Filtration and crystallisation
Separation techniques
The production of a chemical does not necessarily produce a A substance that consists of only one element or only one compound. sample of the chemical. It may contain A substance, usually unwanted, that is present in another substance. or start as a Two or more substances that are not joined together. The substances can be elements, compounds, or both. of substances. Separation techniques are used to separate the useful product from any impurities or Something that is produced as a waste product..
Separation techniques work by using differences in the A description of the appearance of a substance or how it acts without involving chemical reactions. For example, state, melting point, conductivity, etc. of substances in the mixture:
- an Unable to dissolve in a particular solvent. For example, sand is insoluble in water. substance in the solid state may be separated from a substance in the liquid state by Method used to separate an insoluble solid from a liquid using a physical barrier such as paper.
- liquids with different The temperature at which a substance rapidly changes from a liquid to a gas. may be separated by A separation technique which involves a solution being heated so that the solvent evaporates before being cooled to form a pure liquid.
Separation processes are not always completely successful the first time. Repeated processes are sometimes needed to achieve acceptable purity.
Learn more on filtration, evaporation and crystallisation in this podcast.
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Filtration
Filtration is used to separate an insoluble solid from a liquid. It is useful for separating sand from a mixture of sand and water, or In chemistry, a substance is in excess if there is more than enough of it to react with another reactant. solid A substance that reacts together with another substance to form products during a chemical reaction. from a Mixture formed by a solute and a solvent..
Filtration works because the filter paper has tiny holes or pores in it. These are large enough to let simple A collection of two or more atoms held together by chemical bonds. and A substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution. Electrically charged particle, formed when an atom or molecule gains or loses electrons. through, but not the much larger A general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. of undissolved solid.
Image caption, 1. One beaker contains a mixture of solid and liquid, the other contains a funnel with filter paper
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Crystallisation
The process of producing crystals from a solution by evaporating the solvent. is used to produce solid A solid containing particles (atoms, molecules or ions) joined together to form a regular arrangement or repeating pattern. from a solution. When the solution is warmed, the The liquid in which the solute dissolves to form a solution. evaporates leaving crystals behind. For example, crystallisation is used to obtain copper sulfate crystals from copper sulfate solution.
Image caption, 1. A solution is placed in an evaporating basin and heated with a Bunsen burner
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To obtain large, regularly shaped crystals:
- put the solution in an evaporating basin
- warm the solution by placing the evaporating basin over a boiling water bath
- stop heating before all the solvent has evaporated
The evaporating basin may then be left, allowing the crystals to grow.