Chemistry Unit 2.1

Mixtures

As you recall from the previous unit, there are two classifications for matter based on composition and properties—a pure substance or a mixture. A pure substance cannot be further broken down or purified by physical means. Each substance has its own characteristic properties that are different from the set of properties of any other substance. On the other hand, mixtures are combinations of two or more pure substances in which each substance retains its own composition and properties. In this lesson, you will learn more about how mixtures are made, and what the different types of mixtures are.

Classifying Mixtures

Almost every sample of matter that we ordinarily encounter every day is a mixture. The combined substances in mixtures can be mixed in varied proportions. In a mixture, substances do not react with each other.

For example, take a mixture of salt and pepper. Salt and pepper are physically mixed together, but they do not combine chemically to form a new substance with new properties. Each component in the salt and pepper mixture retains its original properties. The salt remains salty, while pepper remains spicy. Also, we can make an infinite number of different mixtures of salt and pepper by varying the relative amounts of the two components used.

Since the components of a mixture are combined physically, the components can be easily separated by physical means as well. In order to separate the two components in the salt and pepper mixture in Fig. 2.1.1, you could treat it with water to dissolve the salt, collect the pepper by filtration, and then evaporate the water to obtain the salt in solid form. The techniques illustrated are some examples of separation techniques that can be employed in mixtures. Other ways of separating mixtures include sieving, decantation, distillation, and chromatography, to name a few. These techniques will be further discussed in the succeeding lessons.

A mixture can be classified based on the distribution of the components. It can be classified as a homogeneous mixture or a heterogeneous mixture.

Homogeneous Mixtures

A homogeneous mixture is a type of mixture that has uniform composition and properties. The components of a homogeneous mixture are spread out evenly all throughout. It has only one phase; thus, the components cannot be distinguished from each other. Homogeneous mixtures are also called solutions. Shown below are some examples of homogeneous mixtures that you may encounter in your daily life.

Other examples of solutions include saltwater; some alloys, which are homogeneous mixtures of metals in the solid-state; and air (free of particulate matter or mists). Air is a mixture of gases. It is mainly nitrogen, oxygen, argon, carbon dioxide, water vapor, and some trace amounts of other substances in the atmosphere.

Heterogeneous Mixtures

The second type of mixture has components distinguishable from one another, with each portion having recognizably different properties. Such a mixture, which is not uniform throughout, is called a heterogeneous mixture. A heterogeneous mixture has varying composition and properties. It may have two or more phases, and the individual substances are visually distinct. Shown below are some examples of heterogeneous mixtures that you may encounter in your daily life.

Other examples of heterogeneous mixtures include mixtures of salt and pepper (in which two components with different colors can be distinguished easily from each other by sight), foggy air (which includes a suspended mist of water droplets), and vegetable soup.

Solutions: Homogeneous Mixtures

Homogeneous mixtures are also called solutions. In a solution, one component is uniformly dissolved and scattered in another component to form a homogeneous composition. It consists of two components: the solvent and the solute. Regardless of the states of the pure substances mixed together, the solvent is the one that is more abundant in solutions. It is the dissolving medium. The solute is the component present in less amount in a solution. It is the substance being dissolved.

When a solute dissolves in a solvent, the solute particles break apart into very small pieces and get evenly distributed in the bulk of the solution. This results in a mixture having a consistent appearance and composition all throughout. The particles of a solution are too small to be seen by the naked eye and to be distinguished from each other.

Consider brine as an example of a homogeneous mixture. Its components are salt and water. In this mixture, salt is the solute and water is the solvent. Once you dissolve the salt in water and stir it vigorously, a brine solution is prepared. This liquid mixture has a uniform appearance.

Types of Solutions

Solutions may be classified based on the physical state of the solvent—namely, solid, liquid, and gaseous solutions.

Liquid solutions are solutions wherein the solvent is in the liquid phase. It is the most common type of solution. It could be a solid-liquid solution, liquid-liquid solution, or a gas-liquid solution. Seawater is an example of a liquid solution. Salts of calcium and sodium are the solutes, while water is the solvent. Listed in the table below are some examples of liquid solutions.

Table 2.1.2. Some examples of liquid solutions

Solid solutions are solutions wherein the solvent is in the solid phase. For example, alloys are solid solutions because they are made up of metal mixed together with another metal. Listed in the table below are some examples of solid solutions.

Table 2.1.3. Some examples of solid solutions.

Gaseous solutions are solutions wherein the solvent is in the gaseous phase. Air is an example of a gaseous solution. When it is free of particulate matter or mist, the air is a mixture of gases composed of several gases like oxygen, argon, carbon dioxide, water vapor, and some trace amounts of other substances that are all dissolved in nitrogen gas, the most abundant gas component in the atmosphere.

Suspensions and Colloids: Heterogeneous Mixtures

Recall that heterogeneous mixtures are mixtures whose composition and appearance is not consistent all throughout. The components of a heterogeneous mixture are visually distinct from each other. It can have two or more phases. Heterogeneous mixtures can be further classified based on the distribution of its components. There are two types, namely suspensions and colloids.

