Chemical reactions, also called chemical changes, refer to processes that alter chemical compounds, making new substances out of one or more old substances. To put that another way, the atoms of a substance are rearranged when a chemical change takes place. In contrast to physical changes, chemical changes cannot usually be reversed, unless an equivalent chemical reaction takes place.

The chemical system’s energy changes when a chemical change happens. Endothermic reactions are chemical reactions that absorb heat, while exothermic reactions are chemical reactions that give off heat.


Signs Of Chemical Change

Fireworks are examples of chemical changes. The light and heat produced are the results of chemical reactions that produce new compounds. Photo: nidan via Pixabay, URL:, CC0

There are various physical phenomenon that can indicate an occurrence of a chemical change. Indications of a chemical change include:

  1. Light Emission – Certain chemical reactions produce light as a result of the reaction.
  2. Bubbles – When a chemical change occurs in a liquid solution, it very often produces gas bubbles, which can be seen in the liquid.
  3. Change Of Color – A substance changing in color is an excellent indication that some form of chemical change has happened. Color changes are particularly noticeable in transition metals. An example of how a change in color indicates a chemical change is copper, which turns from a bronze color to a greenish color when it oxidizes.
  4. Odor Change – Chemical reactions can release molecules with certain scents, this is especially true of volatile chemicals.
  5. Change Of Temperature – Chemical reactions necessitate a shift in the energy of an object, and this energy change is frequently detectable as a change in temperature.
  6. Formation Of Precipitate – Precipitate refers to solid particles that congregate together in a solution. Certain chemical reactions create these solid particles.
  7. Irreversible Changes – While physical changes can often be reversed, chemical changes are basically irreversible in nature.
  8. Change In General Form/Composition – Substantial changes in the composition or makeup of the substance often indicate chemical changes, such as the when a log turns into ash as it burns.

While the above indicators can help you determine if a chemical change took place, chemical changes can take place without the indicators occurring, so this isn’t a foolproof method of detection. As an example of how a chemical change can occur without the presence of the above indicators when iron rusts it changes color and produces heat energy, but the process is very slow and not easily noticeable.

Different Kinds Of Chemical Changes

Chemists separate chemical changes into three different categories: biochemical changes, organic chemical changes, and inorganic chemical changes. Biochemical changes are the changes that happen within the bodies of living organisms. These chemical changes are driven by hormones and enzymes, and examples of such changes are photosynthesis, digestion, nitrogen fixation, fermentation, and the Krebs cycle.


Meanwhile, organic chemical changes are changes that occur when organic compounds alter the structure of another chemical. Organic compounds contain hydrogen and oxygen. Examples of organic chemical changes include halogenation, methylation, crude oil cracking, and polymerization. Finally, inorganic chemical changes are chemical changes that don’t use carbon as a part of the chemical reaction. Inorganic changes are processes like redox reactions, oxidation, and the mixture of bases and acids.

Difference Between Chemical Changes And Physical Changes

You may have heard about another type of change called a physical change. Let’s examine the differences between physical changes and chemical changes. In essence, a physical change does not produce a new chemical substance, while a chemical change does lead to the creation of a new substance.

Physical changes simply alter the shape or other physical attributes of an object/material, the chemical makeup of the material isn’t altered during a physical change. A physical change is something like the freezing, boiling, or melting of a material/substance, while a chemical change is like the rusting or burning of a substance. Since physical changes only alter the form of a chemical and not the composition of the chemical the change can often be undone, but this isn’t true of chemical changes.

Examples Of Chemical Changes

Rust appearing on iron is an example of a chemical change. The rust is the result of iron oxide being created. Photo: cortixx via Pixabay, URL: , CC0

Chemical changes occur when the atoms in the substance/compound are rearranged to create new chemical bonds.

