A chemical reaction transforms starting substances (reactants) into new substances with different chemical identities and properties. This involves breaking and forming chemical bonds, which rearranges the atoms. Because the resulting products have a different molecular structure, they often interact with light differently, leading to a visible color change. This relationship makes color change a frequent, though not always reliable, sign of a chemical reaction.
When Color Change Signals a Chemical Reaction
A shift in color serves as strong evidence for a chemical reaction when it results from a change in the substance’s molecular structure. The color we perceive is based on which wavelengths of light a substance absorbs and reflects back to our eyes. When a reaction forms a new compound, the arrangement of electrons changes, altering the way light is absorbed and producing a new color.
Oxidation, such as when iron metal rusts and develops a reddish-brown hue, is a common example. Iron atoms react with oxygen to form iron oxide, an entirely new substance with a distinct color from the shiny, gray metal. Similarly, the browning of a cut apple is an enzymatic reaction where compounds in the fruit react with oxygen, forming pigments. Chemical indicators, like litmus paper used for pH testing, are designed to change color when exposed to different levels of acidity or basicity, confirming a reaction has taken place.
Color Changes That Are Physical Processes
Color change alone is not sufficient proof of a chemical change because many physical processes cause a shift in appearance without forming new substances. A physical change alters the form or state of a substance, but its underlying chemical composition remains unchanged. For instance, mixing two colored paints or diluting a concentrated dye results in a new color, yet no chemical bonds are broken or formed.
Another physical process involves structural color, which creates iridescent effects in peacock feathers and soap bubbles. This color is caused by light waves interfering with small, repeating surface structures, not by a chemical pigment. Heating solids, like a metal glowing red-hot, is also a physical change; the heat changes the energy state of the atoms, causing them to emit light, but the metal remains the same element. The color change in these cases is reversible and does not produce a new chemical product.
Definitive Proof of a Chemical Reaction
To move past the ambiguity of color change, scientists look for several accepted indicators that confirm the creation of a new substance. One reliable sign is the formation of a precipitate, which is a solid that appears and settles out of a liquid solution when two liquids are mixed. This solid material is a new compound that was not present in the original reactants.
The evolution of a gas is another strong indicator, typically observed as bubbling or fizzing, provided it is not caused by simple boiling or the release of a dissolved gas. This gas formation signifies that atoms have rearranged to create a gaseous product. Another clear sign is a significant, sustained change in energy, often felt as a change in temperature.
An exothermic reaction releases energy, making the surroundings hotter, while an endothermic reaction absorbs energy, causing the container to feel cooler. This energy shift indicates that the chemical bonds in the reactants and products have different energy levels. While color change is a helpful clue, the simultaneous presence of these other indicators provides the necessary evidence to definitively conclude that a chemical reaction has occurred.