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-nails

Rust Experiments

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

1. What rusts? (Preliminary free exploration)

Gather:

  • 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

steel-wool

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.

tea-and-water-pretest

Rust experiment, before set-up.

tea-darker-after15

Check what is happening after 15 minutes.

tea-left-water-right

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.

rust-at-24hours

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.

About.com has more information

73 thoughts on “The Chemistry of Rust (Oxidation)

  1. Post author

    Boom,

    It formed rust at the surface of the water? The most likely scenario is that the acidic conditions caused the iron atoms to go into solution where they reacted with oxygen to form rust. Did you use distilled water or tap water?

  2. Kelly

    What are the independent, dependent, and controlled variables in this experiment? Also, what do you think would be a accurate hypothesis? Thank you!

  3. Post author

    Kelly,

    The hypothesis is what you are predicting will happen. It might be called a tentative explanation or guess. For example, say you want to test the effect of water on rusting. Your hypothesis might be "iron placed in water rusts faster than dry iron."

    The independent variable is the thing the scientist changes or manipulates. In the example above, water is the independent variable because you changing the amount of water the iron is exposed to.

    The dependent variable is the thing you expect to be different, or in other words, what you are measuring. In the example, this would be the amount of rust produced.

    Sorry if you had trouble leaving a comment. I have comment moderation turned on so it may take a day or two for your message to appear.

  4. Kelly

    Thank you so much, wonderful experiment. Your explanations and responses to comments really helped me understand the material you taught!

  5. Hallie

    Hi, I am doing a science fair project involving masonry nails in different solutions with salt, vinegar, bleach, apple juice and baking soda. So far the baking soda solution has completely stopped the rust from forming and the apple juice solution was the 2nd best at slowing the rust from forming on the nail (all nails were sanded with steel wool). I can't find any information as to why baking soda is stopping the rust from forming. Also do different pHs affect the rust formation. I chose agents with different pH levels. Can you help?
    Thanks so much for this site!

  6. Hallie

    Hi Roberta, I'm not sure we have signed up correctly as we have not heard anything yet. Did you get my question about my science fair project involving rust forming on nails and more specifically why the baking soda solution stopped the rust from forming? We posted it on Jan 17th at 9:54 am. I noticed the previous questions were answered pretty quickly so I'm thinking you did not receive it. We cannot find any info about baking soda and rust. I think it is a chemical thing. Thank you.

  7. Post author

    Hallie,

    You signed up correctly. I just needed to do some thinking about this.

    In general, iron rusts at low pH (acids) and not at high pH (bases), with some exceptions. In acidic conditions the H+ ions react with the metals, making them more susceptible to oxidation (rusting).

    Technically, baking soda is a base (According to About.com, the pH of baking soda is 8.3) http://chemistry.about.com/od/acidsbases/a/phtable.htm and so has more OH- ions in solution, rather than H+.

    On exception to the general rule is bleach, which although it is a base, is also an oxidizing agent. Bleach is one base that promotes rust even at high pH.

    I hope this helps.

  8. OK i get this but what happens if you leave it there for a week you get crystal but why dose the penny form 1879 is the dirtiest penny for the penny in 1789?

  9. Jean Thomas

    Hi. My daughter did a science fair project called "does the change in the concentration of salt in water affect the rate at which iron will oxidize"?

    She had 5 tests:
    1. No Salt
    2. 25grams salt
    3. 50grams of salt
    4. 75 grams of salt
    5. 100 grams of salt

    She used 500 grams of water.

    She put a pure piece of iron in each container. She hypothesized that the container with the most salt would present the most rust. However, the bolt in the container with NO SALT clearly had more rust after 10 days and the container with the least salt (25 grams) showed the second least amount of rust. The water (tap water) was clearly more red in the containers with 1) no salt and 2) the least salt.

    In addition, she measured the mass of the bolts each day. The mass of the bolt in the tap water increased more than two times the mass of the other bolts. She used an accurate scale and dried each bolt before weighing the bolts.

    Could the results be skewed because she used tap water or table salt? Is there some chemical in the tap water or in the table salt the was slowing the oxidation process in the salted containers or accelerating oxidation in the tap water only container?

    Finally, should the increase in salt concentration increase the acceleration of oxidation?

    We are stumped!

    Thank you!

  10. nazrul

    can anyone tell how can i form rust on cotton fibre with in two hour. what type of catalyst have to use.

  11. Milan Abraham

    can you tell me the justification for the hypothesis or a assingment type of thing pls. It would be very helpful

  12. boitumelo

    Can you please help me with the hypothesis and the dependent,independent and conteolled variable of reactions of metals with oxygen-rust

  13. Boitumelo Tlhagale

    Hi
    Im doing a project on reactions of metals with oxygen-rusting carrying out a practical investigation and writing a scientific report.
    -What is the hypothesis?
    -What are the dependent and independent variables ?

    I really need you help.Thank You♡☆♡

  14. Post author

    Boitumelo,

    The hypothesis will depend on the particular experiment you perform. The hypothesis is the tentative explanation you come up with for the specific question you are asking. For example, if you ask, "Does salt makes iron rust faster?" your hypothesis might be that salt does make iron rust faster or it might be that salt does not make iron rust faster.

    Science buddies has a good explanation of variables: http://www.sciencebuddies.org/science-fair-projects/project_variables.shtml

    More about coming up with a hypothesis

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