From electronic device to fizzy soda, our world runs on chemical reactions. So what happens in chemical reactions? In a simplified definition, specific interactions with molecules result in the rearrangement of atoms to create new molecules. However, this magic has restrictions, as will be discussed in the next sections.
What is a chemical reaction?
There are two types of changes in chemistry when discussing matter: physical changes and chemical changes.
A physical change is one that changes an object but does not change its chemical structure. For example, you may boil water into steam, but H2O stays H2 during that process. Similarly, shredding paper would be an example of a physical change. Paper may change from a piece to shreds, but it stays "paper".
Changes in the state of matter of a substance or the formation/separation of substances are common physical changes. Nothing changes on a molecular level, rather, the properties of the substance change.
However, chemical changes, as the name implies, end with a brand-new product. There must be some sort of change on a molecular level with chemical changes! For example, if you leave iron out for too long, it rusts. Rust is a chemical reaction between iron (Fe) and oxygen in the air (O2) to form rust (iron oxide, Fe2O3).
Another example of a chemical change is mixing baking soda (NaHCO3) with vinegar (CH3COOH) to form carbon dioxide and other products. Chemical changes are the focus of AP Chemistry and are described using chemical reactions. The following are observations that are evidence of a chemical change, and therefore, a chemical reaction taking place:
- 💡Heat or light: combustion reactions, as we'll discuss below, are evident with fire.
- 🫧 Formation of gas (and maybe an odor): you may see bubbles.
- 💧 Precipitation: this refers to the formation of a solid when two solutions mix
- 🌈 Color change: using indicators, or chemicals that change color based on pH.
How are chemical reactions represented?
A chemical reaction can be represented using a chemical equation. A chemical equation is a written representation of the substances involved in a chemical reaction and the changes they undergo. Here is how to read a chemical equation:
- The reactants, or the substances that are reacting, are listed on the left side of the equation.
- The products, or the substances that are produced, are listed on the right side of the equation.
- The arrow (→) indicates the direction of the reaction, from left to right.
- The coefficients, or the numbers in front of the reactant and product formulas, indicate the relative amounts of each substance involved in the reaction. For example, in the reaction 2H2 + O2 -> 2H2O, the coefficient "2" in front of H2O indicates that two molecules of water are produced.
- The formulas of the reactants and products represent the chemical compositions of the substances involved in the reaction. The formulas are made up of symbols for the elements present in the substances, with subscripts indicating the number of atoms of each element. For example, the formula H2O represents a molecule of water, which is made up of two atoms of hydrogen and one atom of oxygen.
There are five main types of reactions: synthesis, decomposition, combustion, single replacement, and double replacement. While we will go over these in later sections, let’s do a brief overview of each type:
Synthesis Reactions
Synthesis reactions are the easiest type to understand. Essentially, a synthesis reaction sees two reactants fuse to form products in the form A + B → AB, where A, B, and AB are arbitrary.
An example of a synthesis reaction is 2Na + Cl2 → 2NaCl, as it sees two reactants, Na and Cl2, become one product: NaCl. The synthesis of water from hydrogen and oxygen is another example of a synthesis reaction: 2H2 + O2 → 2H2O.
Decomposition Reactions
Decomposition reactions are the exact opposite: one reactant decomposes into two or more products. This can be represented by the general equation: AB → A + B.
For example, hydrogen peroxide decomposes into hydrogen gas and water in the reaction 2H2O2 → 2H2O + H2. You can also reverse the synthesis of water in the last section to represent the decomposition of water: 2H2O → 2H2 + O2.
Combustion Reactions
A combustion reaction is a special type of decomposition reaction involving organic molecules, which are molecules that are carbon-based. A special class of organic molecules is called hydrocarbons which are only made up of hydrogen and carbon molecules.
When in the presence of heat and oxygen, hydrocarbons combust or burn which is really just the energy being released from a chemical reaction! Therefore, a general combustion reaction sees a hydrocarbon being combusted into carbon dioxide and water in the presence of oxygen.
