A chemical reaction is a process where substances transform into entirely new substances. This change involves the reorganization of atoms, leading to materials with different properties. It is a core concept in chemistry that governs how matter interacts and changes.
Atoms Rearrange and Bonds Transform
During a chemical reaction, original substances, called reactants, undergo change at the atomic level. This transformation begins with the breaking of existing chemical bonds within reactant molecules. Breaking these bonds requires energy to overcome attractive forces, initiating the reaction.
Following bond breakage, freed atoms rearrange into new combinations. These arrangements create new chemical bonds, resulting in different molecules called products. The atoms themselves are not created or destroyed; they are simply shuffled and connected in new ways, much like disassembling building blocks and reassembling them into a different structure.
Consider the reaction where hydrogen gas (H2) combines with oxygen gas (O2) to form water (H2O). The bonds holding hydrogen atoms in H2 and oxygen atoms in O2 must first break. New polar covalent bonds then form between hydrogen and oxygen atoms to create water molecules. This breaking and forming of bonds drives the transformation of matter, yielding new substances.
Telltale Signs of a Chemical Change
Observing a chemical reaction often involves noticing changes that indicate new substances have formed. One common sign is a change in color, as the products possess different light absorption and reflection properties than the reactants. For instance, an apple browning after being cut shows a chemical change due to oxidation, turning from white to brown.
The production of gas is another clear indicator, often seen as bubbles or fizzing. When baking soda reacts with vinegar, carbon dioxide gas is visibly released, demonstrating a new gaseous product that was not present before. A noticeable change in temperature can also signal a reaction, with some reactions releasing heat (exothermic) and others absorbing it (endothermic), making the surroundings feel warmer or colder.
The formation of a solid within a liquid solution, known as a precipitate, also points to a chemical change. When two clear liquid solutions combine and a cloudy solid appears, a new, insoluble substance has formed from the dissolved reactants. Chemical reactions can also emit light, as seen in glow sticks, where internal processes generate luminescence.
Energy and Conservation in Reactions
Every chemical reaction involves an exchange of energy, often as heat, light, or electricity. In exothermic reactions, energy is released into the surroundings, making the environment feel warmer. This occurs because the energy stored in the products’ bonds is less than in the reactants’ bonds, and the excess energy is expelled.
Conversely, endothermic reactions absorb energy from their surroundings, causing the temperature of the environment to decrease. In these cases, the energy required to break the reactant bonds is greater than the energy released from forming product bonds, necessitating an uptake of energy from outside the system.
A fundamental principle governing chemical reactions is the Law of Conservation of Mass. This law states that matter is neither created nor destroyed during a chemical reaction, only rearranged. The total mass of reactants consumed precisely equals the total mass of products formed.
Everyday Examples of Chemical Reactions
Chemical reactions are constantly occurring around and within us. A common example is the burning of wood, a combustion reaction. Wood reacts with oxygen, releasing heat and light, transforming into ash, smoke, and gases.
Another familiar chemical change is the rusting of iron. When iron metal is exposed to oxygen and water over time, it slowly undergoes oxidation, forming iron oxide, commonly known as rust. This process causes the strong, metallic iron to become a brittle, reddish-brown flaky substance.
Within our bodies, the digestion of food represents a complex series of chemical reactions. Large, intricate food molecules like carbohydrates and proteins are broken down into simpler molecules through enzymatic processes in the digestive system. This transformation allows the body to absorb nutrients and extract energy, illustrating how chemical changes are fundamental to biological functions.