StellarChemical Reactions
At its core, a chemical reaction is a process in which substances are transformed into new substances with different chemical identities. This is fundamentally different from a physical change, where the substance may change form but not identity. In a chemical reaction, bonds between atoms are broken and new bonds are formed, leading to rearrangement of atoms into entirely new structures. For example, when hydrogen gas reacts with oxygen gas to form water, the resulting substance has properties completely different from either reactant. This idea of “new substance formation” is the defining feature of a chemical reaction and is what you should always look for when distinguishing chemical changes from physical ones.
In many cases, evidence of a chemical reaction can be observed macroscopically. Common signs include the formation of a precipitate (a solid forming from solution), gas production (bubbling), color change, or energy transfer such as heat or light. However, these observations alone are not sufficient proof; they are indicators that suggest a reaction may have occurred. At the particle level, what is really happening is that atoms are being rearranged. Importantly, atoms are never created or destroyed in a chemical reaction, which leads directly to the law of conservation of mass. This principle ensures that the number of each type of atom must remain the same before and after the reaction, forming the basis for balancing chemical equations.