what do the letters hb stand for on a pencil
Pencil Most manufacturers, and almost all in Europe, designate their pencils with the letters H (commonly interpreted as "hardness") to B (commonly "blackness"), as well as F (usually taken to mean "fineness", although F pencils are no more fine or more easily sharpened than any other grade. also known as "firm" in Japan[68]). The standard writing pencil is graded HB.[69] This designation might have been first used in the early 20th century by Brookman, an English pencil maker. It used B for black and H for hard; a pencil's grade was described by a sequence or successive Hs or Bs such as BB and BBB for successively softer leads, and HH and HHH for successively harder ones.[70] The Koh-i-Noor Hardtmuth pencil manufacturers claim to have first used the HB designations, with H standing for Hardtmuth, B for the company's location of Budějovice, and F for Franz Hardtmuth, who was responsible for technological improvements in pencil manufacture.[71][72]
Rubidium hydride Rubidium hydride is the hydride of rubidium. It has the formula RbH and is an alkali metal hydride. It is synthesized using rubidium metal to react with hydrogen gas. As a hydride of an alkali metal, it is reactive towards even weak oxidizing agents. A redox reaction will occur with chlorine or fluorine and a lot of heat will evolve. Rubidium hydride will react violently with water or air and careful storage is necessary.