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This project should be used to teach about chemical changes.
The reason people cry when an onion is sliced open is because a gas is emitted. It is not important that you demonstrate exactly how the gas is formed at this time, but it is important that everyone understands what is irritating their eyes - a gas that comes out of the onion when it is cut. Tell the students that they are going to try to prepare an onion several different ways to observe if any of the methods reduce the gas, or keep the gas from reaching their eyes.
- Five Onions
- Piece of Bread
- Cutting Boards
- You will need five students to volunteer for this project. They must be warned that there is a good possibility their eyes are going to be affected.
- Have the first student demonstrate what happens when you chop one onion the normal way. Have the student use a cutting board and remind him or her not to let their fingers get too close to the knife. The student should be of an age where they are able to use a knife without getting cut.
- Have the other students write what is happening in their notebooks. They should separate their notebook page into five categories, with category one being, "Cutting an Onion Normally". To go a little further with this experiment you may even want the students to use a stop watch to see how long it took for the student volunteer to begin tearing.
- Have the second volunteer wear a pair of swimming goggles. This is one way to prepare the onion without actually affecting the eyes. Since the onion emits a gas that bothers the eyes it would make sense that wearing goggles would stop the gas from getting in the eyes. Have the students record what happened.
- Give your third volunteer an onion and have him or her place it in the refrigerator. The logic behind this is to see if the temperature changes the amount of gas that is omitted from the onion when it is cut open. After the onion has chilled, have the student begin cutting it. Again, you can use the stop watch to see how long it takes for the student to begin tearing, if at all. Continue to write results in the notebook.
- The fourth volunteer will cut his or her onion under the running water in the sink. See if the running water washes the gas away before it has a chance to reach your eyes. Write down the results.
- The final volunteer will cook his or her onion first. Allow the onion to sit in a pot of boiling water for a few minutes. Then remove the onion carefully and begin cutting it. You may want to wear an oven mitt to keep from scolding yourself. Again, you will be testing to see if temperature has affected the gas level that will come out of the onion. Record the results.
Have the students share if they were surprised by any of the methods. Ask the students to talk about how doing this experiment can benefit them in their everyday lives. They might not think it will benefit them right now if they don't help cook at home, but it will in the future.