(A)  INTRODUCTION - important terms explained for

2.4 Five techniques used in particular separation & purification procedures

FILTRATION,  EVAPORATION,  CRYSTALLISATION, DRYING and DECANTATION procedures

All these terms are explained and the techniques described in detail, and remember,

they often go together in separating certain kinds of mixtures

e.g. separating salt and sand from a 'muddy' sea water mixtures,

or, the preparation of salts by reacting an acid with a base/alkali or using a precipitation reaction.

Filtration, evaporation, crystallisation and drying are four techniques used in the isolation and purification of a solid product from a chemical reaction e.g. How can we separate a mixture of sand and salt? or, how do we separate a salt from a salt preparation?

What do the words solvent, solute, solution, soluble, insoluble and solubility mean?

solvent: the liquid that dissolves something, often water - very useful because it dissolves many things

solute: the substance that dissolves in the solvent

soluble: the property of substance that allows it to dissolve in a solvent

solution: the homogenous mixture of the solute and solvent - the solute has completely dissolved into the solvent

insoluble: the property of substance that stops it dissolving in a solvent, not soluble!

solubility: how soluble a substance is in a solvent (often water) e.g. in terms of mass of solute dissolved in a specific volume of liquid e.g. g/cm³ or g/dm³

Remember, in all the physical separation processes of filtration, evaporation and crystallisation, no chemical reactions are involved, so no new substances are made.

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(B) Filtration uses a filter paper or fine porous ceramic to separate an insoluble solid from a liquid.

The insoluble solid might be an impurity or the product of a reaction.

Filtration works because any tiny dissolved molecule particles OR the liquid molecules are too small to be filtered and pass through the filter paper with the equally tiny water molecules. BUT, any insoluble 'non-dissolved' solid particles are too big to go through and are trapped of 'filtered' out by e.g. a filter paper, but it can be any form of VERY fine mesh.

The filter paper is folded into a cone shape and fitted snugly into the filter funnel.

 Filtration be used to ...

Filtration is an important purification procedure e.g. to isolate the product of a reaction, the product may be insoluble or a crystallised product,

Filtration removes solid impurities or excess solid reactant from a solution e.g. (3) in the right diagram in a salt preparation,

It is used to remove some solids in water for our domestic supply, so you don't get bits of leaves, twigs or grit in your cup of tea!

Insoluble means ,will not dissolve' in the liquid.

Here filtration is used in STEP 3 to separate a desired insoluble compound from the original liquid reaction mixture.

(C) Evaporation means a liquid changing to a gas or vapour.

In a mixture separation procedure, it is the removal of most of the liquid from a solution, usually to cause crystallisation to leave solid crystals in the evaporating dish - but you should not heat some salts to complete dryness - see copper sulfate note.

It can be done quickly with gentle heating or left out to 'dry up' slowly in 'open air'.

The solid will almost certainly be less volatile than the solvent and will remain as a crystalline residue.

Evaporation is often followed by crystallisation (discussion of one, usually involves the other!)

If a solid dissolves its described as soluble (the opposite of insoluble), therefore the only way to recover the solid is to evaporate the solvent, often water.

 

An example application of evaporation: In preparing copper sulfate you can concentrate the solution by boiling-evaporating it in stage 2. Then, after filtering the hot solution into an evaporating dish, in stages 3-4 it can be left to cool and crystallise (further described below). The crystals can be collected from the residual solution (left diagram above).

Alternatively, you can filter the hot dilute solution and then carefully evaporate most of the water to concentrate the solution prior to leaving it to cool and crystallise (right diagram above). You can also use a hot water bath to evaporate a solution to concentrate it. Once the crystals start forming its best to stop the heating and leave the solution to cool and continue to crystallise on its own.

In many salt preparations its important not to completely dry the crystals because some might decompose on further heating. Copper sulfate is an example and you should not evaporate this solution to complete dryness. The lovely blue crystals easily break down to give a dull white-grey powder. This is due to the water of crystallisation being driven off (see Reversible Reactions for more details of this reaction).

If the salt is like sodium chloride, which has no water of crystallisation locked into the crystal structure, its actually ok to evaporate the solution to complete dryness - but leaving a little concentrated solution still makes better bigger crystals.

As a general rule, slow crystallisation from a concentrated solution produces larger higher quality crystals.

You should also appreciate that solubility increases with temperature, so when you cool a hot saturated solution, the solubility decreases, hence the formation of the crystals as the solid comes out of solution.

You can filter sea water to remove any debris and slowly evaporate the water by heating the filtrate in an evaporating dish until salt crystals start forming. This is how 'sea salt' for the dining table is made by evaporating sea water in large metal pans. Salt is also made in hot countries by trapping sea water in pools and letting the sun do all the work to evaporate the water. The salt crystals are then gathered up in a very 'green' energy efficient process!

In a similar experiment you can show that mineral waters (spring waters) also contain dissolved salts e.g. magnesium sulfate is known as 'Epsom Salts' because it was crystallised out of solution by evaporating mineral water from a spring in Epsom in southern England.

(D) Crystallisation

Crystallization can mean a liquid substance changing to its solid form. However, the term usually means what happens when the liquid from a solution has evaporated to a point beyond the solubility limit of the dissolved solid. Then solid crystals will 'grow' out of the solution because the solution is too concentrated for all the solid to remain dissolved at that temperature.

