Today we started unit 2 on the properties of matter. I had assigned HW 1,2,3 from the study guide to be completed after the test prior to class. We went over this HW, which included three main concepts: the difference between a physical property and a chemical property, the difference between an element and a compound, and the difference between a mixture and a pure substance.
To show the physical vs chemical difference, we talked about a piece of paper--what are some properties of it? Color, bendability, weight, mass--these are physical because you don't have to change the substance to note them. The flammability of the paper is a chemical property, because as it burns, it becomes a new substance.
We looked briefly at the periodic chart and could see that hydrogen and oxygen are elements which water (H2O) is a compound which has vastly different properties than either H or O. We talked about ratios (2 parts H to 1 part O for water).
Next we discussed the difference between a heterogeneous mixture and a homogenous mixture. Milk is a great example of both. Homogenized milk is all one color--well mixed--while milk out of a cow is heterogeneous. I also showed students my six year old milk which I keep in a cabinet. Yuck. Salt water is a mixture because it can be separated by simple physical means--there is not a special formula for salt water as there is for salt or water alone. You can prove this with a simple distillation process (or simple evaporation).
Class ended with me burning magnesium in the fume hood to show a dynamic chemical change as it turns into magnesium oxide. Where does the oxygen come from?
Sweet justice.
