Chemical Change
Summary: A chemical change is a transformation in which one or more new substances with different chemical properties are formed through the breaking of existing chemical bonds in the reactants and the forming of new bonds in the products — a process that is generally difficult or impossible to reverse by simple physical means. Tags: chemical-change igcse-chemistry bond-breaking bond-making chemical-reactions stoichiometry Created: 2026-07-18
A chemical change (also called a chemical reaction) is a fundamental process in which the atomic and molecular structure of a substance is reconfigured: the chemical bonds holding atoms together in the reactant substances are broken, the constituent atoms rearrange, and entirely new bonds form to produce one or more products that have a chemical identity distinct from the starting materials. This is fundamentally different from a Physical Change, where the chemical composition remains unchanged — only the physical state (solid, liquid, gas) or appearance is altered, and intermolecular forces (not chemical bonds) are overcome. Because the products of a chemical change are chemically different substances, the change is accompanied by one or more observable indicators: a permanent colour change (e.g. iron turning reddish-brown as it rusts), the evolution of a gas (bubbles forming without heating, as in the reaction of an acid with a metal carbonate), the formation of a precipitate (an insoluble solid emerging when two clear solutions are mixed), a measurable temperature change (exothermic or endothermic), or the emission of light. Crucially, during any chemical change the total mass is conserved — atoms are neither created nor destroyed, only rearranged into new combinations, a principle enshrined in the Law of Conservation of Mass and the quantitative foundation of all Chemical Equations and Calculations. Chemical changes are typically difficult or energetically prohibitive to reverse (e.g. you cannot un-bake a cake or un-rust iron by any simple means), and understanding the distinction between chemical and physical change — and the atomic-level bond-breaking and bond-making that defines the former — is one of the core conceptual pillars of the IGCSE Chemistry syllabus.
What Defines a Chemical Change
A process is classified as a chemical change if it meets the central criterion: at least one new substance is produced, and that substance has chemical properties different from those of the reactants. At the atomic level, this always means:
- Bonds in the reactants are broken — energy is absorbed (endothermic step) to separate atoms from their existing bonding arrangements.
- Atoms rearrange — the now-free atoms reorganise into new configurations.
- New bonds form in the products — energy is released (exothermic step) as atoms come together into more stable arrangements.
This bond-breaking and bond-making sequence is the universal mechanism of all chemical change. Whether the reaction is overall exothermic or endothermic depends on the energy balance between these two steps: if more energy is released in bond making than is absorbed in bond breaking, the reaction is exothermic (ΔH negative); if the reverse is true, it is endothermic (ΔH positive).
Observable Signs of a Chemical Change
In the laboratory, a chemical change can often be identified by one or more of the following observations. Any single sign is suggestive but not definitive — chemists look for a combination of indicators and, ideally, verification that the products have different properties from the reactants.
| Sign | What is Observed | Example |
|---|---|---|
| Colour change | A permanent, non-reversible colour shift that indicates a new substance has formed. | Iron turning reddish-brown as it rusts (Fe → Fe₂O₃·xH₂O); colourless lead nitrate mixing with colourless potassium iodide to form bright yellow lead iodide. |
| Gas production | Bubbles of gas forming in a liquid, without heating the liquid to boiling. | Magnesium ribbon reacting with dilute hydrochloric acid produces hydrogen gas; marble chips (CaCO₃) reacting with acid produce carbon dioxide. |
| Precipitate formation | A solid appearing when two clear solutions are mixed — the solid was not originally present and cannot dissolve. | Mixing silver nitrate solution with sodium chloride solution produces a white precipitate of silver chloride (AgCl). |
| Temperature change | The reaction mixture becomes hotter (exothermic) or colder (endothermic) without any external heating or cooling. | Neutralisation of HCl with NaOH releases heat; dissolving ammonium nitrate in water absorbs heat and the beaker feels cold. |
| Light emission | Visible light, sparks, or flames are produced during the reaction. | Combustion of magnesium in air produces an intense white light; the thermite reaction produces molten iron and sparks. |
| Odour change | A new smell is produced that was not present in the reactants. | The pungent smell of sulfur dioxide gas when sulfur burns in oxygen; the sour smell of vinegar (ethanoic acid) when wine oxidises. |
Important: A substance changing state (melting, boiling, condensing, freezing) is not a chemical change. These are physical changes — no new substance is formed, no bonds are broken or made, and the change is easily reversed by altering the temperature.
