Tin dendrite

A metal tree grows before your very eyes!

Difficulty:
Danger:
Duration:
10 minutes
Tin dendrite

Reagents

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the protective underlay.
  • Observe safety precautions when working with batteries.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

The liquid didn’t spread out to completely cover the Petri dish.

Try placing the Petri dish on a horizontal surface and shaking it gently. If that doesn't help, add 1–2 teaspoons of water.

The springs aren’t touching the solution.

First, try pushing the springs farther down the Petri dish wall.

Secondly, make sure that the solution has spread out to completely cover the base of the Petri dish, leaving no empty areas. If there isn’t enough solution, add 1–2 teaspoons of water.

Nothing happened. Why not?

Don’t worry! This is probably easy to fix.

· First of all, make sure that the batteries have been properly inserted into the battery holder. If everything has been set up correctly but nothing is happening, try changing the batteries.

· Make sure that the springs are touching the metallic parts of the wires, not the insulation.

· Make sure that the springs are touching the solution.

· And don’t forget to check the polarity of the batteries!

The tin chloride solution is bubbling near one of the springs. Is everything OK?

Yes, everything is fine. This spring is acting as an anode in this electrochemical cell. A water decomposition reaction is producing gaseous oxygen (O2), which bubbles through the solution and makes it foamy.

How do I use the batteries?

We highly recommend that you use the same brand of batteries in your devices. Batteries made by different manufacturers can have different specs (voltage, amperage, etc.). If these characteristics conflict, one of the batteries can start to overheat, which can potentially result in a battery leaking and damage to the equipment.

Carefully examine your batteries to determine their polarity and location in the battery holder.

The flat end of the cylindrical batteries is their "–".

The convex end of cylindrical batteries is their "+".

And the polarity must be indicated on the battery case or label!

There is almost always a spring on the negative side of the contact in the battery holder. You need to connect "–" of the battery to the "–" of the battery holder, and "+" to "+". Insert the new batteries into the battery holder, paying attention to the polarity.

After use, immediately remove one battery from the battery holder. If the batteries don’t work, and you did everything correctly, then dispose of them in accordance with the environmental standards of your region.

Step-by-step instructions

Be sure to use fresh AAA batteries and wear protective gloves.

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First, secure the electrodes to your Petri dish.

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Attach the wires to the electrodes.

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Prepare a tin chloride SnCl2 solution.

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Pour the solution into your Petri dish.

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Disposal

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

Scientific description

Batteries are basically electron pumps: they suck electrons in with their "+"  and pump them out from their "-" . When such a pump is connected to a solution via "electric hoses" (i. e. wires), a variety of chemical reactions can take place: by the "-" electrode  some particles will capture the electrons that are pouring out, and by the "+" electrode  some particles will give their electrons away.

In our experiment, tin ions Sn2+  (i.e. tin atoms with two electrons missing) will gladly accept some electrons at the "-" wire and will turn into metallic tin . Meanwhile, the "+" electrode itself , which is made of iron, will give some of its electrons to the "+" wire , leaving some iron ions floating around . This is why the "+" electrode will eventually dissolve if you don't disconnect it.

Why does the tin dendrite grow?

By attaching wires to the springs on the Petri dish edges we connect the tin(II) chloride (SnCl2) solution to the batteries. Once connected, the electric current starts to flow through the solution. Near to one of the springs, a tin reduction reaction takes place:

Sn2+(solution) + 2e→ Sn(solid)

Tin is a metal that precipitates (forms a solid) as solid, elemental tin. The tin dendrite grows in the direction that the electric current flows through the solution; from one spring towards the other.

More details:

The metallic springs are electrical conductors, known as electrodes. A variety of types of electrode is available. It is important to learn about different types that have been invented and to understand the type we use in our experiment. Electrodes may be made of metal, graphite, or polymer material. Some of them participate in chemical reactions that occur in solution (called consumable electrodes), others are chemically inert (non-consumable electrodes). The electrodes in our experiment are inert (non-consumable) metal ones.

Tin forms on one of the springs as a dendrite that grows directly towards the other spring. Why does the dendrite not appear as a sort of tin "island" near the first spring? The answer is that once a bit of tin dendrite has grown on the electrode it can also conduct electricity. Consequently, it acts as part of the electrode immersed in the solution and so each new branch of dendrite grows further into the solution.

It is important to understand that dendrites made from different metal solutions grow in diverse ways because each metal forms unique crystals with unique properties. Tin has a very special crystalline structure and forms long and thin, but strong enough, crystals.

What is electrolysis?

