s,p, d Metal Cation Colors in Aqueous State, Compounds, Flame Test

Some metals show different colors when they form different cations. As an example, copper metal can make two ions as +1 and +2. Most of aqueous copper +2 solutions are blue color and copper +1 solutions are colorless. Also, aqueous solutions of some metal cations like sodium, potassium are always colorless. There are reasons to show colors by metal cations and they are explained in detail in this tutorial.

Metal cation compounds can exist in an aqueous solutions or precipitates. Sometimes according to the state of the compound, color of compound can be changed. As an example, Solid CuCl₂ is yellow color and aqueous CuCl₂ is blue colour.

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Metal cations

Metals form cations when they form compounds such as NaCl, Fe(NO₃)₃, PbCl₂ etc. Different metals form different positive charges (when they form cations). Also some same metal can form different cations such as Fe²⁺ and Fe³⁺.

In this tutorial, we will look what are the colors of these cations shown in different states and different experiments. As an example, most of the aqueous sodium compounds are colorless while sodium +1 ion gives yellow color to the flame test.

Colors of s block cations

In the s block there are two groups; group 1 and group 2.

• Group 1 cations: Group 1 (alkali metals) elements form only +1 ion.
• Group 2 cations: Group 2 (alkaline earth metals) elements form only +2 ion.

Aqueous state

In most times, when compounds of group 1 metal cations are soluble in water, they form colorless solutions . As examples, aqueous NaCl, NaNO₃, KNO₃ are colorless solutions.

When alkaline earth metals cations form compounds which are soluble in water, those are colorless solutions too. As examples, CaNO₃, MgCl₂, Ba(OH)₂ are colorless solutions.

If you want to realize this, take some salt from your kitchen and put them in water. Can you see a difference in color of water?

Flame test of s block cations

Flame test is a good way to identify s block cations. In inorganic qualitative analysis, after first five groups, flame test is used to identify alkali metals.

Flame test colors of alkali metal cations

• Lithium - red
• Sodium - yellow
• Potassium - purple
• Rubidium - red-violet
• Caesium - purple-blue

Flame test colors of alkali metal cations

• Beryllium - white
• Magnesium - none
• Calcium - brick red
• Strontium - crimson to scarlet
• Barium - pale/apple green

s block metal compounds in solid state

Most compounds of s block compounds are white when they are in the solid state.

Examples: Na₂CO₃, CaCO₃, NaCl, NaNO₃, BaSO₄

p block metal cations

As s block cations compounds, soluble compounds (in water) of p block cations are colorless. Some examples are AlCl₃, Al(NO₃)₃, Pb(NO₃)₂ etc.

But, lead +2 ion form colorful precipitates with halide ions.

• PbCl₂: white
• PbBr₂: cream
• PbI₂: yellow

Flame test colors of p block metal cations'

• Aluminum: Silver-white, in very high temperatures such as an electric arc, light blue
• Lead(II): Blue/white
• Tin(II): Blue-white

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d block metal cation colors

Here, we see the beauty of chemistry. Most of these d block cations give different colors in different states, aqueous state, solid compound. When we study colors of d block cations, following are important to know.

• Most of d block metals can form different cations. As examples, iron can form +2 and +3 cations.
• Also, d block metals can show different oxidation numbers. Manganese has +2, +3, +4, +6, +7 oxidation states. According to the oxidation state, color of compound can be changed.

Flame test colors of 3d metal cations

• Chromium(III): Silver-white
• Manganese(II): Yellowish green
• Iron(II): Gold, when very hot such as an electric arc, bright blue, or green turning to orange-brown
• Iron(III): Orange-brown
• Cobalt(II): Silver-white
• Nickel(II): Silver-white (sometimes reported as colorless)
• Copper(I): blue
• Copper(II): green
• Zinc(II): Colorless (sometimes reported as bluish-green)

Now, we are going to learn cations of 3d metals and their colors when they form compounds and complexes.

Cations of iron

Iron forms two cations as +2 or +3 according to the situation. With that, color of the compound is different.

Iron +2 cation

Most of the iron +2 compounds are green when they exists as solid. Also, when iron (II) cation form precipitates like iron(II) hydroxide, iron(II) carbonate, they are green color. If iron(II) cation form aqueous solutions like iron(II) nitrate, iron(II) sulfate, they become green color or colorless according to the concentration.

Iron +3 cation

Also, when iron (III) cation form precipitates like iron(III) hydroxide, iron(III) carbonate, they are brown color. Also, when iron +3 cation form aqueous solutions, such as iron(III) nitrate, iron(III) sulfate, they are yellow brown.

Zinc

Zinc only form +2 cation. Most of It's aqueous solutions are colorless and solid compounds are white. When zinc sulfide is heated, it turns to yellow. When it cools back, again it becomes white.

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