Pure Substances and Mixtures
When we do an experiment, we often end up with a mixture of substances rather than just one. We must know how to separate the mixtures.
A single substance that has no other substances mixing with it is called a PURE SUBSTANCE. If there is something else mixed with it, it is a mixture.
Solid/liquid mixtures
The mixture of dissolved salts in water is an example of a solid/liquid mixture. The mixture is clear and no salt can be seen. We called this sort of mixture a solution. The solid which dissolves, such as salt, is called the solute. The liquid that solute dissolves, such as water, is called the solvent. Solids such as sugar and salt that dissolve are describe as soluble. Solids such as mud and coin which do not dissolve are described as insoluble.
Solute + Solvent = Solution
We often say that a solution that contains a little solute in a given amount of solvent is dilute. We called a solution contains a lot of solute in a given amount of solvent CONCENTRATED.
We called a solution SATURATED when it can dissolved all the solute. Usually cold solvents dissolve less solute than hot solvents.
A saturated solution is a solution which has dissolved all the solute, at a given temperature.
An aqueous is to describe water when it is used as a solvent. Water is the most common solvent, however the are many other solvents which we used in industry and around houses. They are all needed to dissolve substances that cannot be dissolve in water.
Liquid/liquid mixtures
We used to word MISCIBLE when liquids mix together completely in order to form one single liquid. We called those liquids which cannot mix together completely IMMISCIBLE. Oil and water form two separate layers when they are mixed.
The reason which we need to pure substance is because in Chemistry, pure substances are needed to produce drugs or perform experiments in the laboratory. However, most substances obtained from nature are mixtures. Therefore these mixtures need to be separated and purified before we can use them.
Methods of Purification and Separation
Purification and separation of substances are very important techniques in Chemistry. Some of the common methods of purification and separation are explained in the other sections.
Filtration
This is a method which is the most especially effective for separating suspensions, for example mud in 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 called a solid residue. We called the liquid which pass through the FILTRATE.
There are two ways of folding the filter paper for the filtration:
- Fold the paper in half along one diameter then in quarters.
- Fold a fluted filter paper. Fold the paper in half, then open out, after that fold in the same director at a right angles to the original. Fold the paper two more times, the folds being all the same direction and mutually at around 45 degrees. Each section will then individually folded in the opposite direction. As result is a 'FLUTED' which sixteen faces will be produced. It provide a faster rate of filtration.
Crystallization
It is a process of forming crystals. It is also a method for separating dissolved solids from a solution.
Two common techniques of Crystallization are:
- By cooling down a hot concentrated solution.
- Slow evaporation of solution at room temperature.
When a solution of solid in liquid is heated, the liquid will evaporates. The hot vapor that formed can de condensed back to liquid again on a cold surface. We called this method DISTILLATION. Distillation is used for separating a solvent from a solution. We called the liquid collected a distillate.
Evaporation + Condensation = DISTILLATION
The water supply enters the condenser at the lower opening, leaving the upper opening to get a better cooling effect.
Chromatography
In paper chromatography, there are two factors which the movement of each substance in the mixture need to depends on.
- The solubility of the substance in the solvent. The substance moves with the solvent easily if the substance is very soluble in the solvent.
- The adsorption of the substance on the filter paper. Some solids are able to attract other substance strongly and hold them on their surface. This is called ADSORPTION. The substance will not move with the solvent easily if the substance in the mixture is absorbed strongly by the filter paper.
Chromatography can also used to identify different dyes used in food.
Floatation
An improved flotation separation apparatus for separating and classifying diverse, liquid-suspended solids having a plurality of high volume air bubble infusers. A plurality of stationary impinging plates projecting from the interior circumferential wall into the circular cavity and equally spaced circumferentially in series therealong. An injecting stream of water and air impinges upon the impinging plates in series to repeatedly create, divide and subdivide air bubbles as the injection stream transverses the series of impinging plates.
Extraction
Liquid-liquid extraction is a powerful separation technique that falls right behind distillation in the hierarchy of separation methods.
Reasons to use extraction:
Separation not feasible by distillation
Break azeotropes
Energy requirements of distillation are prohibitive
A complex distillation sequence is required
The material is heat sensitive
The material is non-volatile
Here are two videos about separation:
