Mixtures vs Compound
A very important concept in Chemistry is the difference between a mixture and a compound. A mixture is when two or more different substances are placed together without reacting, and thus each of the individual substances would still possess its own characteristics. This is not the same for the production of a compound, where two substances would react to form a new compound, for example, the formation of table salt, NaCl where pure sodium (Na) is explosive when a drop of water comes in contact with it while Chlorine (Cl2) is poisonous and would kill a human being if inhaled for just a few seconds.
The characteristics that distinguish mixtures and compounds can be seen in the following tables:
| Mixtures | Compounds |
| Contains two or more substances | Contains two or more elements chemically bonded. |
| Different composition | Always with the same ratio of elements. |
| Physical Change | Chemical change |
| Properties are the same as the individual substances | New properties |
| Separation by physical | Very difficult to separate. |
Separating Mixtures
There can be different types of mixtures, mainly:
- solid-solid mixtures
- solid-liquid mixtures
- liquid-liquid mixtures
A solid-liquid mixture consists of a solute (the material that will dissolve in teh liquid) and a solvent (the liquid in which the solid will dissolve).
Different mixtures would have to be separated using different techniques, and the techniques that are going to be discussed are:
- filtration
- decanting
- simple distillation
- fractional distillation
- crystallisation
- separating funnel
- sublimation
Filtration
This is a method which is the most especially effective for separating suspensions, for example, mud in the water. We pour the mixture into a funnel fitted with a piece of filter paper. There are tiny holes in the filter paper for the liquid to pass through, the solid particles are too large to do so, therefore the solid particles will stay on the paper as what we call a solid residue. We call the liquid which passes through the FILTRATE.
Decanting
Decantation is a process for the separation of mixtures, carefully pouring a solution from a container, leaving the precipitate (sediments) in the bottom of the container. Usually, a small amount of solution must be left in the container, and care must be taken to prevent a small amount of precipitate from flowing with the solution out of the container. It’s generally used to separate a liquid from an insoluble solid.
Crystallisation
Crystallisation is the separation of a liquid from soluble salt by evaporating and removing the solvent leaving the soluble salt behind as crystals.
Simple distillation
When a solution of solid in a liquid is heated, the liquid will evaporate. The hot vapour that formes can be condensed back to liquid again on a cold surface. We call this method DISTILLATION. Distillation is used for separating a solvent from a solution. We call the liquid collected a distillate.
Evaporation + Condensation = DISTILLATION
The most common set-up for distillation is with the use of a condensor. This set-up condenses the steam even more efficiently. The condenser consists of two tubes, one inside the other. Cool water will pass through the outer tube and steam from the solution will pass through the inner tube.
The water supply enters the condenser at the lower opening, leaving the upper opening to get a better cooling effect.
Separating funnel
Immiscible liquids, such as oil and water can be separated by using a separating funnel. The mixture is put into a separating funnel, shaken and allowed to settle. The oil and water form two separate layer. The liquid with higher density, in this case, water forms the lower layer. Remove the stopper and open the tap after standing. The water runs out through the tap. The oil remains in the funnel and can be run out into another beaker.
Fractional Distillation
When two miscible liquids need to be separated a fractional distillation can be set up, in which a column would be added to the apparatus of simple distillation so that the surface area for condensation would be increased sufficiently as to produce a temperature gradient in the column, so that only the liquid with the lowest boiling point would be obtained while the liquid with the higher boiling point would be condensed back to the round-bottomed flask.
Sublimation
Sublimation is the process by which a solid turns into a gas without first turning into a liquid. This can be used to separate two solids when there is one of the solids that sublime, simply by heating up and condensing the gas on the sides of an inverted filter funnel.
Chromatography
Chromatography is a laboratory technique for the separation of a mixture. The mixture is dissolved in a fluid called the mobile phase, which carries it through a structure holding another material called the stationary phase. The various constituents of the mixture travel at different speeds, causing them to separate.
Solvent Extraction
A reaction mixture can have substrates that are either soluble in water, an organic solvent or both. This means that these substrates can be separated using their solubilities in different immiscible solvents.