MIXTURES - #13

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Separating Mixtures

Many of the substances we use everyday were actually once part of a mixture. Someone somewhere separated that substance from the mixture so we could use it. It turns out that many compounds and elements aren't found in nature in their pure form, but are found as parts of mixtures. Separating substances from mixtures is an important part of chemistry and modern industry. 

Some important chemistry terms are used in this section including mixtures, suspensions, and solutions.  Read carefully.

Chemical Mixtures

One of the main aspects of chemistry is combining different substances. Sometimes combining substances can cause a chemical reaction and bonding which creates an entirely new substance called a compound. However, sometimes there is no chemical reaction or bonding. In this case, a mixture is formed from the combined substances. 

Mixture

A mixture is made when two or more substances are combined, but they are not combined chemically.

General properties of a mixture:

• The components of a mixture can be easily separated
• The components each keep their original properties
• The proportion of the components is variable 

 Types of Mixtures

There are two main categories of mixtures: homogeneous mixtures and heterogeneous mixtures. 

• In a homogenous mixture all the substances are evenly distributed throughout the mixture (salt water, air, blood). 
• In a heterogeneous mixture the substances are not evenly distributed (chocolate chip cookies, pizza, rocks)
  

 

Within the categories of homogeneous and heterogeneous mixtures there are more specific types of mixtures including solutions, alloys, suspensions, and colloids.
Solutions (homogeneous)

A solution is a mixture where one of the substances dissolves in the other. The substance that dissolves is called the solute. The substance that does not dissolve is called the solvent.

• An example of a solution is salt water. These components can be easily separated through evaporation and they each retain their original properties. However, the salt is dissolved into the water to where you can't see it and it is evenly distributed in the water. In this example the water is the solvent and the salt is the solute.

A saturated solution is one containing as much solute as possible without forming a precipitate. This is the maximum concentration of solute.

An unsaturated solution is a chemical solution in which the solute concentration is lower than its equilibrium solubility.

What is the difference between a solution and a mixture?

In chemistry a solution is actually a type of mixture. A solution is a mixture that is the same or uniform throughout. Think of the example of salt water. This is also called a "homogenous mixture." A mixture that is not a solution is not uniform throughout. 

• Think of the example of sand in water. This is also called a "heterogeneous mixture."

Alloys (homogeneous)

An alloy is a mixture of elements that has the characteristic of a metal. At least one of the elements mixed is a metal. 

• One example of an alloy is steel which is made from a mixture of iron and carbon.

Suspensions (heterogeneous)

A suspension is a mixture between a liquid and particles of a solid. In this case the particles do not dissolve. The particles and the liquid are mixed up so that the particles are dispersed throughout the liquid. They are "suspended" in the liquid. A key characteristic of a suspension is that the solid particles will settle and separate over time if left alone.

• An example of a suspension is a mixture of water and sand. When mixed up, the sand will disperse throughout the water. If left alone, the sand will settle to the bottom.

Colloids (heterogeneous)

A colloid is a mixture where very small particles of one substance are evenly distributed throughout another substance. They appear very similar to solutions, but the particles are suspended in the solution rather than fully dissolved. The difference between a colloid and a suspension is that the particles will not settle to the bottom over a period of time, they will stay suspended or float.

• An example of a colloid is milk. Milk is a mixture of liquid butterfat globules dispersed and suspended in water.

Colloids are generally considered heterogeneous mixtures, but have some qualities of homogeneous mixtures as well.

Interesting Facts about Mixtures

• Smoke is a mixture of particles that are suspended in the air.
• Tap water is a mixture of water and other particles. Pure water or H2O is generally referred to as distilled water.
• Many of the substances we come into contact with every day are mixtures including the air we breathe which is a mixture of gases like oxygen and nitrogen.
• Blood is a mixture that can be separated by a machine called a centrifuge into its two main parts: plasma and red blood cells.
• Mixtures can be liquids, gases, and solids.

SEPARATING MIXTURES 

Why do we want to separate mixtures? 

All the way back to Ancient History, industrious humans have separated mixtures in order to obtain the specific substances that they need. One example of this is extracting metal from ore in order to make tools and weapons. We'll discuss some other examples of separation below. 

Separation Processes 

The way in which different substances in a mixture are separated is called a process. There are a number of different processes used for separation. Many of them are very complex and involve dangerous chemicals or high temperatures. A lot of important industries in the world today are based on separation processes. 

Filtration 

One common method of separation is filtration. Filters are used everywhere. We use them in our houses to filter dust and mites out of the air we breathe. We use them to filter impurities from our water. We even have filters in our bodies such as our kidneys which act as filters to get bad stuff out of our blood. 

The filtration process is generally used to separate a suspension mixture where small solid particles are suspended in liquid or air. In the case of filtering water, the water is forced through a paper that is made up of a very fine mesh of fibers. The water that has been run through the filter is called the filtrate. The particles that are removed from the water by the filter are called the residue. 

Distillation 

Another common separation process is called distillation. Distillation uses boiling to separate mixtures of liquid solutions. It takes into account that different substances in the mixture will have different boiling points. 

For example, if you heat salt water the water in the solution will boil before the salt. The water will then evaporate leaving the salt behind. If the steam from the water is collected it will turn back into liquid as it cools. This cooled water will be pure water without any salt. 

Centrifuge In some cases, there are suspension mixtures where the solid particles are too fine to be separated with a filter. In these cases, sometimes a centrifuge is used. Centrifuges are mechanical devices that spin at very high speeds. These high speeds allow the solid particles in suspensions to settle very quickly.  For example, rather than wait for sand to slowly settle to the bottom of water, a centrifuge can cause the sand to settle in a matter of seconds. Some examples of how centrifuges are used include: separating blood into plasma and red cells separating cream from milk separating uranium isotopes for nuclear power plants.

The heavier particles move to the outside of the cylinder as the centrifuge spins allowing the mixture to be separated.

Other Processes 

There are many other separation processes such as sublimation, adsorption, crystallization, and chromatography. Sometimes it takes many stages of processes to get to the final result. 

• One example of this is the processing of crude oil. Crude oil uses many levels of fractional distillation to produce a number of different products including gasoline, jet fuel, propane gas, and heating oil. 

Interesting Facts about Separating Mixtures

• To separate liquid solutions where the substances have similar boiling points, a more complex version of distillation is used called fractional distillation.
• Painting uses the separation process of evaporation. The wet paint is a mixture of color pigment and a solvent. When the solvent dries and evaporates, only the color pigment is left.
• The separation process of winnowing was used in ancient cultures to separate the grain from the chaff. They would throw the mixture into the air and the wind would blow away the lighter chaff, leaving the heavier grain.
• High speed centrifuges can spin up to 30,000 times a minute.
• Many separation processes are occurring constantly in nature.