A chemical change involves a substance transforming into one or more entirely different substances with new properties. This differs from a physical change, where a substance changes its form or appearance, like ice melting into water, but remains chemically the same. Chemical changes are commonly observed in everyday occurrences such as the burning of wood or the rusting of iron.
The Atomic Rearrangement
At the heart of every chemical change is a rearrangement of atoms. During these processes, existing chemical bonds within molecules break apart, and new bonds subsequently form, creating entirely new substances.
The atoms themselves are not created or destroyed; instead, they simply reconfigure into different groupings. Imagine atoms as building blocks that are disassembled from one structure and then reassembled into a completely new one.
This atomic reorganization is what gives the newly formed substances their unique properties, distinct from the original materials. For instance, when hydrogen and oxygen combine to form water, their arrangement into water molecules results in a substance with vastly different characteristics.
Energy in Chemical Changes
All chemical changes involve transformations of energy. Energy is either absorbed from the surroundings or released into them during a chemical reaction. This energy exchange is a universal feature distinguishing chemical transformations.
Reactions that release energy, often as heat or light, are called exothermic reactions. A common example is combustion, where burning materials like wood or natural gas release significant amounts of heat and light. The energy released typically originates from the formation of new, more stable chemical bonds.
Conversely, endothermic reactions absorb energy from their surroundings, frequently causing a noticeable drop in temperature. Photosynthesis, where plants absorb light energy to convert carbon dioxide and water into glucose and oxygen, is a prime example of an endothermic process. These reactions require an input of energy to break existing bonds, leading to products with higher energy content than the reactants.
Observable Signs of Change
Several macroscopic signs can indicate that a chemical change has taken place. One common indicator is a change in color, such as an apple browning after being cut and exposed to air.
Another sign is the formation of a gas, often observed as bubbles appearing within a liquid, as seen when vinegar reacts with baking soda. The production of a solid, known as a precipitate, from two liquid solutions also points to a chemical change. This insoluble solid can either settle at the bottom or remain suspended.
Additionally, a noticeable change in temperature, either an increase (heat released) or a decrease (heat absorbed), suggests a chemical reaction. The emission of light, like that from a glow stick or fireworks, also serves as a strong indication.
The Law of Conservation
A fundamental principle governing all chemical changes is the law of conservation of mass. This law states that in any chemical reaction, mass is neither created nor destroyed. The total mass of the substances present before the reaction, known as reactants, must equal the total mass of the substances formed after the reaction, called products.
This concept directly reinforces the idea that atoms are only rearranged, not lost or gained, during a chemical transformation. For example, when wood burns, the ash, smoke, and gases produced collectively have the same total mass as the original wood and the oxygen consumed from the air. The conservation of mass is a cornerstone of chemistry, ensuring that matter is accounted for throughout any chemical process.