Rainbow plating

Capture a rainbow on a metal plate!

Difficulty:
Danger:
Duration:
15 minutes
Rainbow plating

Reagents

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
  • Remove protective gloves before lighting the splint (step 5).
  • Keep flammable materials and hair away from flame.
  • Keep a bowl of water nearby while working with fire.
  • Place the stove on the cork hot pot stand. Do not touch the stove after the experiment – wait until it cools down.
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

Which plate is iron?

This set contains two types of plates: iron and copper. The copper plates have a characteristic pumpkin-orange color. The iron plates are grey.

How thoroughly should I polish the iron plate?

The shinier the surface, the better. There shouldn't be any streaks, rust, or patches.

Why can't I touch the polished surface without gloves?

Traces of oils from your fingers (even freshly washed) can remain on the metal. The reaction won’t happen in these spots, which can mar the result.

I didn’t apply the sticker very well – can I reapply it without repolishing the plate?

No, you’ll have to polish the plate again. Any glue particles left on the metal can interfere with the experiment.

The copper isn’t appearing on the surface of the iron – or if it is, it’s dark and unpatinated.

First, you should make sure the surface of the iron plate is very well polished (it should be shiny). Second, quickly rinse the iron plate with a solution of CuSO4. Finally, immediately rinse with water as soon as the copper layer appears.

The tap water washed away the copper coating!

Unfortunately, you’ll have to start the experiment over and use gentler running water.

The copper turned white instead of bluish-red when I heated it.

Yes, this can happen. Copper transitions orange – pink – blue – purple – light blue – white. Try again and extinguish the candle when you get the color you want.

I performed the experiment and got a nice keychain. But the copper rubs off so easily!

We recommend coating the keychain with a clear varnish or hanging it where it won’t be subjected to friction.

Step-by-step instructions

Clean the surface of the iron so that it can easily engage in chemical reactions.

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Protect part of the iron plate with a sticker of your choice.

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Generously apply copper sulfate CuSO4 solution to the unprotected section of the plate and observe the reaction. For the best results, rinse it with water as soon as the surface darkens. This will stop the reaction.

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Carefully pat the plate dry and remove the sticker. You can already see the image—but wait, there's more!

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The image you see on the iron plate is formed by a thin layer of copper from the copper sulfate. When heated, copper can react with oxygen in the air to form copper oxides.

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Copper oxides are usually just boring powders, but they look quite spectacular when they form as thin layers.

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Disposal

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

Scientific description

At the micro-level all metals in their metallic forms are similar: positively-charged metal particles, ions , resting in a cloud of negatively-charged electrons . The iron plate in this experiment is no different. But that's not the only way metal particles can exist. The copper ions Cu2+  from the copper sulfate CuSO4 lack this comfy electron cloud, and can wander through the solution on their own. Copper, however, prefers its metallic form far more than iron does. It can easily kick some iron particles out of their electron cloud and into the solution, taking their place and thus turning from isolated copper ions into copper metal.

The copper that you caught with your iron bait didn't look especially interesting, not even shiny! Why did simply heating it make it colorful all of a sudden? As it turns out, copper can give its newly-obtained electrons away to the oxygen O2 in the air when heated, forming copper oxides CuO and Cu2O. CuO is usually a boring black powder while Cu2O is reddish-brown, but interesting things happen when they form extremely thin layers–a thin copper oxide layer  can transmit some light . Some of it bounces off the far side of the film and returns to the surface, but not all the light survives the journey. Thin films stifle some colors and amplify others, and these colors vary depending on the film's thickness. What's left  determines the color you see.

This optical property of extremely fine structures is cleverly employed by some living organisms. In nature, it's sometimes very tricky for an organism to produce a pigment of a certain color, say, blue. But an organism can use easily-obtainable materials of boring colors to form special "light traps" that only reflect certain colors, which usually look much more exciting. Many birds  and bugs  use this trick. Can you picture the green luster on some beetles, or even a blue jay's vibrant plumage? They possess no green or blue pigments—they trick our eyes with an optical illusion.