Chemistry focuses on matter and the transformations it undergoes. A chemical change alters the fundamental composition of a substance, resulting in the creation of new materials with entirely different properties. This process is distinct from a physical change, which only affects a substance’s appearance or form without changing its chemical identity. Understanding these changes and their observable indicators provides the foundation for exploring the many specific examples that occur every day.
What Defines a Chemical Change
A chemical change, also known as a chemical reaction, involves the rearrangement of atoms at the molecular level. Atoms within the original substances break their existing chemical bonds and form new bonds, leading to the creation of new compounds with a different structure and composition. This alteration means that the starting materials, called reactants, are consumed to produce new end materials, called products.
The resulting products possess characteristics that are dramatically different from the initial substances. For example, when hydrogen and oxygen gases combine, they form liquid water, a product with properties entirely unlike the two gases. This process is considered irreversible, meaning that reversing the change usually requires another chemical reaction or significant energy input.
Observable Evidence of a Reaction
Since it is impossible to see molecules rearranging, scientists rely on several physical signs to determine if a chemical reaction has taken place.
- An unexpected color change, such as when a peeled apple turns brown due to oxidation. This happens because new chemical compounds reflect light differently.
- A change in temperature, signaling a transfer of energy. Reactions that release heat are called exothermic, while those that absorb heat are called endothermic.
- The formation of a gas, often observed as bubbling or fizzing, like the carbon dioxide released when an antacid tablet is dropped into water.
- The formation of a precipitate, which occurs when two liquids are mixed and produce a solid material that does not dissolve in the liquid mixture.
- The production of light or a noticeable change in odor, such as the smell of sulfur from a rotting egg.
Categorizing Common Chemical Changes
Combustion
Combustion, commonly known as burning, is a high-temperature reaction between a fuel and an oxidant, usually oxygen from the atmosphere. When wood or natural gas burns, the fuel combines rapidly with oxygen to produce new compounds, typically carbon dioxide and water, along with significant heat and light energy.
Oxidation
Oxidation is a less intense but common reaction involving oxygen without accompanying fire. Rusting is a familiar example, where iron metal reacts with oxygen and water over time to form iron oxide, the reddish-brown flakey material. Similarly, the green patina that forms on copper statues results from copper slowly oxidizing and forming copper compounds.
Decomposition Reactions
Decomposition reactions involve a single, complex substance breaking down into two or more simpler substances. Cooking is a type of decomposition, where heat breaks down the complex proteins and carbohydrates in food, making them easier to digest and changing their taste and texture. A controlled example is the decomposition of sodium bicarbonate (baking soda), which breaks down when heated to produce sodium carbonate, carbon dioxide, and water.
Synthesis Reactions
The opposite of decomposition is a synthesis or combination reaction, where two or more simple substances combine to form a single, more complex product. The creation of water from hydrogen and oxygen gases is a classic example of synthesis. In industry, the formation of ammonia from nitrogen and hydrogen gases is another synthesis reaction used to create fertilizer and other important chemicals.