Chemistry Unit 2.2

A homogeneous mixture is a combination of two or more substances that cannot be distinguished from each other. It has uniform composition and properties. Homogeneous solutions are also called solutions.

A heterogeneous mixture is a combination of two or more substances that can be distinguished from each other. It has varying composition and properties. Heterogeneous mixtures can be further classified either as suspensions or colloids. A suspension is a heterogeneous mixture whose solutes do not completely dissolve. The insoluble particles settle into clumps or layers when left undisturbed. A colloid is a heterogeneous mixture whose solute-like particles are dispersed in a medium.

Separating Homogeneous Mixtures

Evaporation

Evaporation is the phase transition of matter from liquid to vapor. This separation technique is often employed to solid-liquid mixtures where a solid solute is dissolved in a liquid solvent. In this process, a solution is heated until it boils. Once the solution boils, the liquid solvent starts to evaporate and leave behind the solid solutes. The case for solid solvents and liquid solutes separation can also take advantage of this separation technique.

Recrystallization

Recrystallization is a separation technique based on the difference in solubilities of substances in an appropriate solvent at an elevated temperature. Solubility refers to the amount of substance that can be dissolved in a given solvent at a certain temperature. Recrystallization is often applied as a purification technique for solid mixtures.

The method requires dissolving the solid compound in an appropriate solvent at an elevated temperature. The elevated temperature increases the solubility of the solute. This separates the solute of interest with another solute, which has low solubility in the solvent. Then, the solute starts to recrystallize as the solution cools down and being saturated by the solute. In the slow formation of the crystal lattice, the impurities are excluded. The collection of the crystals is then performed by another separation technique, filtration, which will be discussed further later.

The choice of solvent is important in performing recrystallization as a separation technique. An ideal solvent for recrystallization must have the solute be soluble at high temperatures but only sparingly soluble at room or lower temperatures. The impurities must be insoluble in the solvent. Also, no reaction must occur between the solute and the solvent. An ideal solvent is moderately volatile, and its boiling point is lower than the melting point of the solute.

Distillation

Distillation is a separation technique that is applied for homogeneous liquid-liquid mixtures. The separation is based on the difference in boiling points between the two liquid components. There must be a large difference between the boiling points of the components so that this separation technique can work. In this process, the component with the lower boiling point will evaporate first as the working temperature is raised, while the other remains as a liquid. The vapor then condenses and is collected in another container to separate the two components. The illustration below shows a distillation setup separating acetone and water.

There are four common types of distillation processes—simple, fractional, vacuum, and steam. Simple distillation is used when there is a large difference in the boiling points of the components of the solution. It involves a single evaporation and condensation step. Fractional distillation is used when there is a relatively small difference in the boiling points of the components of the solution. It involves a series of evaporation and condensation steps. For compounds with very high boiling points, vacuum distillation is used. Steam distillation is used for compounds that are heat-sensitive. This enables the separation of the components before any components decompose.

Chromatography

Another way to separate liquid-liquid mixtures is through chromatography. It is a separation technique that relies on the differential partition of the components between the two important phases in chromatography: the mobile phase and the stationary phase. The mobile phase is the solvent in chromatography that carries the components throughout the stationary phase, an adsorbent material that holds the solutes still when interactions are favored. These two phases must have opposite polarities.

Chromatography can be classified based on the polarity of its phases. In normal-phase chromatography, the mobile phase is nonpolar, and the stationary phase is polar. On the other hand, in reversed-phase chromatography, the mobile phase is polar, and the stationary phase is nonpolar.

The separation is determined by the two competing processes: the adsorption onto the stationary phase and the solubility in the mobile phase. As the mobile phase runs through the stationary phase, the components of the mixture separate based on these competing processes. Whichever process is predominantly experienced by a molecule is dependent on the strength of its interaction with the stationary phase or the mobile phase. Solutes that have high solubility in the mobile phase will go along with the mobile phase as it travels on the stationary phase. On the other hand, solutes that have low solubility will tend to remain where they are.

Chromatography can also be classified based on the structure of the stationary phase. In column chromatography, the stationary phase is held in a column. The mobile phase passes through the stationary phase by gravity or by applying pressure. In planar chromatography, the stationary phase is supported on a flat plate. The mobile phase moves via capillary action as it runs through the stationary phase upwards. Below is an illustration of a paper chromatography setup, an example of planar chromatography. Here, the paper acts as the stationary phase and ethanol as the mobile phase.

Paper chromatography, as the mobile phase runs through the paper upwards, the components of the ink solution separate into its component dyes. The result of a chromatography experiment is called a chromatogram, which is a visual record of the result of the separation process. One can see from the illustration that the yellow dye component in the mixture has the highest solubility in the mobile phase, while the purple dye component has the least solubility. The chromatogram also suggests that the purple dye has the strongest adsorption on the stationary phase.

Separating Heterogeneous Mixtures

Manual Separation

Heterogeneous mixtures with large, visible components may be separated into its components through manual separation techniques. Manual picking using your hands or tongs can be done in separating the components of these kinds of mixtures. They may be separated into various containers by picking them individually. For instance, you do manual picking when separating different non-biodegradables in a trash bin.

