H, or h, is the eighth letter in the ISO basic Latin alphabet. Its name in English is aitch , or regionally haitch /ˈheɪtʃ/.
The original Semitic letter Heth most likely represented the voiceless pharyngeal fricative (ħ). The form of the letter probably stood for a fence or posts.
The Greek Eta 'Η' in archaic Greek alphabets, before coming to represent a long vowel, /ɛː/, still represented a similar sound, the voiceless glottal fricative /h/. In this context, the letter eta is also known as Heta to underline this fact. Thus, in the Old Italic alphabets, the letter Heta of the Euboe…
For most English speakers, the name for the letter is pronounced as /eɪtʃ/ and spelled "aitch" or occasionally "eitch". The pronunciation /heɪtʃ/ and the associated spelling "haitch" is often considered to be h-adding and is considered non-standard in England. It is, however, a feature of Hiberno-English, as well as scattered varieties of Edinburgh, England, and Welsh English, and in Australia and Nova Scotia.
In English, ⟨h⟩ occurs as a single-letter grapheme (being either silent or representing the voiceless glottal fricative (/h/) and in various digraphs, such as ⟨ch⟩ /tʃ/, /ʃ/, /k/, or /x/), ⟨gh⟩ (silent, /ɡ/, /k/, /p/, or /f/), ⟨ph⟩ (/f/), ⟨rh⟩ (/r/), ⟨sh⟩ (/ʃ/), ⟨th⟩ (/θ/ or /ð/), ⟨wh⟩ (/hw/ ). The letter is silent in a syllable rime, as in ah, ohm, dahlia, cheetah, pooh-poohed, as well as in certain other words (mostly of French origin) such as hour, honest, herb (in American but not British English) and vehicle (in certain varie…
• H with diacritics: Ĥ ĥ Ȟ ȟ Ħ ħ Ḩ ḩ Ⱨ ⱨ ẖ ẖ Ḥ ḥ Ḣ ḣ Ḧ ḧ Ḫ ḫ ꞕ Ꜧ ꜧ
• IPA-specific symbols related to H: ʜ ɦ ʰ ʱ ɥ ᶣ ɧ
• ꟸ: Modifier letter capital H with stroke is used in VoQS to represent faucalized voice.
• ᴴ : Modifier letter H is used in the Uralic Phonetic Alphabet
• American Sign Language grammar
• List of Egyptian hieroglyphs#H
• The dictionary definition of H at Wiktionary
• The dictionary definition of h at Wiktionary
• Lubliner, Coby. 2008. "The Story of H." (essay on origins and uses of the letter "h")
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula H2. It is colorless, odorless, tasteless, non-toxic, and highly combustible. Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all normal matter. Stars s…
Large quantities of H2 are used in the "upgrading" of fossil fuels. Key consumers of H2 include hydrodealkylation, hydrodesulfurization, and hydrocracking. Many of these reactions can be classified as hydrogenolysis, i.e., the cleavage of bonds to carbon. Illustrative is the separation of sulfur from liquid fossil fuels:
R2S + 2 H2 → H2S + 2 RH
H2 is a product of some types of anaerobic metabolism and is produced by several microorganisms, usually via reactions catalyzed by iron- or nickel-containing enzymes called hydrogenases. These enzymes catalyze the reversible redox reaction between H2 and its component two protons and two electrons. Creation of hydrogen gas occurs in the transfer of reducing equivalents produced during pyruvate fermentation to water. The natural cycle of hydrog…
Hydrogen poses a number of hazards to human safety, from potential detonations and fires when mixed with air to being an asphyxiant in its pure, oxygen-free form. In addition, liquid hydrogen is a cryogen and presents dangers (such as frostbite) associated with very cold liquids. Hydrogen dissolves in many metals and in addition to leaking out, may have adverse effects on them, such as hydrogen embrittlement, leading to cracks and explosions. Hydrogen gas leaking into externa…