Chemical Change vs. Physical Change
The distinction between chemical and physical change is a core IGCSE topic. The table below summarises the key differences:
| Criterion | Chemical Change | Physical Change |
|---|---|---|
| Are new substances formed? | Yes — products have different chemical properties from reactants. | No — the chemical identity remains the same; only the physical state or appearance changes. |
| Are chemical bonds broken and made? | Yes — existing bonds in reactants break, new bonds form in products. | No — only intermolecular forces are overcome (e.g. hydrogen bonds during melting); chemical bonds within molecules remain intact. |
| Is the change easily reversed? | Usually not — reversing a chemical change generally requires another chemical reaction and significant energy input (e.g. electrolysis to reverse the decomposition of water). | Usually yes — physical changes like melting, boiling, dissolving, and freezing are reversed by altering conditions (temperature, pressure). |
| Is mass conserved? | Yes — total mass of products = total mass of reactants (atoms are rearranged, not created or destroyed). | Yes — mass is also conserved; the same particles are present before and after. |
| Is energy involved? | Often a measurable energy change (exothermic or endothermic) due to bond breaking and making. | Energy changes are much smaller — they involve overcoming or re-forming intermolecular forces, not chemical bonds. |
| Examples | Rusting, combustion, neutralisation, photosynthesis, thermal decomposition, cooking an egg, digestion. | Melting ice, boiling water, dissolving sugar in tea, cutting paper, changing the shape of clay. |
A helpful diagnostic question: “Can I get my original substance back by filtering, evaporating, distilling, or changing the temperature?” If the answer is yes, the change is probably physical. If you would need to carry out a separate chemical reaction to recover the starting material, the change was chemical.
The Atomic-Level Mechanism: Bond Breaking and Bond Making
The defining feature of a chemical change — what makes it chemical rather than physical — is the breaking and forming of chemical bonds. This is described in detail on the Bond Breaking and Bond Making pages; the essentials as they relate to identifying chemical change are summarised here.
When a chemical change occurs:
- Bond breaking is the first step. Energy must be supplied (endothermic) to overcome the attractive electrostatic forces holding atoms together within the reactant molecules or lattices. The amount of energy required depends on the Bond Energy of the specific bonds involved.
- Atoms rearrange. Once reactant bonds are broken, the individual atoms are free to recombine in new spatial arrangements — this is the “rearrangement” step.
- Bond making is the second step. When atoms come together to form new bonds, energy is released (exothermic) because the bonded state is more stable (lower in energy) than separated atoms.
The net energy change for the overall reaction is:
ΔH = Σ(bond energies of bonds broken) − Σ(bond energies of bonds formed)
If more energy is released in bond making than was absorbed in bond breaking, the reaction is overall exothermic. If more energy is absorbed than released, the reaction is overall endothermic. In either case, the critical point is that bonds have been broken and new bonds have been made — and it is this that defines the change as chemical.
Conservation of Mass in Chemical Change
A law of fundamental importance: during any chemical change, the total mass of the products equals the total mass of the reactants. This is the Law of Conservation of Mass, first articulated by Antoine Lavoisier in the late 18th century.
At the atomic level, the explanation is straightforward: chemical changes rearrange atoms but do not create or destroy them. Every atom present in the reactants is still present in the products — just bonded differently. This is why chemical equations must be balanced: the number of atoms of each element on the left (reactants) must equal the number on the right (products).
Example: When magnesium burns in oxygen — 2Mg(s) + O₂(g) → 2MgO(s)
- If you start with 48.6 g of magnesium and 32.0 g of oxygen (total = 80.6 g), you will produce exactly 80.6 g of magnesium oxide.
- The magnesium and oxygen atoms have rearranged from Mg and O₂ into MgO, but every atom is accounted for.
In open systems, mass may appear to change — for example, a piece of magnesium burning in a crucible may gain mass because oxygen from the air combines with it. This is not a violation of conservation of mass; the oxygen was simply not included in the initial measurement. A properly sealed (closed) system will always show that total mass is conserved.
