One of the advantages of being a homeschooler is that you get to find creative ways to teach. Chemistry can be a dry subject for small kids, but some of the experiments are fun for people of all ages. In order to make science slightly more interesting, we made smoke bombs to explore the world of chemical reactions. Below is the account of one of our experiments.
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Making a homemade smoke bomb is much easier than expected. There are several recipes online describing how to do it, but they often include heating the mixture on the stove. The cautious side of me thinks that sounds like a risky idea. Why put something you are going to light on fire on the stove? I prefer to keep the excess smoke outside, not in my kitchen.
The ingredient list is very simple. You need a container, a fuse, sugar, potassium nitrate (KNO3), and water. Most of those things I had in the house. I did not have any potassium nitrate. However, it is available from most hardware stores in the form of stump remover.
For a container, I cut 6” off the end of an empty Christmas paper tube. I taped one end shut with duct tape so the powdered mixture would stay in place. I have seen many other things used as a containers, but this one was cheap and easy. One other advantage I found is that the cardboard burns, so there is less mess afterwards.
For a fuse, I used a piece of cotton string that was a few inches longer than the cardboard tube. That way I knew there would be a safe length of fuse when time to light it. I wanted to make sure I had time to back a safe distance away. Now that I had my container and fuse done, it was time to move onto the fuel mixture.
I found that the ratio of the sugar and KNO3 was not specific. Alterations in the ratio will change the reaction process, and therefore cause different effects. The more KNO3 the slower it will burn, and the more smoke will be produced. Likewise, less KNO3 will produce more spark and less smoke.
The amount of water needed is directly dependent on the amount of dry ingredients. With too much water, the mixture takes a long time to dry. Not enough water causes the mixture to be hard to form and pack. I found that the best balance was when the mixture was the consistency of brown sugar. This consistency was easy to work with, and only took a day to dry.
Once I had the mixture ready to work with, I placed the wick in the center of the tube and packed the fuel in around it. There was roughly 3” of wick sticking out of the top of the fuel in the tube. Once I had all the fuel packed in the tube, I put the smoke bomb on a shelf to dry.
I learned the hard way to put the devices on a surface that will not burn. I had an unfortunate accident where one of the smoke bombs tipped over and it burned a hole in the yard. When everything went right, they created a tremendous amount of smoke and a tower of sparks nearly six feet tall.
Overall, it was a very enjoyable way to teach the kids about chemical reactions. Enjoyable for both them and me!