Can you think of substances that we meet every day that are absolutely pure - that is, they contain only a single substance and contain no impurities? There are very few. The water we drink contains dissolved minerals and gases as impurities. The petrol we put into cars is a mixture of difterent hydrocarbons.
A mixture consists of two or more substances (elements or compounds) that are not chemically combined
together and can be separated.
Separation of mixtures
There are many ways or methods of separating mixtures, and each method depends on:
the type of mixture
the physical properties of the constituents of the mixture like solubility, differences in boiling points or freezing points, etc.
All the methods involved are physical processes, because in each case the components of the mixture are simply separated from each other and are not changed into new substances.
Solid/solid mixtures
Separating a mixture of two solids is not easy unless one of the solids has a different property to the other. For example, a mixture of iron filings and powdered sulphur is difficult to separate until you realise that iron filings are attracted to a magnet and sulphur is not.
Another classic mixture of solids is sand and salt. Here we can use the property that salt will dissolve in water but sand will not.
When we shake a little salt with water, the salt seems to disappear. No solid salt settles to the bottom of the water on standing, but by tasting the water we know that the salt is still there. The salt mixes evenly with the water and the liquid is exactly the same throughout. We say that salt is soluble and dissolves in water. The mixture of salt and water is a solution. The salt is a solute and the water is a solvent.
A solution is a mixture of two or more substances. When a solute dissolved in a solvent, it gives a solution.
A solute is a dissolved substance. For example salts
A solvent is a substance that dissolves a solute. For example the generally known water solvent
So when a mixture of sand and salt is added to water, the salt dissolves but the sand does not.
Decantation
In decantation process, all you need to do is to pour out the liquid remaining the solid in it. Let's say for example you have a stone in a water in a bowl A and you want to separate the stone from the water, what you need to do is to find another bowl B in which you will carefully pour the water only in Bowl A into which will leave you finally with the stone empty in the bowl A
Filtration
In filtration process, you want to separate a undissolved solid in a liquid. I.e recovering the undissolved solid from the liquid. Let's say you have a particles of garri in a water undissolved in a bowl A, all you need to do to separate them is to get a filter funnel, in which you pour the mixture of water and the undissolved garri in it, you will noticed the water will go down remaining the undissolved garri in the funnel, through that means you have been able to recover and separate the two mixtures.
Flotation
Flotation is a process of separating mixtures and involves separating substances by whether they sink or float. An example of this is oil and water; if they are stirred and left to settle, the oil will rise to the surface of the water. Another example is if sawdust and sand are stirred in water, the sawdust floats and the sand sinks, separating the two substances.
Flotation is also used in the mining industry to separate impurities from mineral ores. This process,which uses bubbles to separate materials based on their relative affinity to water, is called froth flotation.
Froth floatation
Metal ores such as cassiterite are dug out of the ground. The mineral containing tin(IV) oxide is mixed with unwanted impurities. Froth floatation is a method of separating the solid mineral from solid impurities. First the ore is crushed into a powder to increase surface area. It is then mixed with water to form a slurry and a chemical called a surfactant is added.
The surfactant varies according to the scp
being carried out. The surfactant attaches itself to the mineral. When air is bubbled through the mixtures, the particles of mineral and surfactat attach to the bubb and collect in the froth on the surface. Scooping off the froth collects a mineral-rich material and the waste remains at the bottom of the tank.
Separation by freezing
Fractional freezing or trostation is a process used to separate substances with different melting points. It can be done by partial melting of a solid, or by partial crystallisation of a liquid. Fractional freezing is generally used to produce ultra-pure solids, or to concentrate heat-sensitive liquids. For example, fractional freezing is also used in the production of fruit juice concentrates and other heat-sensitive liquids, as it does not involve heating the liquid, which could cause evaporation.
Precipitation
Precipitation is the formation of a solid in a solution during a chemical reaction. Precipitation can be used to separate out impurities. For example, a solution of sodium chloride from the sea may contain the salts of magnesium and calcium. The ions can be removed by precipitation in the following reactions:
Calcium chloride + sodium carbonatecalcium
carbonate and sodium chloride
CaCl, (aq) + Na,CO, (aq) > CaC0, (aq) + 2NaCl (aq)
Magnesium chloride + sodium hydroxide
magnesium hydroxide + sodium chloride
MgCl, (aq) + NaOH-> Mg(OH), (s) +2NaCl (aq)
Chromatography
Chromatography is a process used to separate mixtures of substances dissolved in a solvent. Chromatography means colour writing and was a technique developed in the 19th century for separating and identifying coloured dyes. Paper chromatography is a process for separating coloured substances by using porous paper, e.g. filter paper, but not glazed paper. The substances are moved over the paper bya solvent at different rates, forming a chromatogram.
Solids are usually more soluble in a hot solvent than in a cold solvent. When a hot, saturated solution (i.e a solution that contains the maximum dissolved solute at that temperature) is cooled, some solid is formed as cystals. Crystals have a regular shape, fat sides and sharp edges. This process is called crystallisation.
The shape of most cystals is not perfect . They touch other crystals or the sides of a beaker as they form, and they cannot grow properly. A large perfect crystals forms when a small one hangs by a thread in the middle of a saturated solution. As the solvent slowly evaporates, the solute forms on all sides of the crystal.
However, a crystal formed in this way has thread inside it. A second method of growing a large crystal is to place one crystal on the bottom or a beaker containing a cold saturated solution. Turn the crystal every dav so that a different side touches the glass. The crystal grows
larger and is perfect, or almost so.
Fractional crystallisation
Fractional crystallisation is a method of separating mixtures of substances based on differences in solubility. If two or more substances are dissolved in a solvent they will crystallise out of solution at diferent rates. Crystallisation can be brought about by Changing concentration or temperature. In fractional crystalisation the mixture is added to a solvent and heated. It is then gradually cooled so that, as each constituent components crystallises, it can be removed in its pure form from the solution.
References
. https://courses.lumenlearning.com/cheminter/chapter/methods-for-separating-mixtures/