  1. A prime example of a chemical change is the rusting of iron, the rust that develops on iron is the result of a chemical change because two different substances are creating a new substance. The rusting of iron is iron molecules reacting with oxygen molecules and creating iron oxide, a new compound.
  2. Baking a cake requires altering the chemical makeup of the cake. When the cake is heated in the oven, an endothermic chemical reaction happens, which changes the egg proteins in the soft cake batter, making the cake firm.
  3. Crating caramel from sugar requires a chemical change. Caramel gets its flavor from by a chemical reaction that takes place between amino acids and the sugars when they are heated.
  4. Digesting food is a process that involves chemical changes. Enzymes found in the intestines and in the stomach degrade the macromolecules in food, making them simpler molecules that can be absorbed more easily.
  5. Burning a substance is a type of chemical change. Similar to rusting, burning happens when the molecules in the wood interact with the oxygen in the air. This process releases heat and light energy.
  6. Mixing acids and bases creates a chemical change. Acids and bases create a chemical reaction when mixed together. A notable example of this phenomenon is mixing vinegar and baking soda. Vinegar is an acid, while baking soda is a base. The ensuing bubbling that occurs is a result of the chemical reaction between the two.
  7. Photosynthesis is a process that creates usable energy for plants through chemical changes. Water and carbon dioxide are combined with energy from the sun to form oxygen and glucose. Since new species of chemicals are created by this process, the change that occurs is a chemical change.
  8. Fermentation is a chemical change. The fermentation process is carried out by bacteria or yeast, and sugar is converted into either ethanol or lactic acid through the process. CO2 is also released as a byproduct.
  9. Nitrogen fixation is a type of chemical change. Nitrogen fixation is any process that causes free nitrogen molecules to be combined with other compounds. This creates nitrogen-based compounds that are more reactive than the base compound, and it’s a form of chemical change.
  10. Fireworks create pretty explosions thanks to chemical changes. Fireworks contain different kinds of elements such as iron, aluminum, steel, magnesium or zinc. These flakes of metal are heated up until they burn or shine very brightly.

Examples Of Physical Changes

Melting ice is an example of a physical change. The form of the ice changes into water, but the molecules stay the same. Photo: Fotocitizen via Pixabay, URL:, CC0

To better understand what changes count as chemical changes and which changes count as physical changes, let’s also take a look at some examples of physical changes. Examples of physical changes include:

  • An ice cube melting is a physical change.
  • The boiling of water is another example of a physical change.
  • Shredding paper changes the physical form of the paper.
  • When alcohol evaporates it undergoes a physical change.
  • When a bottle is broken a physical change has occurred.
  • Crushing an aluminum can is a form of physical change.

Discerning Chemical Changes From Physical Changes

To tell whether or not there is a chemical change or a physical change taking place, you should look at the object/material/substance and analyze it for indication of chemical changes. When a chemical change occurs, it often absorbs or releases heat or absorbs/releases energy, and these reactions frequently produce changes in sound, color, odor, or gas. If none of these indicators are observed, a physical change has probably occurred rather than a chemical change. However, you should be aware that physical changes can often result in dramatic alterations to a substance’s appearance, and these dramatic changes don’t mean that chemical reactions have taken place.

Dramatic changes to the form of a substance can make it hard to tell if a physical change or chemical change has occurred. As an example, when sugar is dissolved into water a physical change has occurred, rather than a chemical change. This is because while the sugar has dissolved, the sucrose molecules that comprise the sugar remain the same in their chemical structure. In contrast, dissolving salt into a glass of water is a chemical change because the salt molecules dissociate, becoming ions. Dissolving both sugar and salt into water is the process of dissolving a solid into a liquid, and removing the water enables the recovery of the salt or sugar. Yet despite the similarities, the processes are different, one is a chemical change and the other physical.

About The Author

Daniel obtained his BS and is pursuing a Master's degree in the science of Human-Computer Interaction. He hopes to work on projects which bridge the sciences and humanities. His background in education and training is diverse including education in computer science, communication theory, psychology, and philosophy. He aims to create content that educates, persuades, entertains and inspires.