The Crystal Garden Demonstration

Abstract:
This simple demonstration holds a special place in many chemist’s hearts – certainly mine. The vividness of the chemical reactions that you are witnessing, with little plant-like growths sprouting in different directions at a just visible speed, controlled as if by tiny different minds in each shoot, provides a visual representation of the unique characteristics of many transition metal compounds all in one demonstration. A sort of chemical family photo if you will!

Introduction:
The Crystal Garden is an extremely simple, yet fascinating, demonstration that seems to become alive before your eyes. Small amounts of various metal salts are added to a gloopy solution of sodium metasilicate, causing the metal salt to react, undergoing a double displacement, and forming the metal silicate and the sodium salt.

 

For instance by adding cobalt(II) chloride:
Na2SiO3 (aq) + CoCl2 (aq) 2NaCl (aq) + CoSiO3 (s)

 

This metal silicate forms a hard skin around the rest of the metal salt to prevent further reaction, but soon the osmotic pressure of the sodium silicate solution is too much and the pressure bursts through the skin, causing a jet of metal salt to spray out, react, and form a new hard skin. Thus these tendrils and shoots grow up and out of the metal salt that had sunk to the bottom, and, most amazingly, it accomplishes this at speeds that are just fast enough to be very noticeable – sometimes growing several millimetres to a few centimetres in a few seconds – but not too fast that it is all over in the blink of an eye.

In only a few minutes these growths of, depending on the metal used, varying colours quite quickly find a resemblance to an exotic coral or colourful plant. They then continue on, slowly growing slightly more over days and weeks. Equally as wonderful as witnessing the whole process of your garden developing is the final product, hence thinking ahead I prepared my first in a small glass tank to display.

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Although this is the final product, I have put this photo here to accompany the description below of this wonderful family photo of a handful of the metals who were able to make it to the family gathering.

As mentioned in the abstract, in almost every way the final product of the demonstration is just a mad family photograph. There is Jimmy on the left not keeping still with his luscious dark brown hair, such an energetic young man, always on the move compared to the others! Oh sorry did I say Jimmy? I meant to say iron(III) chloride – quite easy to mix up those names as you can imagine! Back to describing iron(III) chloride, he is always growing, look how many lovely, luscious brown branches he has sprouted; he always was a ladies man: he builds so erratically, you can see how much energy he has in the prime of his youth: although looking thick and tough, he always broke very easily; a soft side of his personality he only shows to those close to him.

Now lets not forget my daughter Natasha! Always a bit clumsy, look at her knock over that cup of tea all over her boyfriend in the photo near the right. Oh sorry, how did I do that again, her name is copper(II) sulfate, Natasha is a chemist friend of hers. You can see the source of the clumsiness of copper(II) sulfate with that wavy and thin body, as a teenager her body is undergoing a lot of change and is quite fragile: she must have knocked over that tea when tripping over; you can see her laying half on the ground as she was half-caught by her boyfriend, cobalt(II) chloride.

Cobalt(II) chloride always loved fashion didn’t he, always dying his hair different colours from purple to blue and then recently green. You can tell this photo is a little old as his hair was when it used to be blue. (more on these colour changes later!)

I could go on for ages, but I will spare you of my ramblings and hope I proved the point of the individuals, the characters, all different people with different personalities and talents, that are the transition metals and their compounds.

Now to put our serious chemist faces on; the safety for these many chemicals I will be using.

Safety:

