The Chemistry of Rust (Oxidation)

That reddish-brown crud we call “rust” is all around us, yet we probably rarely think much about it. It turns out what we call rust is a chemical process that combines iron (Fe) and oxygen (O) to form iron oxide. Thus, by studying rust we are studying chemistry!

The chemical formula is:  4Fe + 3O2 = 2Fe2O3

What is happening? During this reaction the iron atoms are passing electrons to the oxygen atoms, a transfer that is called oxidation. In the process the atoms are bound together.


Rust Experiments

Because it is a slow process, doing experiments with rust takes a few days.

1. What rusts? (Preliminary free exploration)


  • paper clips, small bolts, metal washers and any other small metal objects to check for rusting – let the children brainstorm and gather samples as appropriate
  • include some items that probably won’t rust such as pennies or brass brads
  • container to hold water
  • water

Place a sample of all the objects in a container of water and check them every day for a few days. Leave the rest of the objects nearby or in a similar dry container to compare what happens. See which objects start to show signs of rust and which do not. Let the children touch and smell the objects that have rusted. Do they feel different? Do they smell? Do they look different?

2. What environmental conditions are needed for iron to rust?

Can iron rust in dry air or is water needed? Does the presence of acids, such as acid rain, speed up rust? What about salt? Do the salty roads in winter or salt spray from the ocean really make cars rust faster? What happens when the tannins in tea meet iron/rust? Let’s find out.

Gather for each participant:

  • fine steel wool (from paint stores or home supply centers- see note below)
  • water
  • white vinegar
  • table salt
  • teaspoon measure
  • tea bags, hot water and container for making tea
  • tape and marker for labels
  • 5 beakers or similar containers
  • paper and pen or pencil to record results


Note:  Why fine steel wool? The coarser steel wool you get to clean dishes is stainless steel, which is resistant to rust. For another experiment, get samples of both and try them side by side.

Note 2: The tea isn’t central to the question, but does react quickly which may engage impatient youngsters who might otherwise lose interest. You may definitely omit it.

Prepare the tea by soaking one or two tea bags in hot water in a container such as a tea mug for about three minutes. Stir briskly and discard tea bags.

Make saltwater by adding 2 teaspoons of salt per 8 ounces of water and stirring.

Label the containers:

  1. water
  2. saltwater
  3. vinegar
  4. tea
  5. air

Pour 4 ounces (1/2 cup) or roughly 120 ml of water into the first container. Add 4 oz or 120 ml of saltwater to the second container. Add 4 ounces white vinegar to the third container and 4 ounces of tea to the fourth. Leave the 5th container dry.

Break off pea to marble-sized balls of steel wool and roll into 5 small balls. Try to use a consistent amount for each container. Drop the steel wool into each container. Some may float, which is okay.


Rust experiment, before set-up.


Check what is happening after 15 minutes.


After 15 minutes the tea probably has started to darken. The steel wool will have turned black. In the photograph above the steel wool that was in the tea is on the left and steel wool that had been in plain water is on the right.

What is happening? The tannins in the tea are reacting with the iron and rust in the steel wool to make iron tannate. Iron tannate is very stable and people are investigating its use to prevent metals from rusting.

Check again after 24 hours.


The tea, on the right, has turned black with a concentration of iron tannates. The water, on the left, and the saltwater (not shown) are turning brown and the steel wool is beginning to rust.

The vinegar (center) is still clear and the steel wool is not showing rust. Why not? One reason might be that the vinegar has been setting on a shelf in a closed jar and might not have much oxygen in it. How would you test this?

The dry steel wool is not rusting either. Even though the chemical equation shows that only iron and oxygen are needed, the chemical process actually needs some water or another catalyst to be present to get the reaction going.

Record your results again after 48 hours. What has changed? Use your results to plan more experiments.

Can you tell me…

why we paint metal objects like the San Francisco bridge?


