Matter can be organized into two main categories in physical science: pure substances and mixtures. A mixture is a combination of two or more different substances that are physically intermingled but not chemically bonded. Understanding how these components interact and can be separated is a fundamental concept in chemistry. This physical combination allows the individual substances to maintain their original chemical identities and properties.
Defining Physical Mixtures
A defining characteristic of a physical mixture is that its components retain their individual chemical nature. When salt and water are combined, for instance, the salt remains sodium chloride and the water remains \(\text{H}_2\text{O}\); no new chemical compound is formed. This contrasts with pure substances, which are either elements or compounds.
Unlike a chemical compound, which requires elements to combine in a fixed ratio, the substances in a mixture can be combined in any proportion. You can dissolve a small amount of sugar in water or a large amount, and the result is still a sugar-water mixture. Furthermore, forming a mixture does not involve a chemical reaction, meaning there is no energy change, such as the release or absorption of heat, associated with its creation.
The lack of chemical bonding allows mixtures to be separated by simple physical processes. This is a major distinction from compounds, which require chemical reactions to break the bonds holding the atoms together. The physical properties of the mixture, such as its melting or boiling point, are not fixed but instead vary depending on the ratio of the components present.
A mixture is considered an impure form of matter because its composition is not constant. In a compound like water, every molecule is identical and has a fixed composition. However, in a mixture like air, the percentages of nitrogen, oxygen, and other gases can fluctuate slightly depending on location and time. The physical combination ensures that the properties of the overall mixture are simply a blend of the properties of the individual substances.
Homogeneous and Heterogeneous Mixtures
Mixtures are classified into two categories based on the uniformity of their composition: homogeneous and heterogeneous. A homogeneous mixture, often referred to as a solution, exhibits a uniform composition throughout its entire volume. In these mixtures, the components are mixed at a molecular or atomic level, making it impossible to visually distinguish one substance from another.
The composition of a homogeneous mixture is the same in every sample taken, meaning it consists of a single phase. Examples include saltwater, where the dissolved salt ions are evenly distributed, and air, a gaseous solution of nitrogen, oxygen, and trace gases. Metal alloys, such as brass (copper and zinc), are also considered solid homogeneous mixtures because the atoms are uniformly dispersed.
In contrast, a heterogeneous mixture is one where the composition is not uniform, and the components remain visibly separate. These mixtures contain two or more distinct phases, and you can see the boundaries between the different substances. A simple example is a mixture of sand and water, where the sand particles settle at the bottom, distinct from the liquid above.
Subtypes of heterogeneous mixtures include suspensions and colloids. A suspension, like muddy water, consists of solid particles large enough to settle out over time or be trapped by a filter. Colloids, such as milk or fog, contain particles that are larger than those in a solution but too small to settle out, often giving the mixture a cloudy appearance. The non-uniform nature of heterogeneous mixtures means that different samples taken from the same mixture may have different proportions of the components.
Techniques for Separating Mixtures
The separation of a mixture relies on exploiting the differences in the physical properties of its constituent substances. Since the components are not chemically bonded, methods like filtration, distillation, and magnetism can be used to isolate them. The choice of technique is determined by the specific physical properties that differ most significantly between the substances, such as particle size, boiling point, or solubility.
Filtration is a process used to separate an insoluble solid from a liquid in a heterogeneous mixture. This technique works by pouring the mixture through a porous barrier, such as filter paper, which allows the liquid component to pass through while trapping the larger solid particles. Filtration is commonly used in chemistry laboratories to separate precipitates from solutions or in everyday life to separate coffee grounds from brewed coffee.
Distillation is a method used to separate the components of a solution, often a homogeneous mixture of two liquids with different boiling points, or a liquid from a dissolved solid. The mixture is heated until one component vaporizes, forming a gas, which is then cooled and condensed back into a liquid in a separate container. This process is effective for separating water from dissolved salts, yielding pure water while leaving the solid residue behind.
Evaporation is a simpler technique that separates a dissolved solid from a liquid by heating the solution until the solvent turns into a gas and escapes into the atmosphere. This method is often used to recover the solid component, such as obtaining salt from saltwater, but the solvent is typically lost. Magnetism is used when one of the components in a mixture, such as iron filings mixed with sand, possesses magnetic properties that the others do not. A magnet can simply be passed over the mixture to attract and remove the magnetic substance.