Common Examples of Chemical Change
The following examples span laboratory, industrial, environmental, and biological contexts — all are explicitly referenced in the Cambridge IGCSE Chemistry 0620 syllabus.
| Chemical Change | Reactants | Products | Type / Notes |
|---|---|---|---|
| Rusting of iron | Iron (Fe), oxygen (O₂), water (H₂O) | Hydrated iron(III) oxide (Fe₂O₃·xH₂O) | A slow oxidation reaction; requires both oxygen and water; the reddish-brown rust has completely different properties from metallic iron. |
| Combustion of methane | CH₄ + 2O₂ | CO₂ + 2H₂O | Exothermic; the products are chemically different gases; energy is released as heat and light. |
| Neutralisation | HCl + NaOH | NaCl + H₂O | Exothermic (ΔH ≈ −57 kJ mol⁻¹ for strong acid–strong base); a new substance (water) is formed along with a salt; the acidic and alkaline properties are destroyed. |
| Thermal decomposition of calcium carbonate | CaCO₃ | CaO + CO₂ | Endothermic; requires strong heating (~900 °C); the solid product (quicklime) has completely different properties from the reactant (limestone). |
| Photosynthesis | 6CO₂ + 6H₂O | C₆H₁₂O₆ + 6O₂ | Endothermic; light energy is absorbed and stored as chemical energy in glucose; the most important chemical change for life on Earth. |
| Cooking an egg | Liquid egg white (soluble globular proteins) | Solid egg white (denatured, cross-linked proteins) | The proteins undergo irreversible denaturation and coagulation — a chemical change; the egg cannot be “un-cooked.” |
| Electrolysis of water | 2H₂O | 2H₂ + O₂ | Electrical energy breaks the O—H bonds in water; the products (hydrogen and oxygen gases) are entirely different substances. |
| Precipitation of silver chloride | AgNO₃(aq) + NaCl(aq) | AgCl(s) + NaNO₃(aq) | Two clear solutions produce a white solid — a classic observable sign of chemical change. |
| Respiration | C₆H₁₂O₆ + 6O₂ | 6CO₂ + 6H₂O | Exothermic; cells break down glucose to release energy; chemically, the reverse of photosynthesis. |
”Irreversibility” as a Practical Criterion
Chemical changes are often described as “irreversible,” but this term requires care. Many chemical changes can be reversed — but only by carrying out a separate chemical reaction, often under extreme conditions and with significant energy input. For example:
- Water decomposed by electrolysis (2H₂O → 2H₂ + O₂) can be reconstituted by igniting the hydrogen in oxygen (2H₂ + O₂ → 2H₂O) — but this is a different chemical reaction conducted under different conditions.
- Calcium carbonate thermally decomposed (CaCO₃ → CaO + CO₂) cannot be re-formed simply by cooling the products; you would need to react CaO with CO₂ in a separate process.
A genuinely irreversible chemical change is one where the products are so energetically stable or physically dispersed that no practical reverse reaction exists — cooking an egg, burning a log, or rusting iron fall into this category.
The key distinction for IGCSE: physical changes are reversed by changing conditions (temperature, pressure); chemical changes are reversed (if at all) only by carrying out another chemical reaction.
Chemical Change and the IGCSE Curriculum
Chemical change is not a standalone topic in the Cambridge IGCSE Chemistry 0620 syllabus — it is the thread that runs through every topic. The syllabus introduces the concept early (Section 2: Experimental Techniques and Chemical Reactions) and returns to it in increasingly sophisticated contexts throughout:
- Section 2.1 (Experimental Techniques): Identifying chemical vs. physical change through observable indicators.
- Section 3 (Stoichiometry): Writing balanced equations that represent the atomic rearrangement in chemical change; applying conservation of mass.
- Section 4 (Electrochemistry): Electrolysis as a chemical change driven by electrical energy.
- Section 5 (Chemical Energetics): The energy changes (ΔH) that accompany bond breaking and bond making in chemical change.
- Section 6.1 (Physical and Chemical Changes): Direct comparison of the two types of change; the central concept that chemical changes produce new substances.
- Sections 7–11 (Inorganic and Organic Chemistry): All specific reaction types — acid–base, redox, thermal decomposition, displacement, combustion, addition, substitution, condensation, polymerisation — are specific categories of chemical change.
- Section 12 (Experimental Skills): Designing and evaluating experiments that demonstrate or exploit chemical change.
