Silver Tree in a Test Tube: The Growth of Metal Crystals in Silica Gel
| Polish version is here |
The following article was originally published in the journal for educators Chemia w Szkole (eng. Chemistry in School) (6/2021):
Metallic elements are most commonly found in nature in the form of ores containing their compounds. Only a few noble metals occur in a free state as native elements. Owing to their distinctive properties, including mechanical characteristics, metals are widely used to produce structural materials and machine components. Their chemical properties are also used in various industries.
From a molecular standpoint, metals differ from other elements due to the presence of free electrons within their crystal lattice. These mobile charge carriers are responsible for metals’ excellent electrical conductivity. In their solid state, metallic elements typically exhibit luster, ductility, malleability, and high thermal conductivity. From a chemist’s perspective, it is important to note that metals generally tend to form compounds with basic and nucleophilic properties rather than acidic or electrophilic ones.
The relatively unreactive metals mentioned above are conventionally referred to as “noble” metals. They include the platinum group (platinum Pt, ruthenium Ru, rhodium Rh, palladium Pd, osmium Os, and iridium Ir) along with two elements from the coinage metals group—specifically silver Ag and gold Au (sometimes rhenium Re and mercury Hg are also included).
Metals in the solid state occur in crystalline form. Metal crystals can assume various shapes, one of which is dendritic aggregates of small crystals. They usually resemble branching plant stems.
A common experiment conducted in school and university laboratories involves growing dendritic silver crystals through electrolysis. Under suitable conditions, at the interface between the solution and air, beautiful fractal-like silver crystals form (Phot.1).
The crystals produced in this way are approximately two-dimensional structures, which makes it easier to study the fundamentals of their formation. However, if we want to observe the development of three-dimensional silver crystals, we run into a certain problem—they are so delicate that during growth in solution, they often break under their own weight. Observation is therefore possible only in the case of small crystals, which must be examined under a microscope (Phot.2).
To obtain larger silver crystals with an impressive structure, it is worth using a technique that involves growing them in a gel medium [1].
Experiment
For this experiment, we only need four substances:
- water glass,
- silver nitrate AgNO3,
- copper Cu,
- acetic acid CH3COOH 6–10% (vinegar).
Water glass is an aqueous solution of sodium or potassium silicate. Sometimes it is a mixture of both sodium and potassium silicates. In any case, it can be treated as a system of variable composition with the general formula mMe2O • nSiO2 • xH2O (Me = Na or K; m, n, x are variable stoichiometric coefficients). Water glass is produced by reacting molten sodium/potassium hydroxide or carbonate with silica SiO2 [2]. It has many applications, primarily as a heat-resistant binder (for glues and putties, ceramic materials, casting molds, and for impregnating refractory materials). Although water glass is not highly toxic, commercial products often include sodium hydroxide, making them potentially caustic.
As our silver ion source, we will use silver nitrate. This compound is sometimes still called “lapis,” owing to the alchemical name lapis infernalis (literally “infernal stone” in Latin). It’s worth considering why a substance that appears as white crystals (Phot.3) might have been given such a devilish name.
One reason is undoubtedly that contact between this salt and our skin—once exposed to light—leads to the deposition of metallic silver in the tissue, resulting in black stains that are almost impossible to remove. These discolorations only disappear as the skin peels away, which without methods to speed up the process can take several days or even more than a week. silver nitrate should be treated as a toxic substance that also has a mild caustic effect on skin. Personal protective equipment—lab coat, gloves, and safety goggles—are essential [3].
Pure acetic acid is a colorless, corrosive liquid. It is miscible with water in practically any ratio. In this experiment, we will use a diluted solution (6–10%) known as vinegar.
The last substance we need is metallic copper—a metal with a beautiful reddish hue. We’ll use a relatively thin copper wire; I used wire with a diameter of about 0.4 mm (about 0.016 in). If you’re using wire typically intended for electrical work, you’ll have to remove the insulating coating before starting the experiment.
The wire must be bent so that it can be placed appropriately in a test tube. Of course, you could use another vessel, such as a beaker, but a test tube provides the best visibility thanks to the relatively thin layer of silica gel between the resulting crystals and the side walls of the vessel. Through trial and error, I discovered that a good technique is to wrap the wire around a pen or pencil and then bend it as shown in Phot.4.
To obtain the silica gel, we must first dilute the water glass so that its density becomes about 1 g/cm3 (about 62.4 lb/ft3), close to the density of water. According to the literature, this is achieved by mixing one volume of commercial water glass with eight volumes of water [4]. Be sure to use purified (distilled or demineralized) water to prepare all solutions in this experiment.
Next, place the appropriately shaped wire in the test tube and pour in a mixture of equal volumes of the diluted water glass and 6% acetic acid. The gelation process is fairly slow, taking several hours. If it fails, try experimenting with water glass dilution and with the ratio in which you mix it with acetic acid. The prepared setup should look like Phot.5.
