Reagents

• Copper(II) sulfate
• Iron(III) chloride
• Manganese(II) sulfate
• Liquid glass

Safety

• Put on protective gloves and eyewear.
• Conduct the experiment on the tray.

FAQ and troubleshooting

Step-by-step instructions

Pour the sodium silicate (liquid glass) solution into the Petri dish.

Now apply the solutions of metal compounds (called metal salts) to the surface of the sodium silicate solution.

An ion exchange reaction occurs between the sodium silicate and the metal salts. As a result, insoluble metal silicates form. These resemble jellyfish!

The water will gradually evaporate and leave behind a multicolored layer of insoluble silicates.

Disposal

Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

Why do the jellyfish appear in the liquid glass?

As you drip them into the Petri dish, the solution droplets don’t mix with the liquid glass as you might expect. Instead, they remain isolated, forming these fascinating structures we call silicate jellyfish. You have to agree, some of them look almost like the real thing!

Their intricate structure is largely due to the “splatter” effect as the drop impacts the dense solution from a certain height. Later on, we’ll discuss how adjusting the height of the fall might affect the structure of your jellyfish.

So why don’t the drops dissolve in the liquid glass?

This is primarily due to the viscosity of the sodium silicate solution – the drop is kept quite compact to begin with. This is why you have to thoroughly mix the concentrated liquid glass with water; if you don’t, the jellyfish will fall apart in the less viscous areas.

So what is viscosity? One way of describing it is “lack of ability to flow” – the higher a substance’s viscosity, the less easily it flows. For instance, water is extremely easy to pour, melted chocolate less so, and honey even less so. Consequently, we would say that water is not very viscous at all, melted chocolate is more viscous than water, and honey is the most viscous of the three.

However, this is not the only reason why the drops retain their original form. You would still think that the drops would gradually dissolve in the liquid glass – so what’s stopping them? Honey dissolves in water! And we mixed the liquid glass with water easily! Something seems amiss.

As it turns out, rather specific chemical reactions between the drop and the liquid glass are forming insoluble structures. Below are just some of the reactions involved in the process. Keep in mind that liquid glass is, in essence, sodium silicate Na₂SiO₃.

3FeCl₃ + 3Na₂SiO₃ → Fe₂(SiO₃)₃↓ + 6NaCl CuSO₄ + Na₂SiO₃ → CuSiO₃↓ + Na₂SO₄ CuSO₄ + Na₂SiO₃ + H₂O → Cu(OH)₂↓ + Na₂SO₄ + SiO₂↓

These reactions occur quite fast – in fact, they rapidly form a thin film of insoluble compounds between the solution of liquid glass and the metal salt solutions. This film is what prevents the drops from dissipating!

How does the height of the fall affect the form of the jellyfish?

As a matter of fact, this depends on how well you mix the water and liquid glass. The less carefully you mix the solution, the more likely the drop is to hit a watery or, conversely, a viscous area (where there is more liquid glass). The drops will dissipate less in a viscous environment, and more in a watery area.

However, let’s assume the solutions have been mixed thoroughly. If you drip the metal salt solutions from a height of about 1–2 cm, you’ll get little pellets resembling pieces of caviar. Perhaps the most beautiful jellyfish result from a dripping height of 5–10 cm. If you drip the solutions from about 20–30 cm (that’s a difficult task in itself; the plastic tray might come in handy), the drops will practically break apart upon impact. The higher you go, the more the drop will splatter across the Petri dish. That is, of course, if you hit the mark!

What’s the difference between the four solutions?

This experiment involves four solutions of metal salts: bluish copper sulfate CuSO₄, yellow-orange iron chloride(III) FeCl₃, colorless manganic sulfate MnSO₄, and a yellowish-brown iron sulfate(II) FeSO₄.

Each of the solutions is special in its own way. Let’s start with copper sulfate – a bluish solution of CuSO₄, often called cupric sulfate. These jellyfish are the most stable of the bunch – they will retain their color and form for several days.

Jellyfish made of manganese(II) sulfate MnSO₄ are transparent and look quite striking on a dark background – try placing some black construction paper or something similar under the Petri dish. Also, they gradually change their color, eventually turning pinkish-brown. Why? Each manganese Mn²⁺ ion (from MnSO₄) can still donate at least 2 more electrons, which other atoms would love to poach! That’s exactly what happens: the oxygen in the air О₂ gradually oxidizes the manganese to manganese oxide Mn_O{2}, which gives the jellyfish its color:

Mn²⁺ – 2e^- → Mn⁴⁺ MnSO₄ + O₂ → MnO₂↓ + SO₄^2-

Why do we use two solutions containing iron ions? At first glance, the answer might seem obvious: each compound contains different anions (negatively-charged ions) – one contains chloride Cl^-, and the other contains sulfate SO₄^2-/. But this isn’t the main difference. Look closer – the iron is different! In the case of chloride, it's Fe³⁺; in the case of sulfate, it's Fe²⁺. The difference is in just one electron:

Fe³⁺ + e^- → Fe²⁺ Fe²⁺ – e^- → Fe³⁺

But the difference in properties, including color, is quite significant!

You may also notice small orange stains forming in the drops of the dark iron(II) sulfate FeSO₄ jellyfish – this metal is oxidized by (i.e. loses one of its electrons to) the oxygen in the air. When this happens, their color starts to resemble that of the iron(III) chloride jellyfish!

Another interesting property of iron(III) chloride FeCl₃ is the speed at which it reacts with sodium silicate. Why is this? The ³⁺ next to the iron ion Fe³⁺ means that the iron ion has triple the positive charge of an electron. The other ions involved in the experiment, Cu²⁺, Fe²⁺ and Mn²⁺, all have a lesser charge – twice the positive charge of an electron. Insoluble silicates form when a positively-charged particle of iron (let’s say Fe³⁺) meets a negatively-charged particle of the silicate SiO₃^2-. The greater the difference in charge between the participants of the reaction, the stronger and faster they attract to each other.

This is why the iron(III) chloride reacts with the liquid glass a lot faster than the other salts – so quickly, in fact, that the forming insoluble compounds literally squeeze the drop out onto the surface of the liquid glass.

That’s interesting!

What is liquid glass?

Liquid glass is a viscous, liquid solution of sodium silicate Na₂SiO₃, and rarely potassium silicate K₂SiO₃, in water.

How is it made? One might think that it would be easiest to dissolve silicates in water, but this method is ultimately very expensive. Instead, a concentrated solution of alkali (NaOH) and a mixture of raw materials containing silicon dioxide (SiO₂) are heated at high pressures on an industrial scale.

Structurally, however, this substance isn’t quite sodium silicate Na₂SiO₃. It would be more correct to write Na₂O(SiO₂)_n because the solution still contains quite a lot of silicon dioxide and related substances. Normally, n = 1.4–1.8.

What is liquid glass used for?

In construction, liquid glass is used as a binding solution in the preparation of concrete and cement.

Additionally, flammable materials such as fabric, wood, and cardboard are treated with liquid glass to keep them from catching fire. Liquid glass is often used to reliably glue paper and cardboard.

How does liquid glass react with normal glass?

You should observe caution when handling liquid glass with normal glass. If sodium silicate dries on normal glass, it will render the normal glass opaque.

Why does this happen? It’s quite simple, really – normal and liquid glass are based on very similar silicate compounds. In essence, the normal glass dissolves slightly in the liquid glass, marginally altering its surface structure and making it opaque.