Combustion reactions always follow this format: Hydrocarbon + Oxygen → Carbon Dioxide + Water. For example, CH4 + 2O2 → CO2 + 2H2O is the combustion of methane (CH4). This definition of combustion is why oxygen is necessary for fires to start.
Single Replacement Reactions
Single replacement reactions occur when you have a compound reacting with an element. The general form is AB + C → AC + B. The most common form of a single replacement reaction is one called a redox reaction, where electrons are transferred between atoms. An example is 3Mg + 2AlCl3 → 3MgCl2 + 2Al.
Single = one, replacement = a switch. These reactions involve the replacement of one element in a compound by another element. Only one switch takes place.
Double Replacement Reactions
The most common type of reaction is a double replacement reaction, with the general form AB + CD → AD + CB.
For example, if you mix an acid and a base, you get a reaction that forms water and some type of salt. A formula example would be HCl + NaOH → NaCl + H2O. These reactions are the majority of the reactions you'll see in AP Chemistry, so get used to them!
Double = two, replacement = switch. These reactions involve the exchange of ions between two compounds to form two new compounds. Here, two switches take place.
Practice Questions
Identify what type of reaction each of the following are:
- Zn + 2HCl → ZnCl2 + H2
- 2C8H18 + 25O2 → 16CO2 + 18H2O
- 2H2O → 2H2 + O2
- AgNO3 + NaCl → AgCl + NaNO3
- 2Ni2O3 → 4Ni + 3O2
- 2Na + Cl2 → 2NaCl
- Cl2 + 2NaBr → 2NaCl + Br2
- BaCl2 + Na2SO4 → BaSO4 + 2 NaCl
- C3H8 + 5O2 → 3CO2 + 4H2O
Answers
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Single Replacement - Zinc is "switching places" with hydrogen to form zinc chloride and hydrogen gas.
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Combustion - You can see the general formula of a combustion reaction of a hydrocarbon combusting in the presence of oxygen to form carbon dioxide and water.
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Decomposition - Here, you can see the division of one compound into simpler molecules.
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Double Replacement - This reaction involves the exchange of two ions to form two new compounds: silver chloride and sodium nitrate.
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Decomposition - This is another example of a complex compound being divided, or decomposed, into its simpler components.
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Synthesis - This is the opposite of the last question! You have two elements coming together to form a compound: sodium chloride.
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Single Replacement - In this single replacement reaction, we can see chloride replacing bromide to form sodium chloride and bromide molecules.
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Double Replacement - Another very common reaction where two ions exchange places!
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Combustion - Remember, whenever you see a hydrocarbon coming together with oxygen, it will combust into carbon dioxide and water.
These reactions are just the basics. Once we dive deeper into the unit, we'll go over three important reactions:
- Precipitation Reactions
- Acid-Base Reactions
- Oxidation-Reduction Reactions
🎥 Watch AP Chemistry teacher Wes Winter define the evidence for a reaction and identify the different types of reactions.
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.
| Term | Definition |
|---|---|
| chemical change | A process in which substances are rearranged into new combinations, resulting in the formation of new substances with different properties. |
| mixture | Materials that contain atoms, molecules, or formula units of two or more types, whose relative proportions can vary. |
| phase | A distinct state of matter: solid, liquid, or gas. |
| physical change | A change in matter that does not alter the chemical composition or identity of the substance. |
| precipitate | A solid substance that forms and separates from a solution during a chemical reaction. |
Frequently Asked Questions
What's the difference between a physical change and a chemical change?
A physical change alters a substance’s properties but not its composition—think phase changes (ice melting), dissolution, mixing, or separation by distillation. No bonds are broken or new substances made. A chemical change produces new substances with different compositions; bonds break and form. Evidence of a chemical change includes heat or light release (exo-/endothermic), gas evolution (effervescence), precipitate formation, and/or a color or odor change. On the AP CED this is Topic 4.1 (LO 4.1.A): identify these evidences (physical change = 4.1.A.1; chemical change = 4.1.A.2). For quick review, check the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and more Unit 4 resources (https://library.fiveable.me/ap-chemistry/unit-4). Want practice identifying examples? Try problems at (https://library.fiveable.me/practice/ap-chemistry).