Crystallisation is often done from a hot concentrated solution, because most substance are more soluble the hotter the liquid.

Consequently on cooling a hot concentrated solution, crystals form as the solubility gets less and less and the solubility limit is exceeded, so crystals must form.

You get the best and bigger crystals by not over evaporating the hot solution and allowing the crystals to form as slowly as possible. You can leave a fairly concentrated hot solution to slowly cool and further evaporation takes place to give a good yield of crystals.

Crystals can be filtered from the residual liquid and left to dry, out in the laboratory, in a drying oven or in a desiccator.

Crystallisation is also an important purification method because trace impurities tend to stay in solution. This may due to their solubility limit not being exceeded or the impurity particles will not fit into the regular crystal lattice of the desired crystallising product.

You can do crystallisation experiments with pure salol which melts at 45^oC. Its easily melted in a test tube placed in a large beaker of hot water > 50^oC. You experiment with fast and slow cooling by using a glass rod to place a small amount of salol onto a cold microscope slide and a warm microscope slide respectively. You should find the colder slide cools the salol faster making small crystals. However, you should see larger crystals when the salol crystallises more slowly on a warm slow cooling surface. Incidentally, this experiment illustrates the fast and slow cooling of volcanic lava/magma to form igneous rocks - the small crystals of basalt and the larger crystals of granite.

(E) Drying may be the final stage in the chemical preparation of a product and here in this context it means removing the last traces of liquid (e.g. the solvent) from the solid product (which may be a powder or nice crystals).

There may be traces of solvent in the solid product e.g. water or an organic solvent.

A solid product can be left in a fume cupboard to allow any excess solvent to evaporate.

A solid product can be gently heated in an oven.

A solid product can be dried in a desiccator (large glass vessel with a tight fitting lid), which contains a drying agent like silica gel, which absorbs moisture (water vapour) from the air, which had evaporated from the solid product. It is also a way of keeping a dried product dry! Many chemicals will absorb moisture from the air, which is obviously prevented if they are stored in the dry air environment of a desiccator.

(G) Some detailed examples of using these four physical methods of separation are involved in e.g.

(1) separation of a sand and salt mixture

(2) separation in salt preparations

(1) The sand/salt mixture (it can be impure raw rock salt) is ground up with a pestle and mortar and stirred with water in a beaker to dissolve the salt (1).

You can heat the mixture to speed up the dissolving and increase the salt solubility (2).

The soluble salt dissolves to give a solution and the insoluble sand is filtered off - the sand grains are too large to go through the filter paper (3).

The salt solution (filtrate) is carefully heated in a dish to evaporate some of the water and eventually the salt crystals form from the cooled concentrated solution (4).

You should never evaporate all the water, otherwise the quality of the crystals might be decreased, especially if the crystals contain water of crystallisation.

Here the solvent is water, but other mixtures can be separated using the same sequence of procedures using a different solvent. e.g. copper and sulfur can be separated using an organic solvent like tetrachloromethane which will dissolve the sulfur (hazardous chemical solvent) and you can filter off the copper.

Lawn sand is a mixture of sharps sand and soluble ammonium sulfate. You can separate out the sand and eventually crystallise the ammonium sulfate in exactly the same way as impure salt, described above.

(2a) When the water insoluble base (e.g. a metal oxide) is dissolved in hot acid (1-2), the excess solid base is filtered off (3) and the filtrate solution heated to evaporate the water to produce the salt crystals (4), but stop evaporating when the crystals start forming. (for more details see filtration, evaporation and crystallisation above and making a soluble salt by from an acid with a metal or insoluble base – oxide, hydroxide or carbonate).

(2b) Two solutions of soluble substances are mixed and react to form an insoluble salt. The insoluble salt is filtered off to separate it from the solution, washed with pure water to remove any residual salt solution. The solid is then removed from the filter paper and dried to give the pure dry insoluble salt (pictured below).

The technique of solvent extraction involves using a liquid to dissolve a solid to separate it from a mixture (e.g. in purifying salt in the experiment described above.

AND

in these separation procedures there always loss of product so at a higher level (GCSE/IGCSE/A Level) you need to know about

I'm afraid there is a bit more to it than beakers, filter funnels etc.!!!

A solvent is a liquid that dissolves the substance, in doing so it must completely break the bonds between the particles in the substance being dissolved (the solute). This enables the solute particles to fully mix with the solvent.

The solute is the substance that is being dissolved in a solvent. A solid may dissolve in one liquid (soluble) but not in another (insoluble).

A solution is a mixture of a liquid with something dissolved in it. If you evaporate the solvent away, you should be left with the 'dry' solid solute you dissolve in the first place.

Soluble means the substance (gas, liquid or solid) dissolves in a liquid to form a solution.

Insoluble means a substance won't dissolve in a particular liquid.

Solubility is how much of a substance (the solute) will dissolve in a liquid solvent to give a saturated solution. A saturated solution is one in which no more substance will dissolve in the liquid at a particular temperature.

See alcohols and esters for further discussions of solvents and solutes including the theory of intermolecular forces.

See how these methods are used in making salts methods (a)-(c)

 % reaction yield & reasons for loss of product  *  atom economy  *  % purity of a product