Chemical vs. Physical Change Lab

I created this lab last night and we did it today.  Overall it went fairly well.

physical-chemical-change-lab

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.

Demonstrating a Chemical vs. Physical Change

This is a standard demo, one I did with my 8th grade Physical Science class and it stuck with them.  It uses sugar to show the difference between a physical change and a chemical change.  The first step is to dissolve sugar in water and then evaporating the water over a low flame.  I usually use a beaker over a burner.  The sugar will crystallize out and can be dried and returned to its original form.

The second step involves heating sugar in a test tube until it carmelizes and turns to carbon.  The kids smell the change and associate the smell with a property change.  We try but can’t get the mess to turn back into sugar.

If you haven’t done this before, don’t go by the picture, it’s just a photo I found on the web.  You want to gently heat the test tube with the sugar.  You only need a small amount of sugar (1/2 at the bottom of the test tube) and if you do it slowly and carefully, you will first see the sugar melt and then start to change.  Gently waft the odors to the students as it starts to change.  If you go fast, you will stink up the place.  I often hold the test tube in my hands as I heat the bottom.  It doesn’t get hot if you go slowly.

I usually throw the test tube out, it’s just not worth cleaning it once the change takes place.  If someone knows how to clean it easily, please comment.  Thanks.