Physical vs Chemical Changes
Properties, Reactions, Conservation of Mass & Evidence of Change
Overview
Every change that matter undergoes falls into one of two categories: physical or chemical. This distinction is one of the most fundamental in all of chemistry, and learning to classify changes correctly is an essential skill.
The key question is always the same: does the chemical identity of the substance change? If the answer is no, it is a physical change. If a new substance with different properties is produced, it is a chemical change.
- Chemical identity is preserved
- Only form, shape, or state changes
- Generally reversible
- No new substances are formed
- Original substance can be recovered
- Examples: melting, cutting, dissolving, boiling
- Chemical identity is altered
- New substance(s) with new properties formed
- Generally irreversible
- Involves breaking and forming chemical bonds
- Original substance cannot be simply recovered
- Examples: burning, rusting, cooking, digestion
Physical Changes
A physical change alters the form or appearance of matter without changing its chemical composition. The substance before and after the change is the same substance — it is just in a different state, shape, or arrangement.
Physical changes include all phase changes (melting, freezing, boiling, condensation, sublimation), as well as cutting, bending, dissolving, and mixing. Because no chemical bonds are broken or formed between atoms, the original substance can always be recovered.
Dissolving is usually a physical change. When salt dissolves in water, the ions separate and disperse, but their chemical identity is unchanged. Evaporate the water and you recover the same salt. However, some substances dissolve by reacting chemically with water — in those cases, dissolving is a chemical change.
| Example | What changes | What stays the same | Type |
|---|---|---|---|
| Ice melting | State (solid → liquid) | H₂O molecules | Physical |
| Cutting paper | Shape and size | Cellulose composition | Physical |
| Salt dissolving in water | Arrangement of ions | NaCl and H₂O identity | Physical |
| Gold being hammered flat | Shape | Gold atoms (Au) | Physical |
| Sugar dissolved in tea | Particle distribution | Sucrose molecule | Physical |
| Water boiling to steam | State (liquid → gas) | H₂O molecules | Physical |
Chemical Changes
A chemical change — also called a chemical reaction — produces one or more new substances with different chemical properties. Atoms are rearranged as chemical bonds are broken and new ones are formed. The starting materials (reactants) are converted into different substances (products).
Chemical changes are typically difficult or impossible to reverse by simple physical means. You cannot un-burn wood or un-rust iron by just heating or cooling it.
| Example | Reactants | Products formed | Type |
|---|---|---|---|
| Wood burning | Cellulose + O₂ | CO₂ + H₂O + ash | Chemical |
| Iron rusting | Fe + O₂ + H₂O | Fe₂O₃ (iron oxide) | Chemical |
| Cooking an egg | Liquid proteins | Denatured proteins (solid) | Chemical |
| Baking soda + vinegar | NaHCO₃ + CH₃COOH | CO₂ + H₂O + NaCH₃COO | Chemical |
| Digesting food | Complex molecules | Simpler molecules (sugars, amino acids) | Chemical |
| Silver tarnishing | Ag + H₂S | Ag₂S (silver sulfide) | Chemical |
In chemistry, we write chemical reactions as equations. The reactants appear on the left side of the arrow, and the products appear on the right. The arrow (→) means “reacts to form” or “yields.”
Reactants → Products
e.g. CH₄ + 2O₂ → CO₂ + 2H₂O
Evidence of Chemical Change
How do you know a chemical change has occurred? Since a new substance is formed, it will often have noticeably different properties from the reactants. There are six key indicators to look for — though none on their own is definitive proof. The true test is whether a new substance has formed.
Bubbles or fizzing indicate a gas is being produced. E.g. adding vinegar to baking soda releases CO₂ bubbles. Distinct from air bubbles produced by boiling.
A permanent change in colour (not from mixing coloured liquids). E.g. iron turning orange-brown as rust forms; a copper roof turning green (patina).
A solid forms from two clear solutions being mixed. E.g. mixing lead nitrate and potassium iodide solutions produces a bright yellow precipitate of lead iodide.
Heat is absorbed or released. Exothermic reactions feel hot (combustion, hand warmers); endothermic reactions feel cold (dissolving ammonium nitrate in water).
Some reactions produce light. Combustion reactions glow or flame; chemiluminescent reactions (glow sticks) produce cold light without significant heat.
