Solutions: Where does the solid material go?

In science today, we began to deepen our understanding of mixtures and solutions. We started off comparing mixtures and solutions, realizing that a solution is a mixture where the solid material dissolves. This lead us to our focus question for the lesson, which was “Where does the solid material go when it dissolves in a solution?”

To figure this out, we started by thinking about the claim, “I think that when the salt dissolves, it leaves the water.” The students discussed whether they agreed or disagreed with this claim and why, pulling on their background knowledge about salt water, evaporation and even our field trip to Lantau where we found evaporated salt in the salt water marsh.

Students then began to discuss how we may be able to answer our focus question. We ended up breaking our steps down into the following:

  1. fill a cup with 50 ml of water
  2. use the equal arm balance to weigh the water (after calibrating the equal arm balance!)
  3. add a “scoop” of salt
  4. weigh the salt water solution

We then discussed what our results would tell us about our focus question. If the salt had weight, then it didn’t just “go away” when it dissolves. We found that the salt weighed 3 grams. To be sure that none of the salt had just left the water when it dissolved, we weighed the scoop of salt on its own to find that it weighed exactly 3 grams.

As we move forward, we will continue to work on understanding exactly what is happening when the salt dissolves in the water.

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