December 30, 2015

Science Lesson from the Bilge

Earlier this month we introduced ‘Lec Lat readers to the curious, perfectly square crystals that we found growing in our boat’s bilge. We assumed they were salt crystals, of course, as there’d been a pinhole leak down there around a through-hull fastening. But we had no idea what made these bilge gemstones form into such perfect squares — flattened cubes, actually. So we sought help from our readership.

Bill Ogilvie, of the Cheoy Lee Clipper 36 Dragon Lady, offered this succinct explanation: "Salt crystals form into perfect cubes because the NaCl molecule packs so well. The space between the chlorine atoms is just the right size for the sodium atoms to occupy. This close, regular packing is what causes the perfect cube shape. The more salty water available the larger the size of the crystal."

Although we were a bit freaked out by discovering we’d had a leak in the bilge, we were absolutely fascinated by these ‘gemstones’, some of which were nearly two inches across.

latitude/andy
©Latitude 38 Media, LLC

That made the mystery a bit more understandable. But Charlie Ruppert’s explanation really got our ‘little gray cells’ buzzing: "A key word here is ‘crystal’ — as in ‘crystal lattice’. I believe a common feature of all crystalline solids, like NaCl (sodium chloride) is that the molecular arrangement of molecules within the solid is organized into a regular three-dimensional lattice structure, a regular repeating pattern. In the undergraduate crystallography course I took at Cal, we would measure these angles under the microscope to identify the mineral we were looking at (a salt crystal’s internal angle is apparently 90 degrees). The form of the crystal is determined by the chemical bonds (tiny electrical attractions) used to build the mineral (the crystal structure of a diamond, just carbon as a single element, is going to be different from the crystal structure of table salt, composed of a bond between two separate elements: a sodium and a chlorine atom).

"Small and large crystals are formed simply because new salt crystals coming out of solution in evaporating sea water need to go somewhere, and they would rather stick into formation (because of electrical charge) onto the nearest existing salt molecule rather than end up as an unattached bit of microscopic salt dust.

"So it looks like sodium and chlorine atoms will bond together when enough water evaporates — due to minute electrical charge — and combine and form a salt molecule, which itself would like to bond to another charged salt molecule, and so forth. Because of the asymmetry of the molecule — the salt molecule looks like a tiny battery: one side is positive and the other negative — every salt molecule is naturally attracted to another, but only at a very special orientation to each other.

"That much I recall — and is likely somewhat correct! But what I didn’t know is much more interesting: (from the Web): ‘Sodium chloride together with the next four most abundant salts comprise more than 99% of all dissolved substances in the sea. Although only eight elements make up these five most abundant salts, seawater contains all of Earth’s other naturally occurring elements.’ And, ‘Individual atoms are dissolved in sea water, not salts.’"

Who knew you could discover such fascinating science by lifting up a bilge board? Here’s wishing you an enlightening year ahead.

As every experienced sailor knows, bad things can happen when you don’t — or aren’t able to — keep a constant lookout while underway.
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