A word of caution to educators:

During preparation for this post I came across a couple of references to experiments that promised “fast rust.” These experiment required mixing bleach and vinegar. Mixing bleach and vinegar is not a good idea! The acid reacts with the bleach releasing chlorine gas. In small amounts the chlorine gas reacts immediately with the iron to give iron chloride, which looks like rust. If you add an excess amount, however, toxic chlorine gas might possibly be released. has more information

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18 Responses to The Chemistry of Rust (Oxidation)

  1. Anon says:

    Just wondering where you got this information from:
    “In small amounts the chlorine gas reacts immediately with the iron to give iron chloride, which looks like rust.”
    Because according to wikipedia, iron(II) chloride is a white or green salt, and iron(III) chloride is green or purple/red. Also, iron chloride is soluble, so how did you manage to see the colour?
    Sorry if I’ve misunderstood something.

  2. Roberta says:

    I actually found the information that the reaction might produce iron chloride on this forum: (scroll down near bottom)

    If you check this forum, again near the bottom, AJKOER suggests “My view on the reaction of HOCl and Fe is temporary formation of Fe(ClO)2, which decomposes forming FeCl3 and liberating some Cl2. ”

    I did not see the color, because I didn’t want to mix bleach with an acid. I was just reporting what was shown in videos posted on the Internet of the experiment, and questioned its safety. If you have more information, I would appreciate clarification.

  3. megan says:

    I am doing a science project please help????

  4. Roberta says:

    Do you have a particular question?

  5. Madalynn Sierra says:

    hi, I love this idea I’m in the 5th grade and my teacher said this was an amazing project. This really helped me understand what rust was. So thank you!!!!!!!!!!!!!!

  6. Roberta says:

    Who would have thought rust would be something to study? :-)

  7. kelly says:

    i am helping my daughter with her science project. we took three glass jars filled one with vinegar one with water and one with bleach . we put a steel scrubbing pad in each jar. we set one of the sets in direct sun one in the shade and one set in total darkness i can not find anything that she can read to understand why the bleach is breaking down faster. please help.

  8. Roberta says:

    It is my understanding that the bleach (formula Na-O-Cl) produces oxygen atoms when it breaks down as well as causes oxidation (removes an electron from the iron atom). Oxygen is needed to produce rust (fe2O3).

  9. Sophia Flanagan says:


    For a school chemistry practical we did something similar to this, investigating the effects of corrosion with different variables etc. I just need some help with some of the discussion questions we were given if possible.
    I have to explain why objects corrode slowly when painted, and slowly in the desert. I also have to write the formula for rust and explain why metal object fall apart when they rust.

    Sophia Flanagan

  10. Emma says:

    I am working on an assignment and I was wondering:
    What is the word equation for iron in salt water?

  11. Roberta says:


    Sorry for the late reply, your question got caught in the spam filter.

    Okay, the answers to these questions are in the post. Painting an object helps keep out the oxygen needed for oxidation or rusting to occur. Painting also keeps out the water. The absence of or low levels of water in the desert also helps prevent or slow rusting.

    The equation for oxidation of iron to iron (III) oxide or rust is 3 Fe + 2 O2 → Fe3O4

    The reaction can be speeded up by water, salt or by burning iron wool.

    The iron (III) oxide molecules do not bind to each other like iron metal does, which is why the object falls apart.

  12. Roberta says:


    I’m not sure if this is what your instructor is looking for, but iron plus oxygen in salt water becomes iron (III) oxide or rust. The salt water acts as a catalyst.

  13. Erik says:

    I have to do this lab where we write down the chemical eqaution of vinegar and of steel wool and then we have to write the chemical equation of their product so i was just wondering what the chemical equation of their products is

  14. Roberta says:

    Hi Erik,

    The chemical reaction is the standard chemical reaction for rust (which you can find in the comments above). The vinegar is not actively involved in the reaction from the standpoint that it ends up in the products. It serves to remove the coating from steel wool so the oxygen can get to the iron in the steel, and it may also serve as a catalyst. Rusting takes place a lot faster in water and acidic conditions.

  15. Aye Myat says:

    Can vinegar only help the rate of corrosion because I don’t want to mix it with bleach ( it’s too dangerous) ?

  16. Roberta says:


    When we did the experiment in straight vinegar, the steel wool did not rust as quickly as it did in water, which is likely because it formed iron acetate rather than iron oxide. The vinegar may act as a catalyst in some cases if the steel wool is coated to prevent rusting. If you think it may be coated, briefly dip the steel wool in vinegar, dry it and then add it to water.

  17. ad says:

    this website helps me in my project so thankyou verymuch

  18. Roberta says:

    Glad it was useful for you.

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