Either the taking up of energy from radiation by the medium through which radiation is passing, or the taking up of matter in bulk by other matter. A simple example of the absorption of energy is how sunlight warms a tree's leaves (see illustration). Wavelengths of green light are reflected by molecules in the leaves, making them appear green, while other wavelengths of light are absorbed for use in photosynthesis, with some energy being converted into heat. A simple example of absorption by matter of other matter is the dissolving of carbon dioxide gas into water to create soda water. See also: Carbon dioxide; Chemistry; Energy; Photosynthesis; Physics; Tree; Water

A geminal diether (R¹ = H). Ketals, considered a subclass of acetals, are also geminal diethers (R¹ = C, aliphatic or aromatic). Acetals are (1) independent structural units or a part of certain biological and commercial polymers, (2) blocking or protecting groups for complex molecules undergoing selective synthetic transformations, and (3) entry compounds for independent organic chemical reactions.

A common medicine used to relieve mild to moderate pain and reduce fever. Acetaminophen (N-acetyl-p-aminophenol), or paracetamol, is a white crystalline compound with the molecular formula C₈H₉NO₂ (Fig. 1). It is a member of the aniline family of analgesics (acting to relieve pain) and antipyretics (acting to prevent or reduce fever). Three types of nonopioid analgesics exist: aniline derivatives (such as acetaminophen), nonsteroid anti-inflammatory drugs (NSAIDs, such as aspirin, ibuprofen, and naproxen sodium), and nonacidic pyrazole derivatives (such as phenylbutazone and dipyrone). NSAIDs and pyrazole drugs are both used to relieve pain, reduce fever, and reduce inflammation, while pyrazole drugs can also be used for their spasmolytic (acting to relieve muscle spasms) properties. Aniline derivatives exhibit analgesic and antipyretic, but not anti-inflammatory, properties. See also: Analgesic; Aspirin; Fever; Inflammation; Medicine; Pain; Pharmacology; Pharmacy

A colorless organic acid, CH₃COOH, with a pungent odor and sour taste. Acetic acid (see figure) is the principle component of vinegar. Concentrated acid is called glacial acetic acid because of its readiness to crystallize at cool temperatures. See also: Acid and base; Organic chemistry; Vinegar

A chemical compound, (CH₃)₂CO; the first member of the homologous series of aliphatic ketones. Acetone (see illustration) is a colorless, volatile, and flammable liquid. Acetone occurs naturally in plants and animals. Its physical properties include boiling point 56.2°C, melting point −94.8°C, and specific gravity 0.791. Acetone is an extremely important, low-cost raw material that is used for production of other chemicals. See also: Ketone

An organic compound with the formula C₂H₂ or HCCH. The first member of the alkynes, acetylene is a gas with a narrow liquid range; the triple point is −81°C (−114°F). The heat of formation (ΔH°_f) is +227 kilojoules/mole, and acetylene is the most endothermic compound per carbon of any hydrocarbon. The compound is thus extremely energy-rich and can decompose with explosive force. At one time acetylene was a basic compound for much chemical manufacturing. It is highly reactive and is a versatile source of other reactive compounds.

In the Brönsted classification, an acid is any chemical species, ionic or molecular, capable of donating a proton (hydrogen ion) to another substance; the other substance acts as a base in accepting the proton. For these two interrelated classes of chemical compounds, the precise definitions have varied considerably with the development of chemistry. However, the Brönsted classification is the most used.

One of an important class of reactive organic compounds derived from acids via formal intermolecular dehydration; thus, acetic acid (1), on loss of water, forms acetic anhydride (2).

Chemical compound that changes in physical appearance at a certain concentration of hydrogen cations, or pH. Acid-base indicators, or pH indicators, are used in various industries, such as chemistry labs and medical offices, to determine the concentration of hydrogen cations (H⁺), also known as pH range, of a solution. Acid-base indicators can be any chemical compound or material that changes in physical appearance, typically color, with a change in the concentration of H⁺ ions in solution (Fig. 1). This change occurs due to a large amount of H⁺ present in solution (a low pH) or a small amount of H⁺ in solution (a high pH). Indicators can therefore only reveal a pH range, rather than an exact pH value. Acid-base indicators may be natural or synthetic. Most indicators are weak organic acids used in medical, industrial, and laboratory settings. See also: Hydrogen; pH

One of a large group of organic substances possessing the halocarbonyl group