Substance Hazard Information
Sodium metasilicate 40% solution
(water-glass)
Corrosive/Irritant
Corrosive and irritating to the skin and eyes. Skin contact may produce burns.
Ingestion can be very dangerous.
Cobalt(II) chloride Carc. Cat. 2 (probable carcinogen)
Toxic/Dangerous to the environment
Harmful if swallowed and may be harmful in contact with skin. Irritating to the eyes.
May cause allergy or asthma symptoms if inhaled or in contact with skin.
Iron(III) chloride
Harmful/Irritant
Harmful if swallowed, irritating to skin, risk of serious damage to eyes.
Some suppliers classify anhydrous solid iron(III) chloride as CORROSIVE.
Nickel(II) chloride Carc. Cat. 1 (certain carcinogen)
Muta. Cat. 3 (mutagenic effect unknown)
Repr. Cat. 2 (may impair fertility)
Toxic/Irritating/Dangerous to the environment
It is irritating upon ingestion, inhalation, skin contact, and eye contact. Prolonged inhalation exposure to nickel and its compounds has been linked to increased cancer risk to the lungs and nasal passages.
Copper(II) sulfate Harmful
It is harmful if swallowed, especially saturated solutions. The solid may irritate the eyes and skin. Water added to the anhydrous solid generates heat.
Copper(II) chloride Harmful
It is harmful if swallowed. Solid may irritate the eyes or skin.
Tin sulfate(VI) Low hazard
Basic chromium(III) sulfate Harmful
Harmful if swallowed, irritating to the eyes and skin.
Iron(II) sulfate Harmful
Harmful if swallowed.
Magnesium sulfate Low hazard
  • Gloves and goggles should always be worn when handling chemicals to protect your sensitive eyes and skin.
  • Now, that table of chemicals I used has probably put you off a bit if you are not an experienced chemist, obviously nobody can say, not even the experienced chemist, that they particularly like to regularly work with carcinogenic substances. One of the reasons this is such a great demonstration is its ability to shape itself around the chemists’ inventory – the only substance that is required to set the stage is a solution of sodium metasilicate – the rest of the chemicals are the star actors, the more rich and colourful the variety the more magnificent the performance. If you are not comfortable with handling cobalt(II) chloride or nickel(II) chloride, I definitely recommend to just not use them; they are very easy to handle safely, but it is important they are not mishandled.
  • A dust mask is recommended when working with the dry metal compound powders as the main hazards for most of these when working with just small amounts is usually the inhalation hazard. Make sure you work in a well ventilated area, but avoid any centralised air currents over any powders exposed to the air as this instead can just agitate the powders and cause small particles to become airborne yet stay in the room with you.
  • A lab coat is also recommended when working to prevent bringing any metal compounds that have found a way onto your clothes out of the lab to be spread all around your home. The lab coat will protect your clothes from contamination and if it is itself contaminated this is not an issue as it always stays in the lab.

Experimental:

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Any glass container will do. I recommend experimenting with shapes. I found a rectangular shape, such as this one, provided a great view from the front which was to be the main viewing angle when on a shelf. I can imagine a cylinder could equally work as well when there is no main viewing angle and instead it is looked at from every direction such as in the middle of a table.

Setting the demonstration up is extremely quick as mentioned. I rather arbitrarily measured out 100ml of 40% sodium metasilicate solution using a 250ml glass beaker and poured it into a suitable glass container I had prepared.

IMG_7684[1]
40% sodium metasilicate solution is surprisingly thick and syrupy! This can be useful as it means it will not slosh around as much when nudged, protecting your garden from any potential clumsiness, although low concentrations can have their uses too.

I then measured out three 100ml portions of distilled water in the same 250ml beaker to wash it out and each time added this to the container. This was to dilute the sodium metasilicate solution to 10%.

There is no perfect concentration to use as far as I am aware and it is really a matter of what you prefer. I personally found the 10% allowed the quickest growth, but also the most fragile growth, when compared to 20%. Also the lower the concentration the lower the density and the easier to get the metal compounds to sink to the bottom. I would therefore recommend low concentrations for a quick demonstration, but higher concentrations for a more permanent feature.

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I thought I would share this photo as, because the 40% sodium metasilicate solution has a massive density difference between itself and distilled water, two immiscible layers form. With a bit of manual stirring they quickly mix and dissolve in each other to form one layer, but not first without creating these waves of different densities that manipulate the refraction of light to create what looks like mini-tornadoes.

Done! Yep that is it. If you just wanted to use the concentration of sodium metasilicate you currently possess and not dilute it then you can even skip these steps and simply just dump the lot in a container of your choice.

