Barium

Barium is the element where solubility is the whole story. Barium sulfate is so insoluble — a solubility product around 10⁻¹⁰ — that a patient can drink a litre of it as a "barium meal" and pass it through unchanged, while it blocks X-rays well enough to outline the gut in sharp white. Barium chloride, which dissolves freely, is a potent poison that blocks potassium channels and stops the heart. The metal is named from Greek barys, heavy, and Humphry Davy isolated it in 1808 by electrolysis; today most barium is consumed not in medicine but as drilling mud, where finely ground barite weights the fluid column enough to keep an oil well from blowing out.