This class covers reversible and irreversible changes. This is an introduction to chemical reactions. If this topic is for older kids, you may want to discuss it as reactions that only have one direction and go to completion. Exothermic reactions like combustion fall into this category. Many reactions like those between weak acids or weak bases and water go to an equilibrium and are written with a double sided arrow. For younger kids you just want to get the concept across that some changes can’t be reversed, and some can.
I think kids can easily see that when something solid burns, it can not go back to it’s original state. In this section I highlighted that if a lot of energy is produced in a reaction, then you can’t make the reactants of the reaction from the products without putting a lot of energy in, for example the combustion of hydrogen. When two or more substances are mixed, if a gas is released in the reaction, you can not remake the reactants from the products. Reactions that bubble out gas, for example mixing vinegar (acetic acid) and baking soda illustrate this.
Here are some video clips for examples of irreversible reactions that have been discussed in previous lessons: Fluorine reacting with cotton (4:09), combustion of hydrogen and oxygen (3:50), and dissolving styrofoam in acetone (45:30).
In this section you can talk about ways to separate things that are mixed together. For example with a sieve or a filter, or evaporating the water from a solution. In this part of the class we mixed little beads and water. I had the kids try to separate them with a funnel first and then when that didn’t work they added a coffee filter to the funnel. Next we mixed salt and water. I asked if they could make the salt disappear, and they all knew to stir it. Then I asked them to try to separate it. Even if all the salt isn’t dissolved and some stays in the filter ask them to taste the water that went through the filter to see if the salt is still there. Then ask if they can separate it a different way. Put the cup in the sun and wait until it evaporates to show the kids that the salt is now separated from the water, although the crystals may look different.
For this part I gave the kids each a cookie and a pile of trail mix. I asked them to separate each into the parts they were made from. They could see that they could separate the parts of the trail mix, but the cookie could not be separated into eggs and flour etc.
I showed them pictures of the cookie dough to remind them that the cookie dough was irreversibly changed by heating it. This is in contrast to the trail mix that was only stirred together.
After I did this I asked them if there was a way to irreversibly change the trail mix. You want them to come up with “eating it”. They need to know that the acid in the stomach changes the molecules in the food, and the food no longer has the same properties.
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