Colour of Ions and Precipitate in Chemistry for JEE

Many ionic compounds have a colour. The metal ion present in the compound is responsible for the compound's colour. A transition metal ion is usually responsible for the colour of an ionic compound. PbI₂ is a yellow solid, Al(OH)₃ precipitate and AgCl colour is white. The colour of an ionic compound is frequently different from the colour of an aqueous solution of the constituent ion. The question now arises in our mind about why ions show colour. Compound colours are produced when a substance absorbs certain wavelengths of light while reflecting others. The electrons present in metal ions get excited by accepting the energy and releases its complimentary energy in the form of light to de-excite. Different molecules have various shapes (geometry). Different shapes absorb and reflect various wavelengths. In addition to shapes, temperature and pressure are colour-related parameters. 

Since the 'white colour' is the product, it does not absorb visible light but instead forms a large number of fine crystals that scatter room light. So the 'whiteness' is caused by the light scattering back towards your eyes after it is reflected from the many facets of the many crystals in the ppt. When you use red light to illuminate, the 'white colour' turns red. If the ppt is coloured, light absorbs at some wavelengths while scattering occurs at all wavelengths.

In other words, a small fraction of light is reflected at the surface of a crystal, but with millions of randomly oriented crystals, many reflections occur. The amount of light reflected at each surface depends on the refractive index difference between the crystal and air for the wavelength used.

Precipitate in Chemistry

When two solutions containing soluble salts are combined, a precipitation reaction occurs, resulting in the formation of an insoluble salt. Due to the formation of small aggregations of the insoluble salt, the resulting solution is visibly cloudy. The precipitate is the insoluble salt that falls out of the solution and is filtered. The supernatant is the liquid that remains after a precipitate forms. When the cations from one of the insoluble ionic compounds combine with the anions from the other reactant to form a double-replacement reaction, precipitate forms. Precipitation reaction examples of a double replacement reaction are when potassium iodide and lead (II) nitrate aqueous solutions are mixed, and the following reaction occurs: 

$\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}+2\mathrm{KI} \rightarrow \underset{\text{YELLOW PPT}}{\mathrm{PbI}_{2}}+2\mathrm{KNO}_{3}$

The attraction forces between Pb⁺ and I^- ions are extremely strong, resulting in a yellow precipitate. The reaction's other byproduct, potassium nitrate, is still soluble.

It is important to note that not all ionic compounds react to form precipitates. Furthermore, a precipitate can form under certain conditions. Temperature and pH, for example, can influence whether or not a precipitation reaction occurs. In general, increasing the temperature of a solution increases ionic compound solubility, decreases the likelihood of precipitate formation. The concentration of the reactants is also significant.

Formation of a Precipitate: Example

1. Silver chloride precipitates out of the solution during the chemical reaction between potassium chloride and silver nitrate.

$\mathrm{AgNO}_{3}(a q)+\mathrm{KCl}(a q) \rightarrow ,\underset{\text{white ppt.}}{\mathrm{AgCl}(\mathrm{s})}+\mathrm{KNO}_{3}(a q)$

Ag and K are positive ions while the negative ions NO₃^- and Cl^- displace the position to form an insoluble solid white precipitate of AgCl and a soluble aqueous solution KNO₃. The white colour of AgCl is because silver ion ([Kr]4d¹⁰) don’t have any unpaired electrons which can impart colour by allowed electronic transition.

2. Take two aqueous sodium sulphate and barium chloride solutions. The precipitate reaction between aqueous  Na₂SO₄ and BaCl₂ will be: 

$\mathrm{Na}_{2} \mathrm{SO}_{4}(\mathrm{aq})+\mathrm{BaCl}_{2}(\mathrm{aq}) \rightarrow \mathrm{BaSO}_{4}+2 \mathrm{NaCl}(\mathrm{aq})$

Where Na and Ba are positive ions while negative ions SO₄^2- and Cl^- displace the position to form an insoluble solid precipitate of BaSO₄ and the soluble aqueous solution of NaCl. 

Properties of a Precipitation Reaction 

Precipitates are crystalline ionic solids. They can be colourless or colourful, depending upon the species involved in the reaction. Colored precipitates are most common when transition metals, including rare earth elements, are present. In aqueous solutions or media, the precipitation reaction occurs in an ionic state. A reaction between ions in aqueous solutions produces the product. In aqueous solutions, the precipitates formed at the end of the precipitation reaction are insoluble. Temperature, solution concentration, buffer solution and other factors all have an impact on these processes.

Uses of a Precipitation Reaction

One application of precipitation reaction is to determine whether or not a specific element is present in a solution. Among the applications of precipitation reaction, the first is in wastewater treatment. A chemical is added to wastewater so that pollutants and impurities (such as lead) precipitate out of the water, which is then sent to be purified further. This process can be used in metallurgy where oxalic acid is mixed with seawater and brine water to extract calcium and magnesium as precipitates.

Conclusion

There are a number of ionic compounds that possess a colour. For example, PbI₂ is a yellow solid and hydrated copper sulphate is blue in colour. Generally, the metal ion present in the compound is responsible for the compound's colour. A transition metal’s compounds are generally coloured compounds due to variable oxidation state or unpaired electrons in d orbitals. When a solution containing a specific cation (a positively charged ion) is mixed with another solution containing a specific anion, the formation of an insoluble compound can occur (a negatively charged ion). This insoluble product is called precipitation. A precipitate is the solid that separates.