Chemical vs. Physical Change Lab
Posted October 6, 2008on:
I created this lab last night and we did it today. Overall it went fairly well.
I began the lab by lighting a strip of magnesium to demonstrate a chemical change. All that is left after that very bright flare is white powder. They really liked that demonstration. Then on to the lab.
The first part of the lab, students add salt to about 75 or 100 ml of water. I used a tablespoon because it was handy, but that amount seemed to work out well. They stirred the solution until it dissolved, then heated it over a bunsen burner. As the water boiled off, the salt came out of solution and first formed a ring around the middle of the beaker, then formed crystals on the bottom of the beaker. We had to be careful because as the water went away, the salt started splattering. That’s your clue to shut off the gas.
Next, they heated a small amount of the salt in a test tube to see what would happen. There was no physical change, no smell, no color change.
Last, we repeated the above with sugar. Immediately they see the sugar melt and carmelize. The smell is carmel or marshmallows, but it’s not bad. A few of them take it too far, which is good, because you can talk about the sugar becoming carbon and water.
I was very rushed this morning trying to set up for this lab and do everything else I needed to do. I accidentally switched the salt and the sugar, so when the beakers were boiled, I got candy. Except for one group, who didn’t listen and let it go too far. That one turned into a bubbling cauldron of carbon. Great demonstration, lots of smell and mess.
Does anyone know how to get that carbon out of a beaker or test tube? I cleaned for hours today and I can’t get it all out.