The matter that makes up our world is constantly undergoing transformations. Scientists categorize these transformations into distinct types based on what happens at the molecular level. Most changes we observe fall into one of two categories. The distinction lies in whether the material’s basic identity is preserved or whether a completely new material is generated. Only one category fundamentally alters the core composition of the starting material.
Changes That Only Alter Appearance
This type of change affects a material’s external characteristics without modifying the molecules themselves. These transformations impact the form, shape, or state of matter. The substance at the end is chemically identical to the substance at the beginning, and the molecular structure and chemical formula remain unchanged.
When water freezes, it changes from a liquid to a solid, but the individual H2O molecules are still intact and chemically the same. Grinding a solid into a fine powder alters its texture and surface area, yet it is still composed of the original substance. Dissolving sugar in water is another example where the sugar molecules disperse but retain their identity, which is why the sugar can be recovered through evaporation. These transformations are considered easier to reverse because no chemical bonds were broken or formed to create a new material.
Chemical Changes Form New Substances
The transformation that forms a new substance is known as a chemical change. This process involves the rearrangement of atoms, where the chemical bonds holding the original molecules are broken and new bonds are formed. The result is the creation of products that possess entirely different properties and molecular structures than the initial materials.
Consider the example of combustion, such as burning wood, where the cellulose molecules react with oxygen from the air. The atoms rearrange to form new substances like ash, carbon dioxide gas, and water vapor. Rusting is another common chemical change, where iron atoms react with oxygen and moisture to form iron oxide, a compound with properties vastly different from metallic iron.
Cooking food is a complex series of chemical changes, as the heat causes proteins to denature and starches to break down. This permanently alters the food’s texture, color, and taste. The new substances formed in a chemical change are not easily converted back to the original materials through simple physical means.
Observable Signs of a New Substance
Since we cannot directly observe the breaking and forming of molecular bonds, scientists rely on macroscopic indicators to confirm that a chemical change has taken place. One common sign is the sudden evolution of a gas, which appears as bubbling or fizzing, provided the material is not simply boiling from heat. When mixing baking soda and vinegar, the immediate bubbling indicates the production of carbon dioxide gas, a new substance.
Another strong indicator is the formation of a precipitate, which is a solid material that appears out of a liquid solution after two clear liquids are mixed. An unexpected change in color is also evidence, such as a penny turning green over time due to oxidation. This is distinct from merely mixing two different colored materials.
The reaction may also involve a noticeable change in energy. This manifests as the release of heat and light, seen in a burning fire, or the absorption of heat, which causes the surroundings to feel cold. Finally, the production of a distinct odor, especially one not present in the starting materials, often signals the creation of a new chemical compound.