Why Blood Comes in Different Colours: The Chemistry Behind the Spooky Science
- Kimberley Ross
- Nov 2
- 5 min read
Advanced Higher Chemists, this one's for you! Since it's Halloween season let's dive into one of those 'brilliant' SQA exam"using your knowledge of chemistry" questions that asks why different animals have different colours of blood.
I actually quite enjoy these questions, even though I know a lot of you probably hate them because they seem a bit weird at first. You're probably thinking "how the heck am I meant to know why a human's blood isn't the same colour as a spider's blood?"
But...when you look at the structures in that past paper question, you should notice that all of these are transition metal complexes. That's your big clue! This question is really asking you to explain the colour of transition metal complexes in the context of these different bloods.
The Basics: d-d Electron Transitions
So let's get into the relevant Advanced Higher level chemistry. When it comes to transition metal complexes, the colour is always caused by d-d electron transitions. Essentially, this is where an electron gets promoted from one d orbital to another one.
When energy is absorbed by a substance, electrons are promoted, and the amount of energy absorbed for a d to d transition corresponds to a wavelength within the visible region. However - and this is the crucial bit - it's not the wavelength that's absorbed that is seen, it's actually the complementary colour that is observed or seen i.e. the wavelength being transmitted.
Think about it this way: you don't see the colour that's absorbed, you see all the colours of light that are NOT absorbed. If the colour has been absorbed, it's like it's been removed from the visible light and you can't see it.
Your Colour Wheel is Your Best Friend
This is where your colour wheel becomes absolutely essential.
If a substance absorbs purple light, then we see it as being green. So all those green trees and leaves you see in the summer? They're actually absorbing purple light! In the context of this exam question, we can then say that the leech's blood, that appears green, is absorbing purple wavelengths of light.
Because the spider's blood is appearing blue, that means it's absorbing yellow wavelengths of light.
For human blood that appears red, it's absorbing blue-green light.
Energy Gaps and Wavelengths
The colour of the blood can actually tell you how big that energy gap for the electron promotion must be. This is where it gets a bit more complex, but stay with me.
If you have a shorter wavelength if visible light being observed, then it means a longer wavelength of visible light has been absorbed. We know this as wavelength and energy are inversely proportional i.e. when energy is high, wavelength is short and vice versa. That means if you're absorbing a longer wavelength of light, your energy gap for electron promotion must be smaller. If you've got a large energy gap, it means you're going to absorb a shorter wavelength, and then you'll observe a longer wavelength of visible light.
All of these bloods have different colours, which means they all must have different sized energy gaps. But why are their energy gaps different?
Why Do the Energy Gaps Differ?
Here's the key: this energy gap actually only appears when the d orbitals get split. The d orbitals are usually all degenerate (equal in energy), but certain things cause them to split in energy into this 2-3 formation.
It is possible to get different degrees of d-orbital splitting, so you'll get different energy gaps, which then gives you different colours.
The splitting happens as a result of the repulsion of the d-electrons around the central metal ion. If any negative charge comes towards the central metal ion, the d-electrons are repelled and this is what causes the splitting of their energies.
What Affects the Splitting?
Several factors influence how much the d-orbitals split:
1. The type of ligand attached to the complex
The spectrochemical series (below) can help you work out the degree of splitting that different ligands cause. Strong field ligands are the ones that cause more splitting, but weak field ligands cause less splitting.
2. The oxidation state of the metal
Generally, higher oxidation states cause more splitting. So Fe³⁺ would cause more splitting than Fe²⁺, for example.
3. The geometry of the complex
At Advanced Higher, you wouldn't be expected to know which geometries cause more splitting than others, but the geometry is really related to the coordination numbers. So even talking about the different coordination numbers of the complexes in these blood molecules would be additional chemistry you could discuss.
Why Can the Same Metal Give Different Colours?
Notice that both haemoglobin (red) and chlorocruorin (green) contain Fe²⁺ ions. This demonstrates something really important: the colour doesn't just depend on the metal ion, it critically depends on the coordination environment and the ligands attached to it.
Even a small modification to the ligand structure can change the energy gap enough to change the wavelength of visible light beeing absorbed and therefore change the observed colour from red to green.
One More Thing: Conjugated Systems
If you've already done organic chemistry, you might have noticed that these ligands have conjugated systems. This means the ligands could be contributing to the colour as well through π-electron delocalisation. These extended conjugated systems can affect the electronic transitions and the overall colour observed. But this blog is already getting a bit long so we will leave that for another day!
So why do these bloods have different colours?
Just all focusing on the central transition metal ion and how it's causing the colour, we can take away the following key points to discuss in our open ended answer:
Colour comes from d-d electron transitions
You see the complementary colour to what's absorbed
The energy gap depends on the degree of d-orbital splitting
Splitting is affected by ligands, oxidation state, and co-ordination number (geometry)
Even the same metal can give different colours depending on its environment
So I hope that helps inspire you for your next open ended question!
If you are looking for more resources to help you with Advanced Higher Chemistry, come and join my Advanced Higher Chemistry Revision Resources Google Classroom. It's packed full of video tutorials, self-marking quizzes and lots of other revision goodies!
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