The cars are powered by Kitchen Chemistry:

The above equations are all representations of the same reaction. It is important to note that for a complete reaction the molar ratio of acetic acid to sodium bicarbonate must be kept constant.

The third equation above shows the balanced form of the reaction. As noted, the reaction requires one mole of acetic acid and one mole of baking soda to produce one mole of carbon dioxide gas. A mole is a unit like a dozen, a dozen is 12 things and a mole is 602,200,000,000,000,000,000,000 things. For comparison a mole of donut holes would cover the earth and be 5 miles deep. However, a mole of molecules can be a reasonable amount. One mole of water is only 18 ml. Moles are used in chemistry because it allows comparisons to be made regardless of phase or measurement units.

Our goal in this experiment will be to always use excess vinegar to help make sure all the baking soda is dissolved and reacted. Once we understand the chemistry, we will attempt different chemical ratios to produce enough gas to drive our cars approximately 25 feet.

While the exact details of this experiment are quite open, a general procedure is outlined below. Prior to building the cars, it is a good idea to develop an understanding for how much gas will be produced. Thus the procedure is broken into three sections: gas production, car construction, and car operation (distance traveled).

Gas Production (plastic bottle chemistry)

1. In a plastic bottle (any approx. 750 ml water bottle with a small mouth will work for this) add ~100 ml 5% acetic acid Vinegar. (Vinegar is sold in 4% and 5%.
2. Wrap ~6 gm Baking Soda in a single-ply Kleenex and add to the vinegar.
3. Immediately cover the bottle opening with a balloon and hold it on to prevent leaks. (It is good to practice this with out the reactants once or twice.)
4. Gently shake to bottle to encourage mixing. The reaction produces gas and will blow up the balloon. Measure the balloon diameter with a piece of string or a measuring tape.
5. Repeat a few times with a few different reactant ratios. (The original ratios were chosen as they are a 1-1 molar ratio.)

Car Construction

1. Discuss the important features of the car design (wheels that spin, a way to secure the reaction chamber (plastic bottles), the direction of the jet and the way the car then moves).
   NOTE: Cars can be constructed out of almost anything that allows you to put wheels and axles on a platform. We use either K'NEX™ or LEGO™ kits for simplicity, but wood or a plastic model kit would also work.
2. Allow students to build car.
3. After a test run, checking for wheels that work, bottle secured, and structural stability, reconstruct if necessary.
   NOTE: The concept we are trying to teach here is engineering problem solving - design and re-design. We learned that the "re-design concept" is not well received by 1st graders. However, the "take-apart-and-build-all-over-again" concept does seem to work...with some coaching.
4. Make sure the plastic bottle used as a reaction vessel can hold a good seal and still be easily opened.
   NOTE: We have found that the best "off-the-shelf" plastic bottle and nozzle system is the 700 ml GATORADE™ bottle with the simple "one turn to open" lid. Any flavor or POWERADE™ bottle works fine .

Car Operation

1. Add 100 ml vinegar to bottle attached to car. Add 50 ml water as well; this adds mass to increase the thrust.
2. Measure 6.4 gm baking soda onto a single-ply Kleenex.
3. Add baking soda bundle to bottle taking care to prevent mixing until the bottle is closed and sealed against leaks.
4. Shake the bottle until a large froth forms. Set the car down, open the valve or bottle top, and watch the car drive.
5. Measure the distance traveled.
6. Repeat experiment a few times for statistics. Change the amounts of vinegar, baking soda, and water to tune the car to the desired distance.

http://www.exploratorium.edu/science_explorer/bubblebomb.html
http://library.thinkquest.org/3347/vinegar+bsoda4.html