How can you tell if a chemical reaction actually happened?
You tell a reaction happened by looking for changes that mean new substances formed (CED 4.1.A). Common evidence: energy release or absorption (heat or light), gas evolution/effervescence, a precipitate forming, a permanent color change, new or changed odor, or a change in conductivity—all point to bond breaking/forming and a composition change (keywords from the CED). Physical changes (phase change, dissolution, mixing) change properties but not composition (4.1.A.1), so don’t count as chemical on their own. No single sign is foolproof—e.g., bubbling could be boiling—so confirm with chemical tests or analysis (measure composition, pH, conductivity, mass change) or multiple concurrent signs. For AP prep, be ready to identify and justify evidence on the exam (Learning Objective 4.1.A). Review Topic 4.1 (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and practice problems (https://library.fiveable.me/practice/ap-chemistry).
What are some examples of physical changes that I should know for the AP exam?
Physical changes = changes in properties but not composition (CED 4.1.A.1). Key examples to know for the exam: phase changes (melting, freezing, vaporization, condensation, sublimation), dissolving (salt or sugar in water), mixing or separating mixtures (filtration, chromatography, distillation), and physical cutting/grinding or compressing a solid. These don’t make new substances—only physical form or arrangement changes. Contrast with chemical-change evidence (helpful for multiple choice): heat/light release, color change, gas evolution, or precipitate formation (CED 4.1.A.2). So if salt dissolves and you can recover the same salt by evaporating water, that’s physical; if you see a new solid form or strong gas/heat, suspect chemical. For quick review, see the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and practice >1,000 problems (https://library.fiveable.me/practice/ap-chemistry).
Why does melting ice count as a physical change if it looks totally different?
Melting ice is a physical change because the chemical composition (H2O molecules) stays the same—you’re only changing the phase (solid → liquid). According to the CED (4.1.A.1), phase changes are classic physical changes: properties like shape, density, and arrangement change, but no new substances form. A chemical change would require a composition change (new bonds making new molecules) and likely show evidence such as gas evolution, precipitate formation, color change, or heat/light (4.1.A.2). Melting just adds energy to overcome some intermolecular forces (endothermic), not to break/form new covalent bonds, so no new substance appears. For more on physical vs. chemical changes and AP-style examples, see the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP). Practice identifying changes with problems at https://library.fiveable.me/practice/ap-chemistry.
I'm confused about mixtures - is making a mixture a physical or chemical change?
Making a mixture is a physical change—as the CED says, forming or separating mixtures doesn’t change composition (Topic 4.1, EK 4.1.A.1). For example, dissolving sugar in water or mixing N2 and O2 just changes physical properties (phase, concentration) but the original substances remain the same. However, mixing can be a chemical change if the components react to form new substances—look for evidence listed in 4.1.A.2 (heat/light, gas evolution, precipitate, color or odor change, conductivity change, bond breaking/formation). So: if nothing new forms and composition stays the same → physical; if new substances form (and you see those evidences) → chemical. Review Topic 4.1 on Fiveable for quick examples (study guide: https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and drill practice problems at https://library.fiveable.me/practice/ap-chemistry to see how the exam phrases evidence.
What does it mean when they say "formation of a precipitate" as evidence of a chemical reaction?
“Formation of a precipitate” means an insoluble solid appears when two (or more) aqueous solutions react—a new substance with a different composition comes out of solution. On the AP CED this is listed as evidence of a chemical change (4.1.A.2): unlike a physical change, a precipitate signals the creation of a new compound (for example, Ag+ + Cl– → AgCl(s)). To be sure it’s a chemical reaction (not just cooling/crystallization or undissolved solid), you can write the net ionic equation and consult solubility rules: if ions that were free in solution combine to form an insoluble product, that’s a precipitate. This kind of observation is commonly used on the exam as one piece of evidence of chemical change (Topic 4.1). For a quick review and examples (including net ionic practice), see the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and try related problems on Fiveable (https://library.fiveable.me/practice/ap-chemistry).
