Calcium White

“When it was built, the American presidential residence in Washington, DC, was coated in a damp-repellent mixture of slaked lime and glue, and people started to call it the White House. Tombs were likewise brushed with lime to protect them from the ravages of the weather.”

This is how the section on calcium compounds begins in Periodic Tales by Hugh Aldersey-Williams. (You're reading the fourth blog post on this book, here are the third, second and first.) What an interesting pair of facts – I did not realize that the reason for the white ‘paint’ had to do with the damp-repelling aspects of calcium oxide, more commonly known as lime. Actually calcium oxide works well as a desiccant because of its ability to absorb water. Apparently that is why it was used in burials, reducing the process of putrefaction and decay by absorbing moisture.

We are most familiar with a different white compound, calcium carbonate, also known as limestone. This is the white of chalk, majestically displayed in the white cliffs of Dover. (I’d like to see them with my own eyes someday.) The author makes the interesting observation that we think of writing mainly as black ink on white paper, but many centuries ago it was likely much easier to use white chalk to write on a dark surface. I first taught using chalk on a blackboard, but now use dry-erase markers on a white board. I’m not sure which is worst – chalk dust or organic fumes from the ink or solvent cleaner. That being said, the markers of today do not have the characteristic smell of its predecessors, and good quality white boards do not require frequent treatment and the solvent cleaner.

Chalk powder can be used as markers for any surface where their white contrasts with practically anything on the ground but snow. I learned that the Italian word for calcium, calcio (which spell-check annoyingly changed to ‘calico’ – good thing I noticed), is the same word for football (or soccer for U.S. folks). The author writes, “both meanings derive from the Latin calx, which is not only literal lime but also a metaphor for a goal, an achievement marked perhaps by a chalk line crossed.” Interestingly the root word also gave rise to calcination, meaning roasting in air, a word introduced by the alchemists. Calcination allows one to make lime from limestone, driving out the CO₂ from chalk (CaCO₃) to form lime (CaO). I bet you can write a balanced chemical equation for this process, now that you know the chemical formulae!

I’ve often used calcium carbonate as an example in my non-science major chemistry courses. It features prominently in the topic “Acids and Bases” because of its many applications as an acid neutralizer. I have a standard antacid demo with three flasks containing hydrochloric acid at a pH mimicking stomach acid. The flasks are mounted on stirplates, each with a magnetic stir bar. These give the aura of some exciting lab experiment – students every year think the magnetic stirrers are nifty gadgets! A drop of acid-base indicator is added to the solution. (I typically use bromothymol blue for this demo.) I get three student volunteers and each of them gets a mortar and pestle to grind up their chosen antacid. The students then get to wear safety goggles and lab gloves, and at the count of three they add their powdery substance to the flasks. Everyone watches with bated breath, but usually nothing happens. That’s because the reaction takes time (5-20 minutes depending on the starting material).

At this point I usually continue with the lesson, but the students (especially those in the front row) are keeping a close eye awaiting the colour change as the calcium carbonate neutralizes the base. In the meantime I’m describing why limestone near lakes acts as a buffer against acidification, and the process of liming a lake to prevent its pH from dropping due to acid rain and other processes. It’s also a good time to talk about the sources contributing to acid rain. We also talk about eating chalk if you were having acid reflux but your antacid was nowhere close at hand. At some point, one of the flasks will change colour before the others, leading to a flurry of excitement and cheers. It’s also a good time to discuss what an acid-base base indicator is, and why it changes colours based on pH.

In my General Chemistry course (for science majors), the decomposition of limestone into lime, while releasing carbon dioxide, is one of the early examples in “Equilibrium”. That’s because in this particular case, the equilibrium constant of the reaction is equal to the partial pressure of CO₂. It’s also a good initial system to discuss aspects of Le Chatelier’s Principle. Lime does come up when I’m discussing lattice energies, but up to this point I’ve failed to say anything interesting about it in class. Maybe I’m too focused on the principles behind lattice energies that I forget that calcium oxide is interesting in its own right.

The author pontificates: “Whiteness is freedom from colour and an escape from the rainbow chaos of life. Lime’s whiteness is a scourging simplicity, the purity of an ideal, the finality of a death. Whiting is the action of adding a layer of lime-wash, yet it is also a subtraction, a gesture towards liberation, a brushing away of the earth and the earthly, a disencumberance, a literal lightening and also the lightening of a load. The cleansing and preserving action of whitewashing ritually repeats the throwing of lime into the grave with the corpse. Our bodies decay, our bones are left, picked clean and bleached of all colour. We fade to white.”

But there is a glimmer of light: “Human intention lined in white is not always grimly fateful. Herman Melville in a chapter-long digression from the hunt for Moby-Dick meditates on how ‘whiteness refiningly enhances beauty, as if imparting some special virtue on its own, as in marbles, japonicas and pearls.’ Two of these three, it is no surprise to find, are calcium white. Japonica is the exception: white in nature where it is not mineral – real white horses, white bears, white elephants, the albino and the albatross – is attributable not to calcium but to the arrangement of organic matter in cells in such a way that it scatters light of all colours.”