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In chemistry, a mixture is a material formed by two or more different chemical substances that, when mixed, do not lose their respective identities. That is, they are combinations between compounds and/or elemental substances, which may be in different states of aggregation, and which remain the same substances after being mixed.
On the other hand, a mixture can also be defined as a combination of two or more substances that can be separated by physical methods (that is, methods that do not involve chemical reactions), such as mechanical separation, settling, filtering and drying or evaporation, among others.
Finally, a mixture in chemistry is the opposite of a pure substance, defined as a substance that cannot be separated into simpler substances by physical processes.
Although the substances that make up a mixture maintain their structure and identity, a mixture can be more than the sum of its parts, being able to exhibit unique properties that none of its components have separately. For example, the preparation of a gelatin results in a flexible, translucent substance that, however, has its own shape, unlike water, which is its main component.
Classification of mixtures
Mixtures can be classified, according to the number of phases that make them up, into homogeneous and heterogeneous mixtures. In addition, homogeneous mixtures can occur in different states of aggregation, while heterogeneous mixtures can be classified in different ways depending on the size of the particles that make them up.
Homogeneous mixtures
Homogeneous mixtures are those mixtures between two or more components in which only a single phase is distinguished and whose composition and properties are constant throughout. This means that if we compare two samples of a homogeneous mixture taken from any two points, both samples will look exactly the same, have exactly the same composition and have the same physicochemical properties.
Homogeneous mixtures are also called solutions and can be obtained in different states of aggregation depending on the components it contains. In this sense, we can have:
- Liquid solutions in which a solute, be it solid, liquid, or gas, dissolves in a liquid solvent. A typical example is a solution of sugar or salt in water.
- Solid solutions in which two substances are melted and mixed together to produce a liquid homogeneous mixture, but this is then allowed to solidify to form a solid mixture. Typical examples of this type of mixtures are metal alloys.
- Gaseous homogeneous solutions or mixtures , such as air, which is a homogeneous mixture composed mainly of nitrogen, oxygen, carbon dioxide, and other gases.
heterogeneous mixtures
Heterogeneous mixtures are the opposite of homogeneous mixtures. In these, more than one phase can be easily distinguished either with the naked eye, or through the use of instruments such as microscopes. What characterizes heterogeneous mixtures is that their composition is not uniform throughout, and parts of the mixture that have different properties from others can be isolated.
Easily identifiable examples of heterogeneous mixtures are those in which with the naked eye we notice that there are two or more separate phases. For example, if we look closely at a handful of sand, we immediately notice that it contains a wide variety of particles of completely different colors and characteristics.
Then, if we also mix the sand with water, we can clearly distinguish the solid phase from the aqueous liquid phase. Heterogeneous mixtures can also occur between substances that are in different states such as gases and liquids, gases and solids, solids and liquids, etc.
Although heterogeneous mixtures are often easy to distinguish, other times this is not the case. This is because, many times, the particles that make up the different phases are so small that we cannot distinguish them with the naked eye. However, these generally present characteristics and properties that make it possible to distinguish them from a homogeneous mixture with relative ease.
In these cases, these mixtures are usually classified according to the size of their particles as:
- Coarse mixes , where the particles are large enough to be seen with the naked eye, such as sand or salad.
- Suspensions , in which solid particles too small to see with the naked eye but large enough to settle are dispersed in a liquid or gas. Smoke is a typical example of a suspension of a solid in a gas, while milk is a good example of a suspension (of milk solids and fat) in water.
- Emulsions , which are formed when two immiscible liquids or a gas and a liquid are mixed and one of them (called the dispersed phase) is dispersed as small droplets (or bubbles) within the other liquid (called the continuous phase). Mayonnaise is an example of an emulsion between water and oil, while foams and whipped cream are examples of emulsions between liquids and air.
- Colloids, which are heterogeneous mixtures that appear homogeneous to the naked eye, but are nevertheless made up of very small particles dispersed in a liquid phase. Unlike suspensions, which are opaque and do not allow light to pass through, colloids are translucent, just like solutions. However, colloids are able to scatter light, which true solutions cannot. The classic example of a colloid is gelatin, but most gels fall into this classification.
Homogeneous mixtures versus pure substances
Both homogeneous mixtures and pure substances are completely homogeneous and have a uniform composition throughout their entire length. This makes it sometimes difficult to distinguish with the naked eye when a material is a homogeneous mixture or a pure substance.
For example, let’s imagine that we see two glasses full of water. One contains pure water and the other contains salt water.
How can we tell which is which with the naked eye?
We can’t do it. The only way to know if a homogeneous material is a pure substance or a mixture is by trying to separate its components. If we manage to separate the material into two or more different components through physical processes, then we know that it is a mixture.
For example, if we take a small sample from each glass of water in the previous example and evaporate it, we will quickly see that the salt water mixture leaves a residue that pure water does not, proving that it was a mixture.
References
Concept of. (sf-a). Chemical Emulsion – Concept, phases, types and examples . https://concepto.de/emulsion-quimica/
Concept of. (sf-b). Mixture – What it is, types, characteristics, examples and substances . https://concepto.de/mezcla/
Concept of. (sf-c). Chemical Suspension – Concept, phases, properties and experiments . https://concepto.de/suspension-quimica/
Khan Academy. (nd). Types of mixtures . https://es.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:intermolecular-forces-and-properties/x2eef969c74e0d802:solutions-and-mixtures/v/types-of-mixtures
