Chem Car

Brief

Build a small car powered by the chemical reaction between baking soda and vinegar. The reaction produces CO2 gas. Channel that pressure to push the car forward. The brief was a distance contest. Whoever’s car went the furthest, won.

Process

The reaction chemistry was the easy part. Sodium bicarbonate plus acetic acid produces sodium acetate, water, and carbon dioxide. The CO2 is what powers the car. The hard part is converting a chaotic burst of gas into smooth, sustained forward motion.

Three variables drove the design. Ratio, hole size, and chassis weight.

Ratio. Too little vinegar and the reaction is weak. Too much and you flood the chamber and the reaction stalls in liquid instead of pressurizing the chamber. I ran a series of small tests at different ratios in a sealed test container before committing to one for the actual car. The sweet spot was a slight excess of vinegar over the stoichiometric balance, which kept the reaction running long enough to sustain motion past the initial burst.

Hole size. The reaction chamber needed a nozzle. Too narrow and the pressure built up but did not release fast enough to push the car. Too wide and the pressure escaped instantly and dropped before the car got moving. I iterated through three nozzle sizes and landed on the middle one.

Weight. Every gram of chassis is a gram the CO2 has to push. I cut the frame down to a minimum, kept the wheels light, and balanced the chamber over the rear axle so the weight pressed the drive wheels into the ground for traction.

On test day my car went the farthest in the class.

Skills Built

• Physics tuning under tight constraints. Three variables, all interacting. The only way to find the sweet spot is to test methodically and write down what each test tells you.
• Reaction chemistry in practice. Stoichiometry on paper is one thing. Watching a slight excess of acid keep the reaction going longer than a perfect balance is when chemistry gets real.
• Iterative testing. Build, test, change one thing, test again. Changing two variables at once tells you nothing.
• Weight-versus-strength trade-offs. Lighter is always faster until the chassis flexes and steals energy from the wheels. Knowing where that line sits is intuition you only get from breaking a couple of chassis.

What I would do differently

I would have a wider set of wheels and tires to test. Wheel material and tire grip turned out to matter as much as anything else and I only tried one kind. Also a lighter frame material. The wooden frame I used was overkill for the forces involved.