Suspensions

A suspension is a type of heterogeneous mixture in which the solute particles do not dissolve, but get suspended throughout the bulk of the solvent when left undisturbed. The solutes form clumps or layers that do not easily break into smaller pieces. A separate, distinct layer from the solvent will be formed when the solute settles due to gravity. The separate layer is usually seen at the bottom of the container. Particles of a suspension are larger than particles of a solution.

Sand in water is an example of a suspension. When sand is mixed with water, it will initially look like a hazy mixture. But after some time, the sand will settle at the bottom of the container. It forms two distinct layers of sand and water.

Other examples of suspensions include mud (soil suspended in water), dust (solid soot and ash particles suspended in air), and vegetable soup (vegetable suspended in water).

Colloids

A colloid is a heterogeneous mixture in which a microscopically dispersed substance, insoluble or soluble particles, is suspended throughout another substance. A colloid is made up of a dispersed phase (solute-like particles) and a dispersing medium (solvent-like medium). The dispersing medium can be regarded as a homogeneous mixture. Unlike suspensions, colloids usually do not separate into layers or clumps. Its components may separate, but it will take a very long time to do so. In addition to this, the dispersed phase particles are very light and minute enough to stay suspended in the dispersing medium for a long time, unlike the large particles that settle in suspensions.

Colloids can be considered as in between the homogeneous nature of solutions and the heterogeneous nature of suspensions. Like solutions, colloids appear as though they have a uniform composition. But unlike solutions, their particles are larger and are not evenly distributed throughout the bulk of the dispersing medium. In terms of particle size of the dispersed phase particles, colloids have larger particles than solutions but smaller particles than suspensions.

Colloids, like solutions, can be classified based on the physical state of the dispersing medium. They can be classified either as gaseous colloids, liquid colloids, or solid colloids.

Gaseous colloids are colloids having the dispersing medium in the gaseous phase. The dispersed phase particle can either be a solid or a liquid. A solid aerosol is formed when the dispersed phase is a solid. An example of a solid aerosol is smoke. Smoke contains solid particulate matter or very small particles that are dispersed in the air. A liquid aerosol is formed when the dispersed phase is a liquid. Deodorant spray is an example of a liquid aerosol, where the liquid deodorant solution is suspended in the air when it is sprayed.

Liquid colloids are colloids having the dispersing medium in the liquid phase. A solid, gas, or another liquid can act as the dispersed phase. A solid disperse phase suspended in a liquid is called a liquid sol. An example of liquid sol is ink. The solid, insoluble pigments are suspended in a liquid organic solvent in ink. On the other hand, a liquid suspended in another liquid is called an emulsion. Milk is an example of an emulsion as liquid fats and proteins are dispersed in water. Finally, a gas suspended in a liquid is called a liquid foam. One example is shaving foam. Air is dispersed in the liquid soap medium to form the foamy texture in shaving foam.

Lastly, solid colloids are colloids having the dispersing medium in the solid phase. A gas, liquid, or another solid can act as the dispersed phase. A gas dispersed in a solid is called a solid foam. Styrofoam is an example because gas is suspended in a solid polymer matrix of polystyrene. On the other hand, a liquid suspended in a solid is called a gel. An example of a gel is Jell-O. Liquid water molecules are dispersed in a solid gelatin matrix. Finally, a solid suspended in another solid is called a solid sol. Colored glass is an example of a solid sol because insoluble solid pigments are suspended in the solid matrix of the glass. Listed in the table below are some examples of colloids and their corresponding types.

The observable difference between solution, suspension, and colloid is the solute’s particle size. In a solution, its particles are very small that they cannot be seen by the naked eye. The solute particles are evenly distributed in the solvent. In a suspension, the particles are very large that they can be distinguished by the naked eye. These particles are insoluble. Initially, suspensions may appear homogeneous. But after some time, the particles of a suspension settle down, making it heterogeneous. Intermediate to the particle sizes of solutions and suspensions are that of colloids. Although the particles in a colloid may appear to be evenly distributed, the particles are not totally dissolved, and they do not settle after some time.

The Tyndall Effect

One way to differentiate colloids from solutions and suspensions is by Tyndall effect. Colloidal particles exhibit Brownian motion and cause the Tyndall effect. Brownian motion is the random movement of particles suspended in gas or liquid. This random movement of particles causes the scattering of light or the Tyndall effect. It is usually demonstrated by passing a ray of light through a sample mixture. Through this test, the mixture could be easily identified as a solution, suspension, or colloid.

When light passes through a given mixture, the mixture is a solution. Light will have a narrow path and is not clearly visible. Since solutions have the smallest particle size which the naked eye cannot see, light is not blocked by the particles of a solution.

When light does not pass through a given mixture, the mixture is a suspension. Since suspensions have the largest particle size, light is blocked by the particles of a suspension. Suspensions appear opaque.

When light passes through a given mixture and the light scatters, the mixture is a colloid.

Colloids have an intermediate particle size. Due to its intermediate particle size, some light can pass through, and some are blocked and scattered. Unlike a suspension, colloids are not necessarily opaque. Among the three, only colloids exhibit the Tyndall effect. For example, milk is a colloid composed of fat and proteins suspended in water. It appears homogeneous since the suspended fat particles are too small. However, these particles are too big to be evenly distributed like solutes in solutions. Therefore, these colloidal particles scatter light.