The world around us is constantly transforming, and understanding these transformations is the foundation of chemistry. Changes in matter are classified as physical or chemical. While a physical change alters the form or appearance of a substance, a chemical change is a fundamental process that creates entirely new materials. This transformation drives processes from cooking food to the operation of a car engine.
What Makes a Change Chemical
The defining characteristic of a chemical change is the transformation of starting materials, called reactants, into new substances, known as products. This process is fundamentally different from a physical change, such as melting ice, where the material’s chemical identity remains the same. The atoms of the original substances are simply rearranged to form new molecules with distinct chemical properties.
This molecular-level transformation requires the breaking of existing chemical bonds in the reactants and the formation of new bonds to create the products. Atoms are not created or destroyed during this process, adhering to the law of conservation of mass. The new connectivity between these atoms results in a substance with a completely different set of characteristics.
Visible Indicators of Reaction
Although the true change occurs at the atomic level, a chemical reaction is often confirmed by observable signs. One common indicator is the formation of a gas, often seen as bubbling or fizzing, which is known as effervescence. Mixing baking soda and vinegar, for example, produces carbon dioxide gas that rapidly bubbles out of the solution.
Another clear sign is the formation of a precipitate, a new solid material that appears when two liquid solutions are combined. This solid product often settles out of the liquid mixture, making the solution appear cloudy or opaque. A sudden change in color or the appearance of a distinct odor can also signal that a chemical change has taken place. The browning of a freshly cut apple, caused by a reaction with oxygen, is an everyday example of a color change indicating a chemical transformation.
The Role of Energy
Every chemical change involves an associated transfer of energy, as energy is required to break initial bonds and is released when new bonds are formed. This energy change is a universal characteristic of a chemical reaction. Reactions are categorized based on the direction of this energy flow relative to the surrounding environment.
Reactions that release energy, often as heat or light, are known as exothermic reactions. The burning of a log is a familiar example where stored chemical energy is released as heat, causing the temperature of the surroundings to rise. Conversely, reactions that absorb energy from the surroundings are called endothermic reactions. When an instant cold pack is activated, it absorbs heat, causing the temperature to drop noticeably.
Permanence and Irreversibility
The formation of new compounds often results in a change that is not easily undone by simple physical means. A chemical change is often described as irreversible because the atoms have been rearranged into new, stable configurations. Once a substance has reacted to form a product, the energy required to force the product back into the original reactants is often substantial.
While some chemical reactions can be reversed through specific conditions, they cannot simply be undone by a physical process like cooling or filtering. The rusting of iron, which forms iron oxide, is a classic example of a practically irreversible change. Similarly, once an egg is cooked, the proteins have undergone a chemical transformation that cannot be reversed by simple physical actions.