A Simple, yet Dramatic Chemistry Experiment with Ping Pong Balls

Cellulose nitrate (also known as nitrocellulose or guncotton) is a very flammable substance that is formed by reacting cellulose (also known as dietary fiber) with a mixture of concentrated nitric and sulfuric acids:

Figure 1: Cellulose (polymer on top) reacts with a mixture of nitric acid in the presence of sulfuric acid to form cellulose nitrate (polymer on bottom).

Cellulose is a polymer of C₆H₁₀O₅ units.  In the formation of cellulose nitrate, each C₆H₁₀O₅ monomer is converted to a C₆H₇O₁₁N₃ unit. Notice the high proportion of oxygen in cellulose nitrate. Because of its high oxygen content this substance easily decomposes, releasing a lot of energy in the process:

2 C₆H₇O₁₁N₃ --> 9 CO + 3CO₂ + 7 H₂O + 3 N₂

You can see some experiments with cellulose nitrate in the video below.

This remarkable substance was discovered by Christian Friedrich Schönbein¹. He also made several other contributions to chemistry, including the discovery of ozone.

Schönbein discovered cellulose nitrate by accident while experimenting in his kitchen, which according to some accounts was prohibited by his wife².

During one set of experiments, Schönbein accidentally dropped a mixture of sulfuric and nitric acids on the kitchen floor. He quickly cleaned up the spill with his wife’s cotton apron and hung it over the oven to dry (by the way, cotton is composed of about 90% cellulose). Upon drying, the apron spontaneously burst into flame. It is my guess is that Schönbein was banned from experimenting in the kitchen AFTER this particular incident! (I can speculate with some authority on this matter, given that my own wife often frowns upon my at-home experiments).

The decomposition of cellulose nitrate is a great demonstration to perform in a chemistry classroom. You can prepare cellulose nitrate on your own. A good recipe is here (but please do not attempt this in your kitchen). If you don’t have the time to carry out this procedure (or would rather not work with concentrated nitric and sulfuric acids), you can use ping pong balls as a convenient alternative. Ping pong balls burn very well because they are partly composed of cellulose nitrate³:

Celluloid is a mixture of cellulose nitrate and camphor. Celluloid was once used to make many objects such as combs, dolls, and other such figurines^3,4. It is generally considered to be the first thermoplastic.

With the ubiquitous use of synthetic plastics today, the use of celluloid as a plastic has been essentially phased out. But celluloid is still used to make ping pong balls and guitar picks. Be careful, though. If you attempt to buy guitar picks as a source of cellulose nitrate for burning, make certain they are made of celluloid. This is because the celluloid in guitar picks has begun to be replaced by different plastics. Indeed, recent patent reports how to make celluloid-free ping pong balls that are not as flammable as the celluloid-type⁵. As a result, you might want to stock up on ping pong balls if you plan on using them in your chemistry classroom for some time to come!

1. http://pubs.acs.org/doi/abs/10.1021/ed006p432

2. http://www.oxfordreference.com/view/10.1093/oi/authority.20110803100446575

3. http://pubs.acs.org/doi/abs/10.1021/ed069p311

4. http://cool.conservation-us.org/jaic/articles/jaic30-02-003.html

5. http://www.google.com/patents/US8105183