Notice that the coil-shaped section of the wire forms a base, while the straight, sharpened section protruding upward is roughly centered along the test tube’s axis. The gel layer should extend about 2–4 cm (about 0.8–1.6 in) above the wire’s tip. As you can see, the gel is almost perfectly transparent.
To initiate the process, carefully add a few centimeters of an aqueous silver salt solution (concentration between a few percent and about ten percent; I usually use 8%) on top of the solidified gel. The effect depends significantly on the solution’s concentration, as well as on temperature and the properties of the silica gel. It’s worthwhile to set up different versions of the experiment using solutions of various concentrations. However, keep in mind that, generally, the lower the concentration you use, the longer you’ll have to wait for results. Except for observation times, the entire reaction setup should be kept away from light, since silver compounds can decompose under its influence.
In the first few hours, you may notice a whitish haze moving downward from the surface of the gel where it contacts the silver salt solution (Phot.6). This is caused by the diffusion of ions (including silver ions, Ag+) deeper into the gel, likely along with their reaction with trace chloride ions (Cl-) present as impurities in the water glass, which leads to the formation of a white, light-sensitive precipitate. The small amount of chloride ensures that not all silver ions are consumed in this reaction.
After some time—usually several hours to more than a day—the front of silver ions migrating deeper into the gel will reach the copper wire’s tip and then progress further. Something interesting begins at the wire’s tip: it first turns grayish, and then small, sprout-like structures start to emerge, branching after a while. The structure grows hour by hour, appearing almost alive. It is best observed through a magnifying glass (Phot.7). The last photo was taken 5 days after adding the silver salt solution.
In the course of the experiment, a structure resembling a miniature tree forms, featuring a “copper trunk” and a branching “crown” composed of metallic silver crystals. It looks especially beautiful under magnification (Phot.8).
Explanation
The reaction occurring during the gelation of the acidified water-glass solution can be written as follows:
The resulting hydrated silica forms a gel in which silver ions can still move.
As the silver ions diffuse deeper into the gel, they encounter metallic copper. We know that chemically more active metals (those with a lower electrochemical potential) displace less active metals (with higher potentials) from their compounds. In the case of silver, its standard potential—measured relative to the hydrogen electrode—is 0.80 V, while for copper, it is 0.34 V [5]. We can write the reaction as follows:
The copper dissolves, forming Cu2+ ions, while silver is reduced to its metallic (atomic) form.
The formation of these branching, tree-like silver crystals can be explained by the phenomenon known as diffusion-limited aggregation [6]. The mobile silver ions in the solution are reduced on the metal’s surface to insoluble—and thus immobilized—silver atoms.
Similar phenomena also occur in nature. The processes described are not merely a laboratory curiosity—aggregates of the mineral pyrolusite often resemble plant fossils, though they are entirely abiotic in origin.
Because convection is inhibited in this system, and the ions moving through the gel come into contact with a precipitating factor, it is sometimes possible to observe an additional phenomenon known as Liesegang rings, an intriguing example of self-organization [7].
References:
- [1] Ples M., Metaliczne rośliny. Krystaliczne dendryty srebra (eng. Metallic Plants – The Beauty of Crystalline Silver Dendrites), Chemia w Szkole (eng. Chemistry in School), 3 (2015), Agencja AS Józef Szewczyk, pp. 6-10 back
- [2] Korzeniowska M., Wpływ struktury uwodnionego krzemianu sodu jako spoiwa mas formierskich na własności żelu krzemionkowego w wysokich temperaturach, online: https://winntbg.bg.agh.edu.pl/rozprawy2/10073/full10073.pdf [22.11.2021] back
- [3] Ples M., Nie tylko srebro - światłoczułe związki w fotografii (eng. Not Only Silver: Light-Sensitive Compounds in Photography), Chemia w Szkole (eng. Chemistry in School), 1 (2018), Agencja AS Józef Szewczyk, pp. 35-41 back
- [4] Chojnacki J., Techniki krystalizacji I. Krystalizacja z żelu, online: https://chem.pg.edu.pl/documents/175187/63147198/Krystalizacja%20z%20%C5%Bcelu [22.11.2021] back
- [5] CRC Handbook of Chemistry and Physics 88th, CRC Press, 2008 back
- [6] Falconer K., Techniques in Fractal Geometry, John Willey and Sons, 1997 back
- [7] Ples M., Porządek z chaosu. O pierścieniach Lieseganga i samoorganizacji (eng. Liesegang Rings and Self-Organization: Order Emerging from Chaos), Chemia w Szkole (eng. Chemistry in School), 1 (2016), Agencja AS Józef Szewczyk, pp. 15-19 back
All photographs and illustrations were created by the author.
Addendum
The growth of silver crystals under the described conditions is relatively fast, but it still takes at least several dozen hours to complete. To allow us to observe the details of this process, I created a time-lapse video. Individual frames (as single photographs) were captured approximately every 3 minutes. When played back at the standard rate of 25 frames per second, this results in a significantly accelerated view of the process. Additionally, after each photograph was taken, the test tube was rotated slightly to provide a complete view from all angles.
Marek Ples