A chemical reaction is a fundamental process where initial substances, known as reactants, are transformed into new substances called products. This transformation alters the chemical identity, composition, and characteristics of the materials. Unlike a physical change, this process is the basis for nearly all changes observed in the natural world, from burning wood to metabolism.
The Core Change: Atomic Rearrangement
The transformation at the heart of a chemical reaction is the rearrangement of atoms. Reactant molecules collide with sufficient energy, causing the existing chemical bonds that hold their atoms together to break apart. Once these old bonds are broken, the separated atoms or molecular fragments move around and form new connections.
This process involves the movement of electrons to create new chemical bonds, resulting in the formation of entirely new product molecules. The atoms themselves are not created or destroyed; they simply change partners and structural positions. For instance, a carbon atom that was bonded to an oxygen atom in one molecule might rearrange to bond with two hydrogen atoms in a product molecule. This reorganization of the original atoms gives the products their distinct chemical properties compared to the starting reactants.
Observable Signs of a New Substance
A chemical reaction is often confirmed by visible indicators that a new substance has been created. These observable changes demonstrate that the chemical identity of the starting materials has been replaced by new products.
- A change in color, such as when a clear solution turns yellow or iron rusts from gray to reddish-brown.
- The formation of a solid, called a precipitate, when two liquids are mixed. This insoluble solid separates from the liquid solution.
- The production of a gas, often observed as bubbling or fizzing. For example, mixing baking soda and vinegar produces carbon dioxide gas.
- The release or absorption of heat, which can be felt as a change in the temperature of the reaction vessel.
Energy Exchange During Reaction
All chemical reactions involve a change in energy because energy is required to break existing bonds and released when new bonds form. The overall energy change is the difference between the energy absorbed and the energy released. Energy must be put into the reactants to break their existing bonds. Conversely, energy is released when atoms form new, more stable bonds to create the products.
When the energy released from forming new bonds is greater than the energy required to break the old bonds, the reaction is called exothermic. Exothermic reactions release energy, often as heat or light, causing the temperature of the surroundings to increase. If the energy required to break the bonds is greater than the energy released, the reaction is endothermic. Endothermic reactions absorb energy from the surroundings, which causes a noticeable drop in temperature.
What Stays the Same: Conservation Laws
While the chemical compounds change entirely, two fundamental properties remain constant throughout the reaction. The Law of Conservation of Mass dictates that the total mass of all reactants must exactly equal the total mass of all products. Mass is neither created nor destroyed during the process, which is why chemical equations must be balanced.
This conservation of mass directly relates to the conservation of elements. The identity and total number of individual atoms of each element remain unchanged from the beginning to the end of the reaction. For example, if a reaction starts with 12 carbon atoms and 24 hydrogen atoms, the products, regardless of their new structure, must collectively contain exactly 12 carbon atoms and 24 hydrogen atoms. The atoms simply rearrange their connections without altering their fundamental nature.