Colour of Salts and Their Solutions: In chemistry, salt is defined as a chemical compound that consists of ions, i.e., a positively charged cation and a negatively charged anion gathered, which results in a compound with no net electric charge. A common example of salt is table salt \(\left({{\text{NaCl}}}\right),\) a molecule of which has a positively charged sodium ion \(\left({{\text{N}}{{\text{a}}^ + }} \right)\) and a negatively charged chloride ion \(\left({{\text{C}}{{\text{l}}^ – }} \right).\)

Among different elements of the periodic table, most of the transition elements form-coloured salts due to the presence of unpaired electrons in the d-orbital – for example, iron, copper, manganese, etc., form-coloured salts. Mostly, pure salt is pure white or even translucent; the colours they gain are from natural elements integrated into the salt crystals as they are produced. Salts formed from transition elements exhibit colour as these electrons get excited and jump to a higher energy level when they receive light energy; after exciting, they fall back in the region of the visible light spectrum, which gives respective colour to that salt.

Colour of Ions of Salts

The colour of salts is exhibited due to the ions present in them. For example, salts of copper are of different colours when copper ions combine with different anions. The salt of copper (II) carbonate is green, the salt of copper (II) oxide is black, and the salt of copper sulphate is blue. Salt of other metal ions like the salt of sodium, calcium, magnesium, aluminium, zinc, lead, ammonium are white in solid-state and colourless in aqueous solution. Similarly, Salt of chloride, sulphate, nitrate, carbonate ions is white in solid-state and colourless in aqueous solution. Salt of iron such as iron sulphate, iron nitrate, and iron (III) chloride is green in both solid and aqueous solutions. Let us discuss the colour exhibited by different salts one by one.

Colour of Aluminium Salts

Aluminium is a metal having atomic number \(13\) and very little density. Some salts of aluminium and their colours are given below:

Aluminium salt	Colour
Aluminium Sulphate \(\left({{\text{A}}{{\text{l}}_2}{{\left({{\text{S}}{{\text{O}}_4}} \right)}_3}} \right)\)	White crystals
Potassium Aluminium Sulphate \(\left({{{\text{K}}_2}{\text{S}}{{\text{O}}_4}{\text{.A}}{{\text{l}}_2}{{\left({{\text{S}}{{\text{O}}_4}} \right)}_3}.24{{\text{H}}_2}{\text{O}}} \right)\)	White crystals
Aluminium Ammonium Sulphate \(\left({{\text{AlN}}{{\text{H}}_4}{{\left({{\text{S}}{{\text{O}}_4}} \right)}_2}.12{{\text{H}}_2}{\text{O}}} \right)\)	White crystalline powder
Sodium aluminium fluosilicate \(\left({{\text{N}}{{\text{a}}_5}{\text{Al}}{{\left({{\text{Si}}{{\text{F}}_6}} \right)}_4}} \right)\)	White powder
Sodium fluoro aluminate \(\left({{\text{N}}{{\text{a}}_3}{\text{Al}}{{\text{F}}_6}} \right)\)	White solid or powder

Colour of Copper Salts

Copper salts are mostly blue. Copper sulphate is blue in colour due to the light energy being used to promote or excite electrons that are in the atom of the copper when it is combined with other ions, such as the sulphate or carbonate ions and so on.

1. Copper (II) sulphate, also known as copper sulphate, is the inorganic compound with the chemical formula \({\text{CuS}}{{\text{O}}_4}{\left({{{\text{H}}_2}{\text{O}}} \right)_{\text{x}}}\) where \({\text{x}}\) ranges from \(0\) to \(5.\) Among all, the pentahydrate form \(\left({{\text{x}} = 5}\right)\) is the most common. The pentahydrate \(\left({{\text{CuS}}{{\text{O}}_4}.5{{\text{H}}_2}{\text{O}}} \right),\) the most encountered salt, is bright blue.
2. Copper (I) chloride, also known as Cuprous chloride having the chemical formula \({\text{CuCl,}}\) is a white salt sparingly soluble in water and completely soluble in concentrated hydrochloric acid.
3. Copper (II) nitrate is also blue, and copper (II) carbonate is green.

Colour of Manganese Salts

1. Manganese (II) sulphate has the chemical formula \({\text{MnS}}{{\text{O}}_4}.2{{\text{H}}_2}{\text{O}}\) exists as a pale pink deliquescent solid and is a commercially significant manganese (II) salt.
2. Manganese (II) chloride is a pale pink salt of manganese.

Colour of Salts and Their Aqueous Solution

We have learned about different colours of salts, and when these inorganic salts are dissolved in water, they mostly result in the same-coloured solutions.

Colour of Copper Salts and Their Solutions

Copper compounds exhibit different colours in different solutions like:

1. Copper oxides are black.
2. Copper hydroxides are mostly pale blue and gelatinous in nature.
3. Copper sulphate crystals are bright blue compounds.
4. Copper carbonate is a bluish-green powdery solid.
5. Copper chloride is commonly present as a light green solid salt.

Colour of Iron Salts and Their Solutions

1. The ferrous sulphate crystals are light green which on heating, the colour changes from light green to white, and on further heating, the white substance turns to dark brown solid. Ferrous sulphate crystals also contain water molecules and are also known as ‘Green vitriol’, having the chemical formula \({\text{FeS}}{{\text{O}}_4}.7{{\text{H}}_2}{\text{O}}{\text{.}}\)
2. Ferric chloride (Iron (III) chloride) or \({\text{FeC}}{{\text{l}}_3}\) is a salt having orange to brown, black colour. It is a soluble inorganic compound in water and forms a yellowish-brown solution.