A new or different smell often signals a new substance. E.g. meat cooking, milk souring, or the distinctive smell of sulfur dioxide when a match is struck.
These indicators are evidence of a chemical change, not proof. A colour change might just be mixing two differently coloured solutions physically. A temperature change can occur during dissolving (a physical process). Always ask: has a new substance with new properties been formed?
Conservation of Mass
One of the most important laws in chemistry governs what happens to mass during a chemical reaction. French chemist Antoine Lavoisier established this principle experimentally in the late 18th century.
In any chemical reaction, matter is neither created nor destroyed — it is only rearranged. The total mass of the substances entering a reaction equals the total mass of the substances produced. Atoms are simply reorganised into new arrangements; none are lost or gained.
This law has a profound practical consequence: it is why we balance chemical equations. A balanced equation has the same number of each type of atom on both sides. We will explore equation balancing in detail in Lesson 1.8.
When wood burns, the solid residue (ash) weighs far less than the original wood. This seems to contradict conservation of mass — but it doesn’t. The “missing” mass has become gases: carbon dioxide and water vapour that escape into the air. If you could collect all the gases produced and weigh them, total mass would be conserved.
The starting substances in a chemical reaction. They appear on the left side of a chemical equation, before the arrow.
The new substances formed during a chemical reaction. They appear on the right side of the equation, after the arrow.
A reaction that releases energy (usually as heat) to the surroundings. The products have less energy than the reactants. E.g. combustion, rusting, respiration.
A reaction that absorbs energy from the surroundings. The products have more energy than the reactants. E.g. photosynthesis, dissolving ammonium nitrate.
Introduction to Reaction Types
Chemical reactions can be grouped into broad categories based on what happens to the reactants and products. You will study each of these in depth later in the curriculum — for now, learn to recognise the basic pattern of each.
-
Synthesis
Two or more substances combine to form a single new substance. Also called a combination reaction.
A + B → AB | 2H₂ + O₂ → 2H₂O -
Decomposition
A single compound breaks down into two or more simpler substances. The reverse of synthesis.
AB → A + B | 2H₂O → 2H₂ + O₂ -
Combustion
A substance reacts rapidly with oxygen, releasing heat and light. Complete combustion of hydrocarbons produces CO₂ and H₂O.
fuel + O₂ → CO₂ + H₂O | CH₄ + 2O₂ → CO₂ + 2H₂O -
Single Displacement
One element replaces another in a compound. Only occurs if the replacing element is more reactive than the one it displaces.
A + BC → AC + B | Zn + CuSO₄ → ZnSO₄ + Cu -
Double Displacement
Two compounds exchange partners (ions) to form two new compounds. Often produces a precipitate, gas, or water.
AB + CD → AD + CB | NaCl + AgNO₃ → NaNO₃ + AgCl↓
Worked Examples
Key test: Is the chemical identity of the substance the same before and after?
Apply Conservation of Mass: Mass of reactants = mass of products.
4.0 g + 32.0 g = 36.0 g of water produced
No matter is created or destroyed — all 36.0 g of reactants become 36.0 g of product.
Practice Questions
Q1. Which of the following is a chemical change?
Q2. The Law of Conservation of Mass states that in a chemical reaction:
Q3. A student mixes two clear solutions and a yellow solid forms at the bottom of the flask. This is evidence of:
Q4. The reaction 2H₂O → 2H₂ + O₂ is best classified as:
Q5. 12 g of carbon reacts completely with 32 g of oxygen to form carbon dioxide. What is the mass of CO₂ produced?
Key Takeaways
- A physical change alters form, shape, or state but not chemical identity. The original substance can be recovered.
- A chemical change (reaction) produces one or more new substances with different properties. It is generally irreversible.
- Evidence of chemical change: gas production, colour change, precipitate formation, temperature change, light emission, odour change.
- The Law of Conservation of Mass: mass of reactants = mass of products. Atoms are rearranged, not created or destroyed.
- Reactants appear before the arrow in a chemical equation; products appear after.
- Exothermic reactions release energy; endothermic reactions absorb energy from surroundings.
- The five basic reaction types are synthesis, decomposition, combustion, single displacement, and double displacement.