Can you give me specific examples of chemical changes that produce heat or light?
Chemical changes that release heat or light are classic evidence of a chemical change (CED 4.1.A.2). Specific examples: - Combustion (exothermic): burning methane or gasoline—you get flames and hot gases (heat + light). - Metal oxidation in air: burning magnesium ribbon (Mg + O2 → MgO) gives an intense white light. - Thermite reaction (Fe2O3 + Al → Al2O3 + Fe): huge localized heat and bright sparks—used to weld rails. - Fireworks: rapid redox reactions produce heat, bright colored light from excited metal ions (emission spectra). - Chemiluminescence (cold light): glow sticks—chemical reaction emits light with little heat. - Bioluminescence: fireflies—enzyme-driven light emission. On the AP, you should identify these as exothermic processes (heat released) or light emission (photon release) as evidence a new substance formed. For a quick topic review see the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and try practice problems (https://library.fiveable.me/practice/ap-chemistry).
How do I know if a color change means a chemical reaction occurred or if it's just a physical change?
A color change can be evidence of a chemical change, but it isn’t conclusive by itself. Per the CED (4.1.A.2), a chemical change means composition changes—look for other signs: gas evolution (bubbles/effervescence), precipitate formation, heat or light release, odor change, or a lasting conductivity/pH change. If the color change reverses with simple physical actions (dilution, cooling, mixing), it’s probably physical (like dilution of a colored solution or a phase change). To be sure, try a follow-up test: filter to see if a solid formed, measure temperature, test conductivity or pH, or use spectroscopy/chemical tests to detect new species. On the AP exam you’re expected to “identify evidence of chemical and physical changes” (Learning Objective 4.1.A), so mention at least one additional corroborating observation when you claim a chemical reaction. For a quick review, see the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and practice problems (https://library.fiveable.me/practice/ap-chemistry).
What's the deal with gas formation - why is that evidence of a chemical change?
Gas formation is a useful clue because it usually means new substances were made—not just a phase change. The CED lists “formation of a gas” (gas evolution, effervescence) as evidence of a chemical change: bonds are broken and new bonds form, so composition changes (for example, CaCO3 + HCl → CO2(g) + ... or Zn + 2H+ → H2(g) + Zn2+). A simple phase change (like boiling water) doesn’t change composition; gas from boiling is still H2O. To decide on the exam, ask: did the reaction produce a new molecular species (CO2, H2, SO2, etc.) or just vaporized the original substance? If it’s the former, that’s chemical change evidence. This fits LO 4.1.A and appears as basic reasoning on multiple-choice or free-response items. For a quick review, check the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and try practice problems (https://library.fiveable.me/practice/ap-chemistry).
Is dissolving salt in water a physical or chemical change and why?
Dissolving table salt (NaCl) in water is a physical change. According to the CED (4.1.A.1), a physical change alters properties or phase without changing composition. Solid NaCl dissociates into Na+ and Cl– ions, but those ions are the same chemical species as before—you haven’t made new compounds. What changes are the interactions: ionic lattice bonds are broken and ion–dipole attractions with H2O form. You’ll see evidence like increased conductivity and possibly a small temperature change, but those aren’t proof of a new substance (contrast with gas evolution, precipitate, color change, or new bonds indicating a chemical change per 4.1.A.2). For AP review, this fits Topic 4.1 (Introduction to Reactions); check the Topic study guide for more examples (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP). For extra practice, try problems at (https://library.fiveable.me/practice/ap-chemistry).
I don't understand how phase changes work - can someone explain solid to liquid to gas?
Phase changes (solid → liquid → gas) are physical changes: the substance’s composition stays the same, but its physical properties change (CED 4.1.A.1). What changes is how particles move and how strongly they stick together (intermolecular forces) versus their kinetic energy. In a solid particles are tightly packed and vibrate; when you add heat (endothermic process) their kinetic energy increases until they overcome some intermolecular attractions and melt to a liquid. Add more heat, and with enough energy particles escape the liquid’s attractions and vaporize to a gas. Cooling is the reverse (exothermic). Evidence it’s a physical change: phase change, temperature plateau during melting/boiling, no new substances formed (CED keywords: phase change, endothermic, exothermic, physical change). For AP practice, you should be able to identify these signs on labs/graphs—see the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and try problems at (https://library.fiveable.me/practice/ap-chemistry).
