Science Project Idea: Cooking In Paper
The concept of the high heat capacity of water is shown by boiling water and cooking over an open flame in a paper cup and a paper lunch bag.
One of my favorite demonstrations in science class is the one I do explaining the high heat capacity of water. Generally I start out by stating that water has a heat of vaporization of 540 calories per gram. I always explain that this extremely high. About this time I have to wake my students up somehow before they all die of boredom, so I light a bunsen burner on my demonstration table.
I then set a paper cup I happened to have with me on fire by holding it with a set of thongs in the flame over the bunsen burner. Now that I have their attention (Flames, loud noises, and rapid color changes always get students attention),I take another paper cup, seemingly identical to the first one, and hold it over the burner and wait while nothing happens. I had prepared this cup by putting a few milliliters or ounces of water in it before class. As the students start to get curious as to why this cup has not caught fire I start to get into the explanation of heat capacity again.
I ask the students to guess why this cup didn't burn. They guess all sorts of things but before I answer to affirm or disaffirm any of their answers, I call them (the students) to come up and examine the cup. I let them see that there is water in the cup and aside from this fact this cup is exactly the same as the other which they just saw burn to ash in an instant. I then pour all but a few drops of water out of the cup (this is to delay its inceneration a few second)and return it to a position over the flame. Within a few seconds this cup is in ashes like its counterpart.
Again I pose the question: Why didn't the cup burn at first and then burn later? Usually the class comes up with the answer that it had something to do with the water. The most often heard answer is that the water must have wet the cup. When I point out that paper cups are generally waterproof they are then on the road to understanding heat capacity. There is then one more step in this first part of the demonstration.
I put another paper cup, identical to the others used, on a stand set up over an unlit burner. I have a quantity of water in plain view and I pour a little of it in the cup and light the burner, at the same time I start explaining that water's ability to hold and absorb heat is tremendous. As the water starts to be reduced near total evaporation I replenish it. I then continue to tell the class that it is this "heat capacity" of water that is responsible for the lack of change in the paper cup although its been over an open flame for several minutes. At this point depending on the amount of water in the cup I either allow the water to totally evaporate or remove the cup, pour out the remaining water and replace it over the burner. At this point I state that lacking the water used to absorb the heat what is going to happen to the paper cup and how long will it take? By now the concept of water holding a great deal of heat is pretty well understood.
I don't let it go that easy though. This is where the second part of my little lesson starts. Producing a regular paper lunch bag, a pencil,and an egg, I start with another demonstration. Stating nothing, I open the lunch bag place it upright on the demonstration table. I then crack the egg and empty its contents into the bag. I then take the pencil and use it to roll the bag closed by forming a handle. I then, holding it by the pencil move the bag over the burner flame and then start to cook the egg.
This usually gets the point across before I ask what they think will happen. The fun part is they never let me overcook the eggs. There is a problem though, I can only cook eggs scrambled or sunny-side up, and I like mine easy over!