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Types of Chemical Reactions

Chemical reactions are happening all around you — in the air you breathe, the food you cook, the fuel that burns, and even inside every living cell. Although they can appear mysterious at first, chemical reactions follow clear patterns that help us understand how substances change.

As you move through this article, we'll dive deeper into questions like:

  • What is a chemical reaction?
  • What are the 5 types of chemical reactions?
  • What is synthesis in chemical reactions?
  • What happens in a combustion chemical reaction?
  • How does a decomposition chemical reaction work?
  • What happens in the chemical reaction with vinegar and baking soda?

And much more! Get ready to explore the fascinating world of chemistry with Omni. 🧪

A chemical reaction occurs when substances transform into something new, and this idea forms the basis of the chemical reaction definition used in chemistry. Whenever two or more particles (in the form of atoms, molecules, or ions) interact in a way that changes their composition, a chemical reaction takes place. Think of iron left in damp air: it reacts with oxygen to form iron oxide (rust). Rust is not just "dirty iron", but it's a completely new substance with different chemical properties.

This situation is very different from a physical change. When water freezes into ice, it looks different but remains the same substance. A chemical reaction, however, creates something genuinely new.

Chemical changes appear everywhere around you. A classic example is the chemical reaction between vinegar and baking soda, which releases carbon dioxide. Metals reacting with oxygen form oxides, and a combustion chemical reaction (like burning wood or fuel) gives off heat and light. Some reactions are triggered by light, such as photosynthesis, one of the most important photochemical reactions in nature.

Heat speeds up reactions by increasing the frequency of particle collisions, and in living organisms, enzymes act as catalysts that control numerous reactions within the cells. To explore how fast a reaction proceeds, try our rate constant calculator.

Writing chemical reactions

Chemists represent these transformations using a chemical equation. On the left, you'll find the reactants — the substances you start with. On the right are the products — the new materials that have been formed. Coefficients tell you how many units of each substance participate in the reaction, and chemists may add labels such as (s), (l), (g), or (aq) to indicate physical states, though these are not required in every equation. These conventions help describe everything from the basic 5 types of chemical reactions to more advanced reaction patterns.

For example, a reaction describing iron turning into rust might look like this:

4Fe+3O2+6H2O→4Fe(OH)3\mathrm{4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3}

The above chemical equation is a simplified model of the rusting process.

Chemical reactions can be grouped into five main types* based on how atoms and ions rearrange during the process: synthesis, decomposition, combustion, single-replacement, and double-replacement. Below, you'll find each type explained in simple terms, along with clear examples that show how these reactions play out in real life.

*Other teaching schemes may group them differently.

Synthesis (or combination) reaction

A synthesis in chemical reactions happens when two or more substances combine to form one new product. It's like chemistry's version of building something from scratch, for example, the synthesis of sodium chloride (table salt) from sodium and chlorine:

2Na+Cl2→2NaCl\mathrm{2Na + Cl_2 \rightarrow 2NaCl}

Decomposition reaction

A decomposition chemical reaction occurs when a single compound breaks down into two or more simpler substances. Most decomposition reactions need an energy boost to get started — heat, light, or electricity often provides the push that breaks the compound apart. The classic decomposition example is the breakdown of hydrogen peroxide to water and oxygen:

2H2O2→2H2O+O2\mathrm{2H_2O_2 \rightarrow 2H_2O + O_2}

Combustion reaction

When a substance reacts vigorously with oxygen, producing heat of combustion and often a bright flame, you’re witnessing a combustion chemical reaction. In every combustion process, oxygen must be one of the reactants, because the reaction cannot occur without it. Combustion releases large amounts of energy in the form of heat and light. For example, burning propane gas (C3H8) in your gas grill produces carbon dioxide and water vapor:

C3H8+5O2→3CO2+4H2O\mathrm{C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O}

Single replacement reaction

In a single replacement reaction, one element steps in and takes the place of another in a compound. Practically, it's like a chemical "swap", where a more reactive element pushes out a less reactive one. An example of single replacement chemical reaction is zinc replacing hydrogen in hydrochloric acid, forming zinc chloride and releasing hydrogen gas:

2HCl+Zn→ZnCl2+H2\mathrm{2HCl + Zn \rightarrow ZnCl_2 + H_2}

Double replacement reaction

A double replacement reaction occurs when the positive and negative ions in two ionic compounds swap partners, forming two new compounds. Double-replacement reactions typically occur in aqueous solutions but only if one of the products forms a solid precipitate, a gas, or a molecular compound, such as water. A common example that forms a visible black precipitate of PbS (or lead(II) sulfide, see our chemical name calculator futher details) is:

Pb(NO3)2+K2S→PbS+2KNO3\mathrm{Pb(NO_3)_2 + K_2S \rightarrow PbS + 2KNO_3}

While the 5 types of chemical reactions above are the most commonly taught patterns, they aren't the only way to classify chemical changes. Chemists use several different classification systems, depending on what they want to emphasize: energy flow, electron transfer, acid-base behavior, or the formation of precipitates. Below are some other important types of chemical reactions:

  • Redox reactions — reactions in which electrons are transferred between substances, changing their oxidation states.
  • Acid-base reactions — a proton (H+) is transferred from an acid to a base. They're among the most common and important reactions in chemistry. A familiar example is the chemical reaction with vinegar and baking soda, where acetic acid (vinegar) reacts spectacularly with baking soda (a base) to produce fizzy carbon dioxide gas:
NaHCO3+CH3COOH→ CH3COONa+H2O+CO2\begin{align*} \rm Na&\rm HCO_3 + CH_3COOH \\ &\rm \rightarrow\ CH_3COONa + H_2O + CO_2 \end{align*}

  • Neutralization reactions — an acid and a base react to form water and a salt.

  • Precipitation reactions — two aqueous solutions form an insoluble solid called a precipitate.

  • Photochemical reactions — chemical changes triggered by light energy, such as photosynthesis.

  • Hydrolysis reactions — water breaks chemical bonds, splitting a molecule into two or more parts.

  • Polymerization reactions — small molecules (monomers) join together to form long chains called polymers.

The difference is that a chemical reaction creates a new substance, while a physical reaction changes only the form or state of the original material. According to the chemical reaction definition, atoms rearrange and new products form; in a physical reaction, the composition remains the same. Melting ice is a physical process, whereas rusting iron is a chemical one.

A double-replacement reaction is the type of chemical reaction in which two elements (or ions) switch places between compounds. In this process, the ions essentially trade partners, forming two new compounds. These reactions often occur in aqueous solutions and may produce a precipitate, gas, or water.

A precipitate is a solid that forms when two aqueous solutions react and create an insoluble compound. It appears as a cloudy, powdery, or chunky solid that settles or floats in the liquid.

We can tell that a chemical reaction has occurred if there is evidence of a new substance forming. Common signs include color change, gas production, heat or light release, formation of a precipitate, or a noticeable change in odor. These indicators show that atoms have rearranged and new products have been created.

This article was written by Joanna Śmietańska-Nowak and reviewed by Steven Wooding.