What are all the signs I need to look for to identify a chemical reaction on the AP test?
On the AP, look for these CED-backed signs that a chemical change occurred (Learning Objective 4.1.A): - Formation of a gas (bubbling/effervescence) not due to boiling. - Formation of a precipitate (solid appears when two solutions mix). - Color change that isn’t just dilution or a physical mix. - Release or absorption of energy (heat or light)—exothermic/endothermic. - New odor or a persistent smell change. - Change in conductivity (new ions formed or lost). - A change in composition: bonds broken/formed—evidence from balanced equations or net-ionic work. Remember: phase changes (melting, freezing, dissolving) and simple mixing are physical changes (4.1.A.1). No single sign is 100% proof—the AP likes multiple lines of evidence and connecting observations to equations (use net ionic when applicable). For a quick CED-aligned refresher, see the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP). Practice identifying these signs with problems at (https://library.fiveable.me/practice/ap-chemistry).
Why do chemical changes create new substances but physical changes don't?
A chemical change makes new substances because the atoms rearrange: bonds in the reactants break and new bonds form to give products with different compositions and properties. A physical change only alters a substance’s form or phase (like melting, dissolving, or mixing) without changing which atoms are bonded to each other, so composition stays the same (CED 4.1.A.1). On the AP exam you should recognize evidence that a chemical change occurred: heat or light released/absorbed, gas evolution (effervescence), precipitate formation, or a persistent color/odor change (CED 4.1.A.2). Think of H2 + Cl2 → 2 HCl (chemical: bonds break and new H–Cl bonds form) versus ice melting (physical: H2O molecules stay H2O). For more AP-aligned review, see the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and practice questions (https://library.fiveable.me/practice/ap-chemistry).
Can a substance go through both physical and chemical changes at the same time?
Yes. A single sample can undergo a physical change and a chemical change at the same time. The AP CED distinguishes physical changes (phase change, dissolution, mixture formation) from chemical changes (new substances formed; evidence: gas evolution, precipitate, color change, heat/light). Both can occur together—for example: - Burning wood: water and volatile compounds evaporate (physical phase changes) while the wood’s organic matter chemically reacts with O2 to form new substances (CO2, ash—chemical change with heat/light). - Iron left outdoors: flakes break off or change shape (physical) while Fe oxidizes to rust (chemical; color change, composition change). On the exam, use the CED evidence list to justify which change(s) occurred: point out phase/physical signs and chemical signs (gas, precipitate, color, energy). For extra review, see the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and get more practice problems at (https://library.fiveable.me/practice/ap-chemistry).
How do I remember the difference between composition changing vs properties changing?
Think: “composition” = chemical, “properties” = physical. - Chemical (composition changes): atoms/bonds are rearranged so new substances form. Look for evidence from the CED: gas evolution, precipitate, color change, light or heat—these suggest bonds broke/formed and composition changed. Mnemonic: C in chemical = C for composition changed. - Physical (properties change): composition stays the same; only physical properties (phase, shape, solubility, mixture separation) change. Examples: melting, boiling, dissolving salt in water, filtration, distillation. Mnemonic: P in physical = P for properties only. On the AP exam, identify which observable signs are evidence of a chemical change (CED 4.1.A.1–4.1.A.2). If you’re unsure, ask: “Did atoms form new compounds?” If yes → chemical; if no and only phase/mixture changed → physical. For a quick refresher, check the Topic 4.1 study guide (https://library.fiveable.me/ap-chemistry/unit-4/intro-for-reactions/study-guide/LNQzVvZYjXxZMmXDubVP) and practice more problems at (https://library.fiveable.me/practice/ap-chemistry).