Chemists categorize substances based on how their constituent parts are combined. Matter is fundamentally assembled as either a mixture or a compound, and understanding this distinction is central to chemistry. A compound is a pure substance formed when two or more elements are chemically joined. A mixture results from the physical blending of two or more substances. The nature of their interaction, resulting composition, and separation methods are fundamentally different.
The Nature of Their Combination
The most significant difference lies in how components interact at the atomic level. In a compound, elements are held together by strong chemical bonds (ionic or covalent). This bonding involves a chemical reaction, often with an associated change in energy, resulting in an entirely new substance. For example, hydrogen and oxygen gases combine to form water (H2O), a liquid with properties completely unlike its starting materials.
Components in a mixture are only physically intermingled and are not chemically bonded. No chemical reaction occurs when a mixture is formed, and each component retains its original chemical identity and properties. For instance, when salt and water combine to create saltwater, the salt and water molecules remain dispersed but chemically unchanged. The final mixture’s properties are essentially a blend or sum of its individual parts.
Composition Ratios and Chemical Notation
A compound is characterized by a fixed and definite composition, meaning elements are present in a precise proportion by mass. This adherence to the Law of Definite Proportions ensures that every molecule of a specific compound is identical. Water, for example, always has two hydrogen atoms for every one oxygen atom, represented by the chemical formula H2O.
Mixtures do not adhere to any fixed ratio, allowing components to be combined in virtually any proportion. The amount of sugar added to tea can vary dramatically, yet the sweetened liquid remains a mixture. Because a mixture has a variable composition, it does not have a unique chemical formula. Instead, its makeup is described by listing the substances that were blended, highlighting the physical nature of the combination.
Methods of Separation
The difference in combination dictates the methods required to separate substances back into their original components. To break a compound down into its elements, the strong chemical bonds must be broken. This requires significant energy input through chemical means, such as using an electrochemical process like electrolysis to separate water into hydrogen and oxygen.
Mixtures, since they are only physically combined, can be separated using relatively simple physical techniques. These methods exploit differences in the physical properties of the components, such as boiling point or particle size. Common separation techniques include:
- Distillation, which uses different boiling points to separate liquids.
- Filtration, which separates insoluble solids from liquids.
- Using a magnet to separate iron filings from sand.
- Evaporation to recover salt from water.