Potassium (K, Z = 19).

Potassium is a soft, silvery metal that reacts extremely vigorously with water, and tarnishes rapidly in air. Its name is derived from the English word “potash,” for potassium carbonate, a compound found in high concentrations in wood ashes. The symbol “K” is derived from the Latin name for the element, kalium.

Potassium is essential for plant growth, and is heavily used in fertilizers. In the body, potassium plays a vital role in the contraction of muscle tissue; the movement of sodium and potassium ions in nerve cells plays a major role in the transmission of nerve impulses.

Potassium chloride, KCl, is used in salt substitutes (mixed with sodium chloride to improve its flavor), and in fertilizers; massive amounts of potassium chloride are used in lethal injections to cause rapid death by cardiac arrest. Potassium carbonate, K₂CO₃, also known as potash, is used in the manufacture of glass. Potassium superoxide, KO₂, reacts with carbon dioxide to produce potassium carbonate and oxygen gas; it is used in rebreathers and respiration equipment to generate oxygen, and is also used in mines, submarines, and spacecraft.

Calcium (Ca, Z = 20).

Calcium is a silver-colored, relatively soft metal. The name of the element is derived from the Latin word for lime, calx. It is found in the Earth’s crust at a concentration of 4.1%, making it the 5^th most abundant element. 

Calcium salts forms the hard parts of the bodies of most living creatures, from the shells of marine organisms and the coral of coral reefs (in the form of calcium carbonate, CaCO₃) to the bones and teeth of land-dwelling creatures (in the form of hydroxyapatite crystals, [Ca₃(PO₄)₂]₃·Ca(OH)₂).

Calcium chloride is a deliquescent (it absorbs enough water from the air that it dissolves in the solution), and is used to remove moisture from the air in damp basements.

“Hard water” contains dissolved minerals having +2 or +3 charges, such as calcium and magnesium; these salts cause some soaps and detergents to precipitate out as “soap scum”; these minerals precipitate out over time to form “scale” in water heaters and pots. The calcium can be removed by water softeners, which exchange the calcium ions for sodium ions, which have +1 charges, and do not readily precipitate out.