Results:
I want to interrupt the rest of this post with text as little as possible so you can witness this demonstration and these photos in all their beauty. I will continue to add a few sentences though about what is happening. I hope you enjoy!
(For reference the time between photos is anywhere from a few seconds to a minute depending on how much action was occurring.)

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A single lump of blue copper(II) sulfate pentahydrate crystals was added to the left, with some anhydrous powder added to the right. As you would expect the powder grew substantially faster.
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A large ball of purple cobalt(II) chloride hexahydrate was added, forming shoots immediately with little bubbles on the end for which I am not sure why these bubbles form.
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I had attempted to add some green chromium(III) oxide to prove that, not being soluble, it won’t participate in this reaction which it doesn’t but I managed to get it everywhere.
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A relatively large ball of orange-brown iron(III) chloride was placed on the emptier left.
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The iron(III) chloride is super jumpy and loves to randomly and extremely quickly sprout growths when you are least expecting it.
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The greens and blues of the cobalt(II) chloride are now starting to show, whereas the main pillar in the middle is still stubbornly slightly purple. This is due to the surrounding waters effect on the structure of the cobalt. A single tiny pale green crystal of iron(II) sulfate was dropped barely to the right of its brother iron(III) chloride.
img_77091-e1518472293533.jpg
It is a race to reach the top and it seems to be a draw between the three contestants, getting to the top all in their own unique ways. A little dark green basic chromium(III) sulfate was added at this point between the iron(III) and cobalt(II) chloride.
img_77111-e1518472311864.jpg
For a lighter green a little nickel(II) chloride was sprinkled on the far left. A couple long, thin, white tendrils have grown up from the iron(II) sulfate, almost mimicking the look of a sprouting seed, although one that is very confused and is growing roots upwards!
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The green nickel(II) chloride has already grown substantially yet the dark green basic chromium(III) sulfate has barely move at all interestingly. Yellowish tin sulfate had a scoop added a little to the right near the front.
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A few sprinkles of white magnesium sulfate fertiliser were added to the left, producing a bit more white in this colourful jungle. The brown iron(III) chloride has also decided he is bored and has begun growing more tentacles again.
The dark green basic chromium(III) sulfate has finally gone full speed ahead and produced a knobbly seaweed structure, occasionally spewing out little pieces of itself everywhere like a volcano as it grows. The most boring was instead one of the most interesting! Also a little while ago a copper(II) sulfate pillar had collapsed under its weight but has begun growing straight towards the surface again. A truly beautiful scene.

I will leave you with a few close up photos of some small-scale specimens of this demonstration, and a wonderful quote from the book ‘Doctor Faustus’ by Thomas Mann, which I found quoted in the heartwarming book I am currently reading, ‘Uncle Tungsten: Memories of a Chemical Boyhood’ by Oliver Sacks:

“I shall never forget the sight. The vessel… was three-quarters full of slightly muddy water – that is, dilute water-glass – and from the sandy bottom there strove upwards a grotesque little landscape of various coloured growths: a confused vegetation of blue, green, and brown shoots which reminded one of algae, mushrooms, attached polyps, also moss, then mussels, fruit pods, little trees or twigs from trees, here and there of limbs. It was the most remarkable sight I ever saw, and remarkable not so much for its appearance, strange and amazing though that was, as on account of its profoundly melancholy nature. For when Father Leverkühn asked us what we thought of it and we timidly answered him that they might be plants: “No,” he replied, “they are not, they only act that way. But do not think the less of them. Precisely because they do, because they try to as hard as they can, they are worthy of all respect.”
– Thomas Mann

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Cobalt(II) chloride. Notice the purples, pinks, blues and greens.
IMG_7667[1]
Copper(II) sulfate. The tiny bubbles on the end of some of the pillars are quite cute!

 

 

 

Further watching or reading:

They have quite a beautiful and extremely well filmed crystal garden video:
https://www.beautifulchemistry.net/reaction/

YouTube channel Periodic Videos: Chemical Garden – Periodic Table of Videos

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