Terrible Tooth Decay: Chemical or Physical

Introduction:

Purpose: The students will set up and observe a science experiment where an egg is placed in vinegar for a three days.  The students will observe changes in the egg (and vinegar) and then conclude whether the changes represent a physical or chemical change.   This change also simulates a tooth that is being affected by tooth decay.

In a physical change, some properties (like shape, or phase) of the material change but the material itself is the same before and after the change. The change can also be "undone". In a chemical change the materials (substances) that are present at the beginning of the change are not present at the end; new substances are formed and the change cannot be "undone".  While you can't always tell for sure whether a change is physical or chemical, there are some signs that normally indicate that a chemical change has occurred.

National Science Standard: Unifying Concept

Nature is predictable and we can use evidence to explain and understand it.

Georgia Performance Standard: Students will explain the difference between physical and chemical changes. S2P2, S5P2

Materials:  One egg (or egg shell) per group        vinegar        plastic cups      Piezo Popper kit (opt)

                   Small plate     Paper Towels                bread           aluminum foil    Binaca Breath Spray

Exploration:

1. Each group of students should obtain a plastic cup (or jar) and an egg from their teacher. 
2. As a group, discuss what you think will happen when the egg is placed into the vinegar. Write down your prediction of what you think will happen. Recognize that the changes will occur slowly over the course of several days.
3. Place the egg gently into the vinegar and make careful observations of any changes that you can see. Record your observations.

1. Place plastic wrap over the cup and store it in a safe place.
2. Observe the eggshell everyday for 3 days. Record you observations and discuss your ideas about what you think.
3. On the third day, gently take your egg out of the vinegar, and clean it off with a little water and a paper towel. Discuss what you think happened to the shell. Be very careful with your egg because it will break easily.
4. Return your egg to your teacher and pour the remaining vinegar down the sink.
5. Wash your hands.

  Egg Observations:

Essential Questions:

1. Explain what you think it was that caused the changes in the egg.

2. Do you think that the change to the egg was a chemical change or a physical change? Explain.

3. What part(s) of your body do you think an eggshell is like?

4. What do you think would happen if you did not brush your teeth?

5.   What do you think the food in your mouth does to your teeth when you are sleeping? 

Explanation:

Upon completion of the exploration, have students share their thoughts, predictions and ideas about the experiment. 

There are many plausible explanations that can be developed by students. In a physical change, some properties (like shape, or phase) of the material change but the material itself is the same before and after the change. The change can also be "undone". In a chemical change the materials (substances) that are present at the beginning of the change are not present at the end; new substances are formed and the change cannot be "undone".  While you can't always tell for sure whether a change is physical or chemical, there are some signs that normally indicate that a chemical change has occurred.

Here are some common examples of physical and chemical changes:

In this experiment, the acid (acetic acid) in vinegar chemically reacts to breakdown the eggshell (which is made of calcium carbonate).  New substances are formed and one of them is carbon dioxide gas that can be observed as tiny bubbles on the surface of the shell. 

  This reaction is very similar to one that occurs between acid that is produced in your mouth and your teeth. As is turns out, there are about 50 different types of bacteria that live in your mouth. These tiny bacteria want an easy place to live where they can multiply and prosper. Hard surfaces are ideal for this and the only hard surface in your mouth is your teeth. So bacteria naturally collect on your teeth and some of them produce mucus that helps the bacteria to stick on your teeth as dental plaque. The good news is that the bacteria themselves do not eat away at the teeth. The bad news is that a few of them also produce acids like vinegar that do eat away at your teeth. These acids are produced when the bacteria feed on carbohydrates and other sugary foods that we eat.  Like the vinegar and the egg (only much more slowly), these acids chemically react to breakdown the calcium (enamel) in your teeth.  We can protect our teeth by limiting our sweet intake, eating healthy and brushing regularly. 

Teachers Guide: This is an excellent long-term science activity that kids love. Make sure that students handle the egg carefully on the final day. Although the membrane of the egg is surprisingly tough it will occasionally break. If this happens, make sure that kids wash their hands and that the area is cleaned thoroughly with soap. Without the shell, the eggs are pretty transparent and resemble reptile (or dinosaur) eggs.  Since eggs are single cells, this activity can also make a good lead in to the idea that all living things are made of cells.

Extension:

As an extension activity, you can demonstrate one of the most exciting types of chemical reactions -- an explosion.  Explosions generate excitement like few other topics in science and they are happening all around us all the time. An explosion is a very fast chemical reaction that produces both gases and heat.  In most explosions, solids or liquids are converted instantly into gases and, as this happens, the amount of space (volume) the particles take up increases greatly.  The rapid expansion of particles also produces a shock wave that accounts for most of the destructive force of an explosion.  Most explosions require only a fuel of some sort, an ignition source (spark), and a good source of oxygen.  

Explosions can occur accidentally in places like coal mines and lumber yards where tiny particles of fuel get into the air.  We have also learned how to control explosions and use them in places like our car engines  where hundreds of little explosions occur each minute as gasoline and air and a spark are combined.  In this case, the motion (energy) from the explosions get converted into the rotation of the wheels.

The Binaca Blast:

In this simple demonstration, you will use a surprising fuel source - breath spray (the alcohol in it), a grill lighter (spark), and the oxygen in the air to ignite a simple, safe, but dramatic explosion.  Have your students carefully observe the reaction and look for signs that indicate that this is indeed a chemical reaction.  Make sure not to point the canister toward students and don't allow your students to run over and pick up the canister until you check to see that the reaction is complete.   After doing the demonstration, you can discuss why airport security people are so careful about what they allow passengers to carry on flights (especially during a time of terrorist threat).  If breath spray and other common substance such as perfume can be explosive then the need for safety is paramount.  

Procedure: 

1. Obtain a Piezo Popper Kit from Educational Innovations www.teachersource.com and construct the Piezo Popper according to directions.

2. Place your breath spray and the film canister into a glass of hot water to warm them up.

3. Warn your students that it will make a loud noise and that they should cover their ears.

4. Add two (and only two) sprays to the canister and attach it to canister top. Caution: To avoid a flaming missile do not use more than two sprays.

5. Point it away from any people and push in the button.  Repeat if needed. The film canister often travels over 20 feet. 

6. Walk over and pick up the canister and make sure the reaction is complete.