Electrolysis is a process by which chemical reactions are induced by an electric current. Electrolysis takes place when current flows through a solution (or a fusion) of certain substances, known as electrolytes. Electrolytes are substances that can split readily into ions in solution. For example, ammonia chloride (NH4Cl) forms two ions in aqueous solution:

NH4Cl → NH4+ + Cl

Electrolysis provides us with a way to form new substances, which evolve from substances presented in solution under the influence of an electric current. For example, metallic copper (Cu) precipitates from copper sulfate solution (CuSO4). Such products can also be formed by secondary electrode reactions, for example, oxidation of tin(II) chloride (SnCl2) to tin(IV) (Sn4+) gives oxygen (O2), which evolves as a gas from water during electrolysis.

More details:

What is electric current? It is a flow of charged particles. These particles are electrons (electronic current) or ions (ionic current).

As the electric current flows through the wires it sets up a negative charge on one of the electrodes (springs) and a positive charge on the other. This makes the ions in the solution move between the springs. The positively charged electrode is called the anode and negatively charged particles (anions) move towards this spring. The negatively charged electrode is called the cathode and positively charged particles (cations) move towards this electrode. This is how ionic current forms in solution.

In our experiment, tin deposits on the cathode and grows into a beautiful dendrite:

Cathode: 2Sn2+(solution) + 4e→ 2Sn(solid)

More reactions occur at the anode. The main process is the formation of oxygen (O2):

Anode+: 2H2O – 4e → O2 + 4H+

At the same time, a secondary reaction between the evolving oxygen and tin chloride (SnCl2) takes place:

2SnCl2 + O2 + 2H2O → 2SnO2 + 4HCl

You should note that the oxygen, which interacts with the tin, is not actually an O2 molecule. It is more like atomic oxygen — a very reactive particle that participates in this reaction. It oxidizes tin(II) (Sn2+) to tin(IV) (Sn4+). A white precipitate of tin(IV) oxide (SnO2) forms.

These are not all the reactions that occur near the anode, but there is no point in making our story more complicated, so let's skip them.

Why do we use SnCl2?

We have chosen tin chloride because of some specific properties of tin. Firstly, tin crystals form thin and long structures; this is because the speed of crystal growth in one direction is faster than the speed of crystal growth in other directions. Secondly, tin is a bright, glittery, silver color, which is why the dendrite looks spectacular. Tin is also a ductile and soft metal, which decreases the chances that the dendrite will break (owing to its own weight or detachment from the spring). In fact, the tin dendrite floats on the surface of the liquid. Moreover, metallic tin is inert enough to oxygen and moisture from the air, so it will not decompose during the experiment. Finally, the tin reduction reaction is fast enough to be able to watch the dendrite grow.

Can dendrites be obtained from other salt solutions?

Tin is the best for this experiment. Many metals form a precipitate as an electric current flows through a solution of their salts, but only tin provides such a spectacular dendrite. Some metals, such as lead (Pb), would flake off the clip surface, layer by layer. Other metals, such as copper (Cu) or silver (Ag), would form weak porous structures that would collapse under their own weight. Each metal has unique characteristics, which results in different crystalline lattices and, consequently, crystals with unique properties.

Follow up

How can I control the growth of the tin dendrite?

Actually, pretty easily! Begin the experiment as shown in the instructions. When the dendrite starts to grow from one of the springs, start moving the other spring along the wall of the Petri dish, ensuring that it stays immersed in the tin chloride solution. The growing dendrite will try to “reach” the moving spring via the shortest path possible. You can also take it off the Petri dish wall completely to move it across the dish. Just make sure that spring stays in contact with the solution!

How will diluting the SnCl2 solution influence the experiment?

Before starting the experiment, dilute the tin chloride solution with water in a 1:1 ratio. Continue the experiment, following the instructions.

Diluting the solution lowers the concentration of tin ions, which slows the rate of dendrite growth. The dendrite “branches” will be fewer and will grow perpendicularly to each other. The branches will also be much thinner.

That’s interesting!

Tin and humans: a long relationship

Humans discovered and began using tin a long time ago. As tin’s melting point is a little higher than 230 oC (446 oF), it was enough to add a stone containing tin to some charcoal (cassiterite mineral contains tin in the form of an oxide, SnO2) to discover drops of the molten metal in a fire pit.

Tin is one of the key components of bronze, which was the most durable alloy known to human beings before the discovery of iron. The importance of tin and bronze cannot be overemphasized: a whole historical period was named after bronze — the Bronze Age (which lasted about 2000 years!).