Tin dendrite

Metal trees grow before your very eyes!

Difficulty:
Danger:
Duration:
15 minutes
Experiment's video preview
Chemical formula

Reagents

Safety

  • Wear protective gloves and eyewear.

  • Conduct the experiment on the tray.

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

What should I do if the dendrite does not grow?

Make sure the batteries are connected properly in the battery block (check the polarity and ensure that the batteries fit well into the holder).

Check that the crocodile clips are securely connected to the battery block wires. Also, make sure that all the clips are securely connected to their own wires.

Change the batteries; perhaps one of them is not working.

One of the clips may not be in contact with the solution. In this case, see the next question.

What should I do if the clips do not touch the solution?

Try to move the clips further down into the Petri dish.

If the tin chloride solution is not distributed evenly on the bottom of Petri dish, the clip may be connected to an “empty” zone. Usually, the tin chloride solution (10 mL) is sufficient to cover the bottom of the Petri dish. However, surface tension can prevent the solution from completely covering the bottom. Try again to distribute the solution in the dish. Still does not work? Take a plastic glass from the starter kit and measure 1–2 teaspoons of drinking water into it. Pour the water into the tin chloride solution.

The tin chloride solution bubbles near one of the clips. Is this OK?

Yes, everything is in order. This clip acts as an anode in this electrochemical cell. A water decomposition reaction takes place to produce gaseous oxygen (O2). It bubbles through the solution and makes it foamy.

Why should I not combine batteries from different manufacturers in one holder?

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

One of the clips dissolves during electrolysis. What should I do? How can I repeat the experiment?

If you leave the electrochemical cell working for some time, one of the clips will start to dissolve. This is an ordinary electrochemical reaction, which is neither dangerous nor disturbing the experiment. However, the part of the clip immersed in the solution will gradually dissolve.

How can I repeat the experiment (or do the follow-ups) if this has happened? First, disconnect the clips from the battery block and then follow one of these two options:

Option 1: Take the clip out of the solution, wash it in water and dry with a paper towel. Now connect the “dissolved” clip to a wire from the battery block. You can now use the undissolved clip to continue your research!

Option 2: Turn the clip around by placing the dissolved part on the outside of the Petri dish. Make sure that the “undissolved” part of the clip is immersed into the solution.

Step-by-step instructions

  1. Pour out all the sodium hydrosulfate NaHSO4 into the vial with tin chloride dihydrate SnCl2 · 2H2O.

  2. Take one of bottle nozzles and insert it into the bottle with the mixture.

  3. Take a red cap and close the vial with tin chloride dihydrate SnCl2 · 2H2O. Shake the vial for about 1 min.

  4. Pour out the resulting tin (II) chloride solution SnCl2 from the red-capped vial into a Petri dish. Add 2 drops of liquid soap into the Petri dish.

  5. Evenly distribute the liquid on the bottom of the dish, gently tilting it from side to side.

  6. Take two crocodile clips and fix them on the opposite sides of the Petri dish. Make sure that the end tips of the clips are immersed in the solution.

  7. Insert 4 AAA batteries into the battery holder. Now, connect the two unused crocodile clips to the wires of the battery holder. Wait 1–2 minutes.

  8. Watch a tin dendrite grow from the one clip toward the other!

Graphical step-by-step instruction

Expected result

A wonderful dendrite grows from a colorless salt solution.

Disposal

Dispose of the experiment residues along with your regular household trash.

Scientific description

Why does the tin dendrite grow?

By attaching the crocodile clips to 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 clips, 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 clip towards the other.

More details:

The crocodile clips 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 clips as a dendrite that grows directly towards the other clip. Why does the dendrite not appear as a sort of tin "island" near the first clip? 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 coper 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 (clips) and a positive charge on the other. This makes the ions in the solution move between the clips. The positively charged electrode is called the anode and negatively charged particles (anions) move towards this clip. 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 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 clip). 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

We have prepared some ideas so that you can continue this experiment. We advise you read them all and choose the most interesting for you.

