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Why are the crystals formed 'blue'?

When a solution gets evaporated, why are the crystals 'blue'?

Like for instance, copper(II) sulfate-5-water.
Anhydrous copper sulfate is white and hydrated copper sulfate is blue.

So how come when you remove water (or evaporate) copper(II) sulfate-5-water, the crystals formed are blue and NOT WHITE? :s-smilie:
Original post by Adorable98
When a solution gets evaporated, why are the crystals 'blue'?

Like for instance, copper(II) sulfate-5-water.
Anhydrous copper sulfate is white and hydrated copper sulfate is blue.

So how come when you remove water (or evaporate) copper(II) sulfate-5-water, the crystals formed are blue and NOT WHITE? :s-smilie:


The crystals are hydrated but the excess solution has evaporated.
Original post by Adorable98
When a solution gets evaporated, why are the crystals 'blue'?

Like for instance, copper(II) sulfate-5-water.
Anhydrous copper sulfate is white and hydrated copper sulfate is blue.

So how come when you remove water (or evaporate) copper(II) sulfate-5-water, the crystals formed are blue and NOT WHITE? :s-smilie:


Coz its the Heisenburg method
Reply 3
Original post by Mayhemβ„’
The crystals are hydrated but the excess solution has evaporated.


I see, thanks!:smile:
You haven't removed the water from the crystal structure yet. Do so required baking.
Reply 5
It's been ages since I did this, so i may be wrong but I think its to do with when its hydrated, ionic bonds are formed and through the donation of electrons over bonds, partially filled shells are left. then electrons here can be excited and absorb certain wavelengths of light - resulting in the colour.

EDIT: (so if it absorbs everything but blue light wavelengths, it will appear blue)

in anhydrous there aren't these bonds so it's just white because there are no partially filled shells

same applies to things like precious gems, just depends on what metal is contaminating the crystal.
(edited 8 years ago)
Reply 6
(and the water is left because it hasnt fully evaporated)

if you left the crystal in a drying environment for a period of time they'd go back to white
Original post by luuucyx
(and the water is left because it hasnt fully evaporated)

if you left the crystal in a drying environment for a period of time they'd go back to white


No, the water is held in the crystal lattice as water of crystallisation. It acts as a kind of cement that holds the lattice together. The water coordinates into vacant orbitals of the copper ion.

Each copper ion is then surrounded by water ligands which produce an electrostatic (or ligand) field making the copper 3d electron orbitals non-degenerate.

This allows energy to be absorbed by electrons promoting from lower energy to higher energy 3d orbitals.

This energy is in the visible region of the spectrum meaning that the salt appears to be the complementary colour.

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