A solid substance that has been separated from a liquid in a chemical process is called a precipitate. That process is defined as precipitating. Precipitates can be coloured or not. Forming precipitates and colours of precipitates are used to identify anions, cations, and compounds.
Therefore, CaCO3 and BaSO4 are called as precipitates and both of them are are white colour precipitate. they .
In this tutorial we cover following lessons.
If there is a precipitate in the aqueous solution, we can see it easily. Because precipitate is in the solid phase and deposited at bottom of the solution after kept it sometime to settle down.
Colours of precipitates help to identify compounds. We can decide which ions (cations or anions) are in the compound by comparing colours of different precipitates.
Example: Compare AgCl and AgBr precipitates. Ag+ ion is common in both compounds. But their colours are different due to different halide ions. AgCl is a white precipitate and AgBr is a light yellow precipitate.
Now we are going to list all precipitates according to the s block, p block and d block and colours.
Alkaline earth metals forms both precipitates and solutions.
Most of 3d block metals precipitates show colours. Therefore we can use their precipitates colours to identify 3d metal ions. Below, we are listing precipitates of 3d metals with their colours according to the anion.
Chromium hydroxide ( Cr(OH)3 ) is dissolved when excess NaOH is added and give green colour solution. Cr(OH)3 is an amphoteric compound.
Most of the 3d metal sulfide are not soluble and they have different colours too.
Carbonate, sulfate, sulphite, phosphate, sulfide, chloride, bromide, iodide and more anions form precipitates with some metal ions. Now we consider about those precipitate of anions and those precipitates colours.
Solubility of carbonates have a variation because there are soluble and insoluble carbonates. But when we study deeply about solubility of metal carbonates, most of the carbonates are insoluble in water.
From alkali metals, only lithium forms insoluble carbonate. Li2CO3 is a white solid precipitate compound.
All alkaline earth metals forms insoluble carbonate.
Now we list precipitates of carbonate ion with their colours.
Most compounds of sulfate ion ( SO42- ) are soluble in water. Some precipitates and their colours are listed below. Sulfate precipitates are not soluble in dilute acids.
BaSO4 - barium sulfate - white
SrSO4 - strontium sulfate - white
CaSO4 precipitates in higher concentrations and it is also a white precipitate.
Precipitates of sulfite ions are dissolved in dilute acids.
Silver phosphate ( Ag3PO4 ) is yellow precipitate and dissolve in dilute nitric acid and ammonia.
Barium phosphate ( Ba3(PO4)2 ), the white precipitate dissolve in HCl.
Ferric phosphate - FePO4 - yellow precipitate - dissolve in HNO3 and not dissolve in CH3COOH.Identify phosphate ion in compounds - qualitative analysis
Chloride, bromide, iodide ions form precipitates with Ag+ and Pb2+ ions.
ksp value give a very clear clarification about precipitation
According to the ksp value, we can decide whether a solid will be precipitated or not.
But, here we don't focus on ksp value. We mainly present you a list of precipitates and their colours.
Accoring to the anion or cation, colour can be vary.
Chloride ion (anion) is common for both compounds. But their colours are different.
Lets consider lead +2 ion. PbI2 is yellow colour precipitate and PbCl2 is white precipitate. So when anion vary, color of preciptate also vary.
All nitrate compounds are soluble in water. That means, there are no precipitates of nitrate compounds. Also this is same for nitrous ion.
Nitrate ion: NO3-
Nitrous ion: NO2-
Question of precipitates and colours
Most metal hydroxides are insoluble except alkali metals and Sr, Ba.
Magnesium, calcium, zinc, lead, aluminium ions will give white precipitates with OH- ions.
Assume that, there are two compounds which are not soluble in water and have same colour. Then we have to find a another method to separate those kind of compounds.
From s block elements, some lithium compounds form precipitates. Also sodium biocarbonate has low solubility in water.
All carboates of s block group 2 elements are precipitates.
Sulfates of Ba2+ and Sr2+ are precipitates. Hydroxide of Mg2+ is a precipitate.
Cannot. Some precipitates have same colour. As an example, both SrSO4 and BaSO4 are white precipitates. Therefore, we cannot identify or separate SrSO4 and BaSO4 by considering only colour.
Aluminium carbonate is not a stable compound and does not exist.
White. SrSO4 and BaSO4 are white precipitates.
AgI, PbI2 are yellow and AgBr is yellow light. CdS and SnS2 are also yellow precipitates.
Solubility of NaHCO3 is low. This is used in sodium carbonate production. NaHCO3 is heated to take Na2CO3.
Manganese hydroxide ( Mn(OH)2) is a pink colour precipitate.
White cream precipitate.
In the laboratory, we can produce precipitates such as CaCO3 easily. Send CO2 gas to the Ca(OH)2 aqueous solution. Then we can see, a white precipiates forms at the bottom of the solution.
Lithium form some precipitates than other alkali metals. Lithium carbonate, lithium fluoride and lithium phosphate are precipitates.
Precipitates are in many colors. Alkali earth metal precipitaes are white while 3d metal precipitates are have so many colours.
Some metal sulfides are yellow colour. So adding some cations to aqueous sulfide ion solution, you can see yellow colour precipitates.
BaCrO4 is a yellow precipitate.
Precipitates have different colours.
According to the anion, colour of precipitate can be varied at sometimes. As an example, PbCl2 is a white precipitate and PbI2 is a yellow precipitate.