Another manual separation technique is sieving. Separation by sieving takes advantage of the difference in particle size of solids. A sieve is a layer of holes that allow only certain sizes of particles to pass. An everyday example of a sieve is a kitchen sifter which is used for separating large clumps from fine flour particles. This technique is also applicable in separating sand from gravel before preparing a cement mixture.

Some heterogeneous mixtures contain metallic components. These metallic components may be separated from the mixture by using a magnet. Iron and other metals are attracted to magnets. An example of a mixture that may be separated using a magnet is iron filings and sulfur. When a magnet is placed near the mixture, the iron filings will be attracted to it, which will cause its separation from the sulfur.

Filtration

Solid-liquid suspensions and colloids require more tedious techniques to be able to separate their components. Filtration is a process of separating solids from liquids by allowing the mixture to pass through a filtering material. Filters can be used to separate suspended solids from liquids. This is provided that the filter paper used has holes small enough for suspended solids not to pass through.

A common example of filtration is when you separate coffee grounds from brewed coffee. The coffee grounds are separated from the brewed coffee because its particles are too big to pass through the holes of the coffee filter. The coffee grounds collected on the filter paper is known as the residue while the brewed coffee is known as the filtrate.

The filter paper to be used in filtering suspensions can be prepared in two manners: simple conical (3:1) or fluted filter paper. A 3:1 filter paper is a simple folded conical filter paper that is commonly used when filtering suspensions in which the solids are to be collected. It is prepared by taking a round piece of filter paper and folding it in half, then folding it again in half. The twice folded filter paper is opened to form a hollow cone that can be used for filtration. It is called 3:1 since the sheets of filter paper in the cone form must be separated by taking the three sheets together, leaving the other sheet alone.

On the other hand, a fluted filter paper is used in gravity filtration when the filtrate or the liquid component is to be collected. It is prepared by taking a round piece of filter paper and folding it in half in multiple ways possible. It allows air to enter the flask along its sides to equalize pressure, thereby increasing the speed of filtration. It also provides a larger surface area through which the solvent can seep through. To show you how a fluted filter paper is prepared, watch the video below.

Sedimentation and Decantation

Sedimentation is the process in which suspended solids will eventually separate from liquids by gravity. Once the solid particles settle at the bottom, the liquid may be separated through decantation. Decantation is the removal of the liquid component from solid sediment by pouring the liquid out of the container gently to avoid the solid particles to suspend again. It can be pumped out using a syringe or other suctioning device. Decantation may be aided with the use of a stirring rod to direct the flow of the solvent and prevent splashing.

Decantation may also be used to separate mixtures with liquids that are immiscible or do not mix well. An example of this is a mixture of oil and water. Since the water settles at the bottom, slowly pour out the oil from the container to separate it from water. An easier way to separate oil from water is by using a separatory funnel . It is used to separate two liquids with different densities.

The components of solid-liquid suspensions and even colloids can be separated by centrifugation. Centrifugation is a process in which the suspension or colloid is rotated at very high speeds. It uses a machine called a centrifuge that rotates the sample. Centrifugation is often performed on suspensions or colloids in which the dispersed particles are small and do not settle at the bottom easily even when it is left undisturbed. Initially, the particles are randomly dispersed in the medium, as shown in the figure below. But after loading the sample in the centrifuge, the centrifugal force pushes down the solids. After the process, the solid particles are separated from the liquid as they accumulate at the bottom of the container. The liquid can then be decanted to separate the solids from the liquid. Different liquids with different densities can be separated into layers using centrifugation as well. The layers can be separated from one another.

Applications of Separation Methods

Separation techniques find great use in our daily lives and in a variety of different industries. In this section, some examples of applications of separation techniques in various industrial processes will be enumerated.

In the chemical and pharmaceutical industry, separation techniques are used to purify chemicals to acceptable standards. For example, in order for a medicine to be acceptable for use, it has to be purified to remove extremely toxic chemicals that were used to manufacture it. Medicines like paracetamol or aspirin, are recrystallized from solution to bring it to acceptable standards.

In the petroleum industry, distillation is often used as a method to purify products. A refinery is an industrial site that is used to refine substances like oil, alcohol, and other substances. Crude oil is distilled to gasoline, kerosene and other petroleum products in order to be acceptable for applications. Petroleum products are used as fuel, which is essential to the production of many other products and supports a comfortable way of modern living.

In the wastewater industry, separation techniques such as sedimentation, centrifugation, microfiltration, and distillation are used to recover water from used water. By doing so, harmful substances are removed before releasing the wastewater to the environment. Household water treatment includes the processing of tap water to separate water from contaminants, making it suitable for drinking. It may include separation methods such as filtration, sedimentation, and use of activated carbon. In this way, tap or faucet water becomes more suitable for drinking because of the removal of particulates, odor, color, and even some harmful, less persistent bacteria. However, it is advisable to check the water quality first before drinking any treated water.

In the field of health and medicine, centrifugation is used to separate the components of blood or urine for further laboratory testing. In blood centrifugation, the blood extracted from the patient is placed in a centrifuge tube. This will be placed in a centrifuge for a few minutes until the components are completely separated. The liquid component of the blood will be filtered to separate the blood cells. Then, the laboratory technician will be able to determine the count of blood cells through a microscope. The result of this observation can tell whether the patient has an infection. This is also how urine testing is done.