What is a Precipitate?
A precipitate is a solid that is formed from a solution, either by the crystallization of a dissolved substance or by the formation of an insoluble substance through a chemical reaction.
The first case occurs when a solid substance, which was initially dissolved in a solvent such as water, becomes less soluble. This can happen when:
- The temperature of the solution is lowered.
- Some precipitating agent is added.
- Another solvent is added in which the solid is less soluble.
When its solubility becomes less than its concentration, the solution becomes supersaturated and a precipitation reaction occurs.
The second case occurs after any chemical reaction that generates an insoluble or very poorly soluble product in the solvent. Once this product begins to form, the solution quickly becomes saturated and the solid begins to precipitate.
The precipitation reaction is the chemical process that leads to the formation of a precipitate. Depending on how the precipitate is formed, the reaction can be written in two different ways:
- If the precipitation occurs because the solubility of the solid in the solvent was decreased (by cooling or adding another solvent, for example), then the precipitation reaction only involves the solute and would look like this:
For example , if sodium acetate (CH- dissolved 3 COONa) in hot water and then allowed to cool, the precipitated sodium acetate according to the following equation:
- If precipitation occurs because a precipitating agent was added, then the precipitation reaction will involve both the solute and the precipitating agent.
For example , silver ions (Ag + ) can be precipitated from a solution of silver nitrate (AgNO 3 ) by adding a solution of sodium chloride or NaCl (precipitating agent). The precipitation reaction in this case is given by:
Mechanism of precipitation
Regardless of the precipitation reaction, the formation of the precipitate always occurs in two stages called nucleation and growth:
At the beginning of the precipitation reaction, small particles of precipitate are formed, which are called nuclei, which is why this stage is called nucleation.
After nucleation, new particles are no longer formed, but the nuclei already formed begin to grow until the precipitation stops.
Types of precipitate
The rate at which precipitates form affects the solid’s particle size and properties. Depending on the size of the particles and the nature of the compound that is precipitating, three types of precipitates can be distinguished:
It is the simplest type of precipitate. In this case, the precipitate consists of large solid particles with a diameter greater than 0.1 microns.
Crystalline precipitates are formed when the precipitation reaction occurs slowly. This allows few solid particles to form during nucleation, and each of these particles has enough time to grow and form large, heavy crystals that fall to the bottom of the solution.
These particles are almost always crystalline solids with flat, brilliant faces, similar to the facets of a diamond, and can almost always be seen with the naked eye.
They are precipitates made up of much smaller particles, between 0.01 and 0.1 microns in diameter. This type of precipitates is formed when the precipitation reaction occurs very quickly, generating many particles during nucleation which do not have time to grow.
Due to their size, these particles are very light and remain suspended in the solution, forming a stable colloid. For this reason, they do not settle to the bottom of the solution and the solution appears cloudy rather than clear.
In addition to this, the particles are so small that they make it through most of the filters used to separate precipitates from solutions.
When the particles are even smaller than those of the caseous precipitates, that is to say with diameters less than 0.01 microns, the precipitates are called gelatinous. The reason is because the solid takes on an appearance similar to jelly or jam.
These precipitates are also considered colloids. In fact, gelatin is a typical example of a colloid, and these precipitates take their name from it.
Characteristic examples of gelatinous precipitates are the hydrated oxides of some metals. These oxides absorb a large amount of water on their surface which prevents it from forming larger particles that crystallize.
Examples of Precipitate
Sodium acetate (CH 3 COONa)
As mentioned above, sodium acetate can be precipitated from a hot saturated solution which is allowed to cool. If allowed to cool slowly, a crystalline precipitate forms in which long white needle-like crystals are observed.
Cadmium (II) Sulfide (CdS)
Cadmium sulfide is a highly insoluble salt that is formed immediately when cadmium (II) ions are mixed in a solution with another containing sulfide ions.
Copper (II) sulfate pentahydrate ( CuSO 4 .5H 2 O)
When dissolved in water, copper (II) sulfate forms deep blue hydrated ions. If the water evaporates slowly, beautiful blue crystals of CuSO 4 .5H 2 O precipitate . These crystals look like precious stones.
Silver bromide (AgBr)
This is an example of a compound that precipitates very quickly, forming a precipitate with very fine particles that are difficult to filter.
Hydrated aluminum (III) oxide (Al 2 O 3 .nH 2 O)
This is a classic example of the formation of a gelatinous precipitate. It is formed when a solution of aluminum nitrate (Al (NO 3 ) 3 ) is made alkaline with sodium hydroxide (NaOH ).
Manganese (II) sulfide (MnS)
Many sulfides are insoluble in water, and manganese sulfide is no exception. This precipitates in the form of a very fine powder.
Silver chloride (AgCl)
Silver (I) ions form very poorly soluble salts with all halogens. The case of silver chloride is a classic example of the formation of precipitates in the laboratory.
Calcium carbonate (CaCO 3 )
Calcium is one of the ions responsible for the hardness of water. When water containing calcium is heated, it reacts to form calcium carbonate that precipitates in the form of a white layer called scale, which is even capable of clogging pipes.
Hydrated iron (III) oxide (Fe 2 O 3 .nH 2 O)
This is another example of a gelatinous precipitate that is formed by the reaction of a solution containing iron (III) ions with sodium or potassium hydroxide.
Magnesium hydroxide (Mg (OH) 2)
Magnesium is the other cation responsible for water hardness, as it can precipitate as magnesium hydroxide in pipes and other water heating systems.
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