A chemical reaction is a process where substances undergo a transformation, rearranging their atoms to create entirely new substances with distinct properties. These fundamental changes in matter are often accompanied by observable indicators, providing clear evidence that a reaction has taken place. Recognizing these signs helps in understanding the dynamic nature of chemistry.
Release of Gas
One common indicator of a chemical reaction is the production of gas. This is often seen as bubbles, fizzing, or a noticeable odor. Unlike boiling, where a liquid simply changes state to a gas, the gas produced in a chemical reaction is a completely new chemical compound.
For example, when baking soda (sodium bicarbonate) is combined with vinegar (an acetic acid solution), vigorous bubbling occurs. This effervescence is due to the rapid formation and release of carbon dioxide gas from the chemical interaction. Similarly, dropping an antacid tablet into water results in fizzing as carbon dioxide gas is generated.
Change in Color
A distinct and lasting alteration in color often signals that a chemical reaction has occurred. This is different from simply mixing two colored liquids to get a new shade, as the color change in a chemical reaction indicates the formation of new substances.
A familiar example is the browning of a cut apple when exposed to air, where oxidation reactions create new pigmented compounds. Another instance is the rusting of iron, where iron reacts with oxygen and water to form iron oxides, resulting in a reddish-brown color. The change in color of litmus paper when testing acids or bases also demonstrates a chemical reaction.
Creation of a New Solid
The formation of a new solid, known as a precipitate, from two liquid solutions is a strong indication of a chemical reaction. This solid forms when dissolved substances react to create an insoluble compound that separates from the liquid. The newly formed solid may appear as cloudiness, flakes, or a dense material that settles.
In everyday life, the formation of soap scum in hard water is an example of precipitation, as minerals in the water react with soap to form an insoluble solid. The curdling of milk when exposed to an acid, such as lemon juice, also demonstrates this phenomenon, as proteins in the milk react to form solid curds. The formation of stalactites and stalagmites in caves involves the slow precipitation of calcium carbonate from mineral-rich water.