The disappearing dendrite

Be very careful to avoid shaking the table or the Petri dish during this experiment. To succeed, the dendrite should be in contact with the clip it grew on. Make sure that the crocodile clips are attached securely to the Petri dish. It is better to grow a rather small dendrite for this follow-up — about 1/3 of the Petri dish in size.

Now, swap the clips on the battery block. To do so, disconnect them both from the battery block and then reconnect them one at a time. Watch the dendrite carefully! It will start to disappear. At the same time, a new dendrite will begin to form on the opposite side.

What happens? We have changed the polarity of our cell. Now, electric current flows through the system in the opposite direction. That is why the old dendrite starts to disappear:

Snsolid – 2e- → Sn2+solution

How can I control the growth of the tin dendrite?

It is no big deal to do this! Follow the instructions at the beginning. When the dendrite starts to grow from one of the crocodile clips, disconnect the other clip from the Petri dish wall and move it along. Ensure that the clip stays immersed in the tin chloride solution! The dendrite will try to “reach” the clip you are moving by the shortest path possible. You can also take the clip off the Petri dish wall completely to move it across the dish. Again, make sure that clip stays in contact with the solution.

Hieroglyphics: a filter paper experiment

Be sure to wear protective gloves when carrying out this experiment. Take regular filter paper (a fine coffee filter or drawing paper would work). Cut a rectangular piece (4 х 8 cm or 1.5 x 3.0 inches). If the paper you use is multilayered and can easily be separated into single layers, it is better to take a single layer. Moisten the piece of paper with the tin chloride solution thoroughly (do this in a clean Petri dish). Avoid the use of too much liquid. Take the paper out of the dish and attach two crocodile clips to it on opposite ends. Watch the paper carefully: a small tin dendrite will gradually appear inside. Try to see the “hieroglyphics” by holding the filter paper up to a light source (but make sure the clips stay connected to the paper). Remember to wear protective glasses!

How will diluting the SnCl2 solution influence the experiment?

Before you start this experiment, dilute the tin chloride solution two-fold with water. Follow the instructions thereafter.

The concentration of tin ions decreases as we dilute the solution, which causes the rate of dendrite growth to slow down. Dendrite “branches” grow less frequently and align perpendicularly to each other. Finally, all branches will be much thinner.

That's interesting!

What do we need a paper clip for?

Let's repeat the experiment. This time we will use a paper clip. Just before you connect the battery block to the crocodile clips, put the paper clip in the tin chloride solution. Place the paper clip in the middle between the two crocodile clips with one loop of the paper clip pointing towards one crocodile clip and a second loop pointing towards the other clip. Repeat the experiment by follow the instructions given.

The tin dendrite grows on one of the paper clip loops. Moreover, it grows very fast!

Why does this happen? The paper clip divides the solution into two parts. The paper clip loops participate in the electrolysis reaction as electrodes (see “Learn more” from the “What is electrolysis?” section). There are now four electrodes in our system: two crocodile clips (cathode and anode) and two paper clip loops (cathode and anode as well). The tin dendrite grows from each of the two cathodes. The paper clip cathode is on the loop that points towards the crocodile clip anode. The paper clip is made of metal, so the electric current, which flows through it, is an electron stream. Electrons move through a metal much faster than ions through a solution. Therefore, in this case, electron flow moves much faster than ionic flow. This is the reason why the dendrite grows more rapidly on the paper clip than on the crocodile clip.

Tin and humans: a long relationship

Humans discovered and began to use tin a long time ago. The melting point of tin is a little higher than 230oC (446oF). It was enough to mix some charcoal with tin-containing stone (cassiterite mineral contains tin in the form of an oxide, SnO2) to discover drops of molten metal in a fire pit.

Tin, as well as copper, is one of the key components for bronze, which was the toughest 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 (it lasted about 2000 years!).