A mixture consists of two or more substances that are physically combined. Unlike chemical compounds, the individual components of a mixture retain their original identities and properties. These combinations are common, found in air, saltwater, or a simple salad.
Mixtures Versus Pure Substances
Mixtures differ fundamentally from pure substances, which include elements and compounds. Pure substances possess a fixed and uniform chemical composition throughout, meaning every part of the substance is identical. For instance, gold is an element with a consistent atomic structure, and pure water is a compound always composed of two hydrogen atoms and one oxygen atom.
In contrast, mixtures do not have a fixed composition; their proportions can vary. The individual substances within a mixture are not chemically bonded together. For example, saltwater can have different amounts of salt dissolved in it, and the salt and water molecules remain distinct. The components of a mixture can therefore be separated by physical means without undergoing a chemical reaction.
Homogeneous and Heterogeneous Mixtures
Mixtures are broadly categorized into two types based on the uniformity of their composition: homogeneous and heterogeneous. A homogeneous mixture exhibits a uniform composition throughout, meaning its components are evenly distributed and indistinguishable from one another. Examples include sugar dissolved completely in water, where the sugar particles are no longer visible, or air, which is a blend of various gases like nitrogen and oxygen that appear as a single substance. Alloys, such as brass (a mixture of copper and zinc), also represent homogeneous mixtures.
Conversely, a heterogeneous mixture has a non-uniform composition, and its components can often be visibly distinguished. These mixtures consist of two or more phases, where each phase has distinct properties. For example, a mixture of sand and water clearly shows separate layers of solid sand and liquid water. A salad with lettuce, tomatoes, and croutons is another heterogeneous mixture because the different ingredients are not evenly distributed. Similarly, granite rock appears as a single stone but contains visible, distinct grains of different minerals like quartz and feldspar.
Methods for Separating Mixtures
Filtration is a common method used to separate an insoluble solid from a liquid in a heterogeneous mixture, such as separating coffee grounds from brewed coffee.
Other physical techniques include:
- Evaporation: Removes a liquid solvent, leaving a dissolved solid behind (e.g., salt from saltwater).
- Distillation: Separates liquids with different boiling points by heating the mixture and collecting the vaporized components separately.
- Magnetism: Separates magnetic materials (e.g., iron filings) from non-magnetic substances.
- Decantation: Separates immiscible liquids (e.g., oil and water) or a liquid from a settled solid by carefully pouring off the liquid layer.