugar dissolving, (d) iron rusting, (e) tearing paper.
Step-by-step solution
1. Step 1
  (a) Ice → water: no new substance → PHYSICAL.
2. Step 2
  (b) Wood + O₂ → CO₂ + H₂O + ash: new substances → CHEMICAL.
3. Step 3
  (c) Sugar in water: still sugar (recoverable) → PHYSICAL.
4. Step 4
  (d) Iron + O₂ + H₂O → rust: new substance → CHEMICAL.
5. Step 5
  (e) Smaller pieces of paper: same substance → PHYSICAL.
Answer
(a) physical, (b) chemical, (c) physical, (d) chemical, (e) physical.
2Balancing an equation
Building confidence• balancing
Question
Balance: __ Mg + __ O₂ → __ MgO.
Step-by-step solution
1. Step 1
  Mg: 1 on left, 1 on right. O: 2 on left, 1 on right. Multiply MgO by 2 → 2 Mg, 2 O on right.
2. Step 2
  Now Mg on left needs to be 2. Final: 2 Mg + O₂ → 2 MgO.
Answer
2 Mg + O₂ → 2 MgO.
3Apparent loss of mass
Stretch• conservation of mass
Question
Calcium carbonate (CaCO₃) is heated in an open dish. The mass of solid in the dish DECREASES. Explain why this doesn't violate the law of conservation of mass.
Step-by-step solution
1. Step 1
  Reaction: CaCO₃ → CaO + CO₂.
2. Step 2
  CO₂ is a gas that escapes from the open dish — it leaves the system.
3. Step 3
  Total mass (CaO remaining + CO₂ in the air) = original mass of CaCO₃. Mass is conserved overall; the system just lost mass to the surroundings.
Answer
CO₂ gas escapes from the open dish. If you weighed CO₂ in the air plus the CaO remaining, total mass = original CaCO₃ mass.

Key Definitions and Keywords — Physical and Chemical Changes

Definitions to memorise and the exact keywords mark schemes credit for physical and chemical changes answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Physical change
Examiner keyword
A change in which no new substance is formed; usually easily reversible.
Chemical change
Examiner keyword
A change in which at least one new substance is formed.
Conservation of mass
Examiner keyword
In any chemical reaction, the total mass of the reactants equals the total mass of the products.
Balanced equation
Examiner keyword
A chemical equation where the number of each type of atom is the same on both sides.

Common Mistakes and Misconceptions — Physical and Chemical Changes

The traps other students keep falling into on physical and chemical changes questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Calling 'dissolving sugar in water' a chemical change.
Why it happens
The sugar 'disappears'.
How to avoid it
Sugar molecules are still there — they have just spread out among the water molecules. Evaporate the water and the sugar reappears. PHYSICAL.
✕Claiming a chemical reaction can break conservation of mass.
Why it happens
Reactions sometimes seem to gain or lose mass.
How to avoid it
Mass changes are always due to a gas entering or leaving the system. Use a SEALED container and the total mass stays the same.
✕Balancing equations by changing subscripts (e.g. H₂O → H₃O₂).
Why it happens
It would balance — but changes the substance.
How to avoid it
You can only change COEFFICIENTS (numbers in front of formulae). Subscripts inside a formula are fixed by the substance itself.

Practice questions

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Physical and Chemical Changes — frequently asked questions

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Physical and Chemical Changes

Short Study Notes in the form of Slides

Short Study Notes — Physical and Chemical Changes

Detailed Notes

What you’ll learn

How it’s examined

1Classifying changes

2Balancing an equation

3Apparent loss of mass

Physical change

Chemical change

Conservation of mass

Balanced equation

✕Calling 'dissolving sugar in water' a chemical change.

✕Claiming a chemical reaction can break conservation of mass.

✕Balancing equations by changing subscripts (e.g. H₂O → H₃O₂).

Practice questions

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Physical and Chemical Changes — frequently asked questions