Unlike the last couple of labs, the Solubility Lab found in Ian Guch’s 24 Lessons that Rocked the World, was a breeze and we actually finished early enough to do a bit of the middle school lab and go over homework!
The solubility lab tests whether water or isopropyl alcohol will dissolve more potassium chloride (KCl). I bought NoSalt at the local grocery store for my source of potassium chloride. It has a few other ingredients but it didn’t affect the experiment. Students write down which they think will dissolve more KCl before we start, and most agreed that water would be the better solvent. They measure 2 grams of the KCl into a small 50ml beaker and then added 10 ml of distilled water. Stir for a minute then write down their observations. They then pour off the liquid, keeping any undissolved KCl in the bottom of the beaker. Beakers were then heated on a hot plate or butane burner (with very low flame) to drive off the remaining water. Once the beaker were cooled, the beaker and remaining residue were massed. Beakers were then cleaned, dried, heated briefly and massed again. The difference in the masses is the mass of the undissolved KCl. Then they repeated the experiment using 10 ml of isopropyl alcohol (91%) and found they had a lot more KCl left over in their beaker.
When I did the lab I found only about 3% of the KCl had dissolved in the alcohol, while over 90% dissolved in the water.
We finished this lab in about an hour so I had the students do the same lab the middle school did yesterday, dissolving chemicals in water and watching the temperature as it dissolves to see if its exothermic (gives off heat) or endothermic (absorbs heat). I had each group just do one chemical, either potassium chloride or calcium chloride (DampRid, purchased at Lowes). The temperature change is pretty drastic for both of those and after the frustrations of the last couple of labs it was nice to do some short and simple labs.