A mixture is formed when two or more substances are physically combined without chemical bonding. The individual components maintain their unique chemical identities and properties, unlike in a chemical compound. Mixtures have a variable composition, meaning the relative amounts of the constituent parts can be changed freely. Mixtures are categorized based on the uniformity of their composition, which determines the terminology used to describe their components.
Identifying Solutes and Solvents
In chemistry, a mixture that is completely uniform at a molecular level is known as a homogeneous mixture, or more commonly, a solution. The components of a solution are described relative to their quantity and function. The minor component is termed the solute.
The solute is the substance that is dissolved or dispersed into another substance. For example, sugar acts as the solute when making sweetened tea, as it is distributed throughout the liquid. Solutes can exist in any physical state, including solids (like salt), liquids (like alcohol), or gases (like oxygen dissolved in water).
The major component of a solution is called the solvent. The solvent is present in the largest amount and performs the dissolving. It determines the physical state of the resulting solution; for instance, water is a liquid solvent that creates a liquid solution, such as seawater. Solutions form when solvent particles surround and separate the solute particles, leading to a single, uniform phase.
In common examples, such as salt water, the difference between the solute and solvent is clear because one is present in a much smaller quantity. Even in gaseous solutions like air, minor components such as oxygen or argon are considered solutes because they are dissolved within the major component, nitrogen. This distinction is fundamental to understanding the behavior and properties of homogeneous mixtures.
Quantifying Minor Components: Concentration
Identifying a substance as a solute is often insufficient for scientific or regulatory purposes; the amount of that minor component must be precisely quantified. This quantitative measure is defined as concentration. Concentration expresses the ratio of the amount of solute to the total amount of solution or solvent, allowing for a standardized way to compare the strength of different solutions.
When the solute is present in relatively high amounts, concentration is often expressed using mass percent (the mass of the solute divided by the total mass of the solution, multiplied by 100). However, for minor components, especially trace contaminants, this unit is impractical due to the extremely small numbers involved. Specialized units are used for these small quantities to clearly communicate the measurement.
Very low concentrations are measured using parts per million (ppm) or parts per billion (ppb). These units express the ratio of the solute to the solution by mass or volume, scaled by one million (\(10^6\)) or one billion (\(10^9\)), respectively. A concentration of 1 ppm means there is one part of the solute for every one million parts of the total mixture.
These units are important in fields like environmental monitoring and public health, where even slight differences in the concentration of a minor component can have significant consequences. For example, the allowable level of arsenic in drinking water is often regulated in the low ppb range, highlighting the need for precise quantification of these trace solutes. The use of ppm and ppb provides a direct metric for communicating the presence of extremely small, yet potent, minor components.
Components in Heterogeneous Mixtures
The terms solute and solvent are reserved for homogeneous solutions that have a uniform composition throughout. When a mixture is non-uniform, meaning its components are not evenly distributed and remain physically distinct, it is classified as a heterogeneous mixture. Examples include suspensions, where particles eventually settle out, and colloids, where particles are fine but remain dispersed.
In these heterogeneous systems, the minor component does not truly dissolve, so the terminology shifts to reflect physical dispersion rather than solution formation. The substance present in the lesser amount, analogous to the solute, is called the dispersed phase. This phase consists of small particles or droplets scattered throughout the rest of the mixture.
The substance present in the greater amount, which provides the background medium for the dispersed phase, is known as the continuous phase or the dispersion medium. For instance, in fog, tiny liquid water droplets are the dispersed phase, while the surrounding air is the continuous phase. Milk is another example, where liquid butterfat is the dispersed phase suspended in the continuous medium of water. This distinct terminology is necessary to describe mixtures where the components retain separate visible boundaries.