Related Pages
- Physical Change — the complementary concept; changes that alter state or appearance without forming new substances
- Bond Breaking — the endothermic first step of every chemical change
- Bond Making — the exothermic second step of every chemical change
- Exothermic Reaction — chemical changes that release energy to the surroundings
- Endothermic Reaction — chemical changes that absorb energy from the surroundings
- Chemical Equations and Calculations — representing chemical change symbolically using balanced equations
- Redox Reactions — a major class of chemical changes involving electron transfer
- Law of Conservation of Mass — the principle that total mass is conserved in every chemical change
- Enthalpy Change — quantifying the energy transferred during chemical change
- Bond Energy — the energy values that determine the net ΔH of a chemical change
- Activation Energy — the minimum energy required to initiate a chemical change
- Reaction Pathway Diagram — visual representation of the energy changes during a chemical change
- Stoichiometry — the quantitative study of reactants and products in chemical change
- Collision Theory — the particle-level explanation for how and why chemical changes occur
- Reversible Reactions — chemical changes that can proceed in both forward and reverse directions
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023–2025), Section 6.1: Physical and Chemical Changes, and Section 2.1: Experimental Techniques
- Harwood, R. & Lodge, I., Cambridge IGCSE Chemistry Coursebook, 5th Edition, Cambridge University Press, 2021, Chapter 2: The Nature of Matter, and Chapter 3: Elements and Compounds
- Gallagher, R. & Ingram, P., Complete Chemistry for Cambridge IGCSE, 3rd Edition, Oxford University Press, 2016, Chapter 2: The Nature of Matter, and Chapter 8: Chemical Reactions
- Clegg, A. et al., Cambridge IGCSE Chemistry Study and Revision Guide, Hodder Education, 2017, Section 2: Experimental Techniques
- Earl, B. & Wilford, D., IGCSE Chemistry, Hodder Education, 2009, Chapter 1: The Particulate Nature of Matter
- Royal Society of Chemistry, “Chemical Change vs. Physical Change,” Education in Chemistry, rsc.org
- BBC Bitesize, “Chemical Changes,” GCSE Chemistry (AQA), bbc.co.uk/bitesize
Common Misconceptions
| Misconception | Correction |
|---|---|
| ”A change of state (melting, boiling, condensing) is a chemical change because the substance looks different.” | Changes of state are physical changes — no new substance is formed. When ice melts, it is still H₂O; the particles are the same, only their arrangement and the forces between them have changed. Chemical bonds within the water molecules are not broken during melting or boiling — only the hydrogen bonds between molecules are overcome. The distinction between overcoming intermolecular forces (physical change) and breaking chemical bonds (chemical change) is a frequently examined IGCSE concept. |
| ”If a solid disappears when mixed with a liquid, it must have undergone a chemical change.” | Dissolving is typically a physical change. When salt (NaCl) dissolves in water, the Na⁺ and Cl⁻ ions separate and disperse throughout the solution, but the ions themselves are unchanged. The salt can be recovered by evaporating the water. Compare this with a genuine chemical change: when magnesium dissolves in hydrochloric acid, it is chemically transformed into Mg²⁺ ions and hydrogen gas — the magnesium atoms have undergone oxidation, and the original magnesium cannot be recovered by evaporation. |
| ”A colour change always means a chemical change has occurred.” | Some colour changes are physical. Diluting a coloured solution (e.g. adding water to orange squash) changes the colour intensity without any chemical reaction. Similarly, heating a metal wire can cause a temporary colour change (red hot) due to increased thermal vibration of atoms — but no new substance is formed. A permanent colour change where the new colour cannot be reversed by cooling or dilution strongly suggests (but does not prove) a chemical change. |
| ”Chemical changes are always irreversible.” | Many chemical changes are reversible in principle — but only by carrying out a different chemical reaction. The thermal decomposition of CaCO₃ → CaO + CO₂ is a chemical change that can be reversed by reacting CaO with CO₂. Electrolysis decomposes water into H₂ and O₂; burning the H₂ in O₂ re-forms the water. True irreversibility (like cooking an egg) arises when the product is so energetically stable, structurally altered, or dispersed that no practical reverse pathway exists. The syllabus expects students to understand that “irreversibility” is a useful practical rule of thumb, not an absolute definition. |
| ”Mass can change during a chemical change (e.g. a log burning leaves a small pile of ash, which weighs much less than the original log).” | The apparent mass change occurs because the system is open — the gaseous products (CO₂, H₂O vapour) escape into the atmosphere and are not weighed. If the burning log were placed in a sealed container with sufficient oxygen and all products (solids, liquids, and gases) were weighed together, the total mass would be exactly conserved. The Law of Conservation of Mass always holds in a closed system; apparent mass changes in open systems are a measurement artefact, not a chemical reality. |