Chemical substance In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Generally, these are called isomers. Isomers usually have substantially different chemical properties, and often may be isolated without spontaneously interconverting. A common example is glucose vs. fructose. The former is an aldehyde, the latter is a ketone. Their interconversion requires either enzymatic or acid-base catalysis. However, tautomers are an exception: the isomerization occurs spontaneously in ordinary conditions, such that a pure substance cannot be isolated into its tautomers, even if these can be identified spectroscopically or even isolated in special conditions. A
Prefixes in Hebrew Prefixes in Hebrew There are several prefixes in the Hebrew language which are appended to regular words to introduce a new meaning. In Hebrew, the letters that form those prefixes are called "formative letters" (Hebrew: אוֹתִיּוֹת הַשִּׁמּוּשׁ, "Otiyot HaShimush"). Eleven of the twenty-two letters of the Hebrew alphabet are considered "Otiyot HaShimush". These letters are Aleph (א), Bet (ב), He (ה), Vav (ו), Yud (י), Kaf (כ), Lamed (ל), Mem (מ), Nun (נ), Shin (ש), and Tav (ת). A mnemonic to remember these letters is איתן משה וכלב ("Eitan, Moshe, v'Kalev"), which translates to "Ethan, Moses, and Caleb." Prefixes in