Colour of Other Salts and Their Solutions

Potassium dichromate is a potassium salt with having the chemical formula \({{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}\) appear as crystals of red to orange colour. The colour of this salt is due to the electron exchange; the orbital character of the electron is moved from the lone pair of oxygen.
Nickel (II) sulphate, \(\left({{\text{NiS}}{{\text{O}}_4}} \right)\) or simply nickel sulphate usually refers to the inorganic salt of green crystals. This is highly soluble blue-green coloured salt and is a common source of the \({\text{N}}{{\text{i}}^{2 + }}\) ion for electroplating.

Uses of Coloured Salts

The colour test of certain inorganic salts is used in the identification of the cations present in them. For example:

1. Blue colour or pale green, as well as the deep green colour of unknown inorganic salt, can be represented as containing copper cation.
2. The green colour is also used for the reference or test of nickel cation.
3. The brown or yellow colour of inorganic salt is characteristic of iron.
4. The deep blue colour of salt mostly represents cobalt ions present in it.
5. Pale pink colour represents manganese ion.
6. Purple or dark green salt represents chromium cation in the given unknown inorganic salt.

Summary

Salts are coloured due to the ions present in them. Copper salts were of different colours when combined with different anions. Like, salt of copper (II) carbonate is green, salt of copper sulphate is blue, etc. Thus, to determine the cation or anion present in salt, its characteristic colour is used for its identification. Salts of other metal ions like the salt of sodium, calcium, magnesium, aluminium, zinc, lead, ammonium are white in solid-state and colourless in an aqueous solution. Whereas salt of chloride, sulphate, nitrate, carbonate ions are white in solid-state and colourless in aqueous solution.

The ferrous sulphate crystals, also known as ‘Green vitriol,’ have the chemical formula \({\text{FeS}}{{\text{O}}_4},7{{\text{H}}_2}{\text{O}}\) is light green, which on heating, the colour changes from light green to white, and on further heating, and Ferric chloride (Iron (III) chloride) is a salt having orange to brown, black colour. Potassium dichromate is a potassium salt with having the chemical formula \({{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}\) appear as crystals of red to orange colour. Nickel (II) sulphate, \(\left( {{\text{NiS}}{{\text{O}}_4}} \right),\) usually refers to the inorganic salt of green crystals.

Among different elements of the periodic table, most of the transition elements form-coloured salts due to the presence of unpaired electrons in the d-orbital. For example, iron, copper, manganese, etc., form-coloured salts. Mostly, pure salt is pure white or even translucent; the colours they gain are from elements integrated into the salt crystals as they are produced. Colours are exhibited by salts formed from transition elements colour as these electrons get excited and jump to a higher energy level when they receive light energy after exciting, they fall back in the region of the visible light spectrum, which gives respective colour to that salt. Thus, mostly transition metals form-coloured salts due to partially occupied d-orbitals.

FAQs

Q.1. What are the colours of different salts?
Ans: The ferrous sulphate crystals, also known as ‘Green vitriol,’ have the chemical formula \({\text{FeS}}{{\text{O}}_4},7{{\text{H}}_2}{\text{O}}\) is light green, which on heating, the colour changes from light green to white, and on further heating, and Ferric chloride (Iron (III) chloride) is a salt having orange to brown, black colour. Potassium dichromate is a potassium salt with having the chemical formula \({{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}\) appear as crystals of red to orange colour. Nickel (II) sulphate, \(\left( {{\text{NiS}}{{\text{O}}_4}} \right),\) usually refers to the inorganic salt of green crystals.

Q.2. What is the colour of most of the aluminium salts?
Ans: Most of the aluminium salts, such as Aluminium Sulphate, Potassium Aluminium Sulphate, Potassium Ammonium Sulphate, Sodium aluminium fluosilicate, etc., are white in colour and mostly form colourless aqueous solutions.

Q.3. Among copper sulphate and ferrous sulphate, which one of the following salts is blue in colour?
Ans: Copper (II) sulphate, also known as copper sulphate, is the inorganic compound with the chemical formula \({\text{CuS}}{{\text{O}}_4}{\left({{{\text{H}}_2}{\text{O}}} \right)_{\text{x}}}\) where \({\text{x}}\) ranges from \(0\) to \(5.\) Among all, the pentahydrate form \(\left({{\text{x}} = 5} \right)\) is the most common. The pentahydrate \(\left({{\text{CuS}}{{\text{O}}_4}.5{{\text{H}}_2}{\text{O}}} \right),\) the most encountered salt, is bright blue, is also known as blue vitriol. On the other hand, ferrous sulphate is light green in colour.

Q.4. The colour of an aqueous solution of a salt is deep green; what is the basic radical of the salt among calcium, cobalt, manganese, and copper?
Ans: The basic radical of the salt having a green coloured aqueous solution is copper. As copper compounds possess different colours in different solutions, like copper oxides, are black, copper hydroxides are mostly pale blue and gelatinous in nature, copper sulphate is a crystalline bright blue compound, but copper chloride is commonly present as a light green salt.

Q.5. Is the cation or an anion responsible for solution colour?
Ans: Salt is a combination of a cation and an anion. Either the anion or the cation can be responsible for the colour of the solution. Some transition metals produce coloured cations, but there are many coloured anions, such as chromate and permanganate ions, that form coloured salts.

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