The Resolution of Mixtures

Mixtures are physical combinations of two or more pure substances (elements or compounds). As a physical combination one should be able to separate these substances from the mixture by physical methods so that no Chemical change can take place during the separation. Substances can be separated from the mixture by taking advantage of any differences the substances have in physical properties. For example, most pure substances have different boiling points (temperature at which a substance will boil). If we can heat a mixture so that the lowest boiling substance will boil off before any other substance begins to boil, we will effectively be able to separate that substance from the mixture. One such laboratory resolution method is known as distillation. There are several different types of distillation.

Types of Distillation

1. Simple Distillation- This form of distillation is best used when you have pure substances that you wish to separate that are 100 degrees or more apart in boiling point. Basically, it consists of a boiling flask in which the mixture is placed. The flask is connected to a distilling head with a thermometer inserted into its top. The distilling head is connected to a water cooled condenser which ,in turn, is connected to a recepticle where the pure substance that was boiled off would be found.

   The flask is usually heated by either a bunsen burner or a heating element called a Heating Mantle. Heating Mantles are used when there is a danger of vaporous substance igniting due to the presence of a burner flame.Other forms of heating are a water bath that is heated by a hot plate surface or a steam bathg where live steam is the source of heating. Milder and more controllable form of heating is the water bath, but it is only capable of heating up to 100 degrees Celsius.

   The flask is heated until the boiling point of the lowest boiling substance in the mixture is reached. Then the substance will vaporize and travel over to the condenser whose inner core is cooled by circulating water from the cold water tap. Water travels in at the bottom of the condenser and comes out at the top. As the hot vapors of the substance come into contact with the water cooled surface of the condenser core, the vapor will lose its energy it gained from being heated in the flask and will condense back into a liquid which will be pure in the lowest boiling substance in the mixture. The closer the boiling points of the substances are in the mixture, the more likely that the vapor coming over into the condenser will be partially of the two substances. If the substances within the mixture are within 60 degrees of one another, simple distillation techniques are not very efficient and a second type of distillation must be considered.

2. Fractional Distillation-Fractional Distillation is similar to simple distillation except for one very important extra item in the set-up. We have a vertical column that is inserted in between the distilling flask and the distilling head called the "fractionating column". This column is ,unlike the condenser, never heated. It is usually filled with glass beads or metal wire mesh inside its central core. Quite often, it is insulated using spun glass which is stuffed into the center of the column. This provides many surfaces for the vaporized substances to condense upon. As the mixture is heated the substances that are nearest to each other in boiling point will both vaporize. The vapor mixture will then rise into the fractionating column and will condense on the relatively cooler surface and the hot condensate will fall back into a lower portion of the column only to be met with hot vapors from below and the hot vapors will re-vaporize the liquid back into the vapor state pushing the mixture vapor even higher up the fractionating column where it will be recondensed and the cycle repeats itself over and over again. Each time the vapor mixture gets richer in the lower boiling substance. By the time it reaches the distilling head at the top of the fractionating column the vapor is pure in the lower boiling substance. Actually, each time there is a vaporization followed by a condensation, a simple distillation is being performed. Fractionating columns are rated in "theoretical plates". A theoretical plate equal to 1 is the same efficiency as one simple distillation. Usually theoretical Plates of 4 or 5 are considered good since that would yield the same efficiency of separation as if you had simple distilled each successive distillate (that which was distilled over) 4 or 5 times in succession.

   Often, the fractionating columns are wrapped in thermal insulation and even heated at the bottom of the column while gradually cooled at the top.

3. Vacuum (Reduced Pressure) Distillation- Often a mixture contains a pure substance which will have such a high boiling point that it will chemically decompose before it reaches its boiling point. This is obviously self defeating. However, boiling points vary according to the atmospheric pressure. The lower the pressure the lower the boiling point will be for the same substance. If the pressure inside the distilling flask could be reduced thereby reducing the boiling point below the decomposition point, then the substance could be effectively separated from the mixture. Vacuum distillation is also used when you have to separate a highly volatile and combustible solvent that has a low flash point. By reducing the pressure inside the flask you could effectively distill such a solvent without using even a heating mantle or hot plate. Just the warmth of the body temperature would be sufficient to separate it!!
4. Steam Distillation- An alternative way of distilling a liquid substance whose normal boiling point (boiling point at 760 mm Hg) is above its decomposition point is steam distillation. A substance must be insoluble (immiscible) in water. It must also have a polar group that could be attracted to the water molecule. Oils and tars are steam distillable. The advantage over vacuum distillation is that you don't have the excessive "bumping" that takes place in the vacuum distillation. The ancient Egyptians marketed a rose oil that was separated from rose petals by essentially a steam distillation process. The rose oil was at one time considered as valuable as its weight in Gold!! The distillate that comes over is actually two immiscible liquids, water and the organic substance. This is referred to as a "co-distillate". The organic by itself might boil hundreds of degrees Celsius but will come over with the steam at or below 100 degrees Celsius.