Scanning electron microscopy (SEM) is a powerful tool that scientists use to look at and understand the structure of tiny materials such as nanoparticles. However, one of the things I have always found fascinating is using SEM as a tool to examine common everyday objects like human hair or paper towel in a new and unique way. One example I found fairly interesting, and the topic of this post, is common table salt and crushed pepper.
These are two materials we see almost every day on our kitchen table, and at least to the naked eye don’t appear to be very interesting. However, when you look more closely (specifically 1000 times normal size as in the SEM image below) you can begin to see that these materials have very interesting and very different shapes and structure which of course leads one to wander, what are salt and pepper really made of and why do they look so different?
So what exactly are salt and pepper made of and why do they look so different? Well let’s first start with salt. Salt is actually a crystalline mineral comprised primarily of sodium chloride (NaCl). Because it is a crystal it has a very regular and well defined atomic structure which leads to its tendency to form cubic structures like the ones seen in the SEM image above. In fact, a newly formed salt crystal will typically have very sharp edges that get worn down by friction during packaging and handling to form a somewhat dulled edge like the ones seen in the image below. Another interesting fact about salt is that although it is a very safe and inert material (and is even essential to life) it is composed of two extremely reactive and toxic elements, Sodium and Chlorine. Elemental sodium, which is a metal, burns violently upon contact with water and is typically stored in oil to prevent its reaction with humid air. Elemental Chlorine, under normal conditions exists as a gas, and is extremely toxic and was even used as chemical weapon during World War I.
Pepper on the other hand (or more specifically the black pepper shown in the SEM Image below) is not a mineral like salt but is actually a fruit. Black pepper is produced by first boiling and then drying the unripe fruit of a pepper plant. This causes the fruit to shrink and the fruit’s skin to harden and wrinkle in a fashion similar to that of a raisin, which is a dried grape. Once the pepper has fully dried and hardened it is then crushed into the powder that is found in your typical pepper shaker. It is this drying and wrinkling process that causes the unique shape and structure you see in the SEM image above on the right, and this grain of pepper is just one of many shattered pieces of pepper that once comprised a whole individual peppercorn.