A type of presumptive test in which a substance is identified by the formation of characteristic crystals when a certain reagent is added. Usually, such tests are conducted using a microscope (microcrystal test). An example is the acetone-chlor-haemin test for blood.
أكمل القراءة »curare
A resin obtained from the bark of South American trees of the genera Strychnos and Chondrodendron that causes paralysis of voluntary muscle. It acts by blocking the action of the neurotransmitter acetylcholine at neuromuscular junctions. Curare is used as an arrow poison by South American Indians and was formerly used …
أكمل القراءة »Curie’s law
The susceptibility (X) of a paramagnetic substance is proportional to the thermodynamic temperature (T), i.e. x = C/T, where C is the Curie constant. A modification of this law, the Curie-Weiss law, is more generally applicable. It states that X = C/(T- θ), where θ is the Weiss constant, a …
أكمل القراءة »cycloaddition
A reaction in which two or more unsaturated compounds form a cyclic adduct or in which a cyclic compound is formed by addition between unsaturated parts of the same molecule. In cycloaddition, there is no net reduction in bond multiplicity. The Diels-Alder reaction is an example. Cycloadditions may be stepwise …
أكمل القراءة »cyclophane
A compound consisting of one or more aromatic rings forming part of a larger ring system in which aliphatic chains of the CH2 groups link the aromatic rings. Compounds of this type have the suffix phane in their names. Depending on the sizes of the (CH2)11 chains, the aromatic rings …
أكمل القراءة »buffer
A solution that resists change in pH when small amounts of an acid or alkali are added over a certain range or when the solution is diluted. Acidic buffers consist of a weak acid with a salt of the acid. The salt provides the negative ion A-, which is the …
أكمل القراءة »buna rubber
A type of synthetic rubber based in polymerization of butadiene (buta-1,3-diene). The name comes from Bu (for butadiene) and Na (for sodium, which was used as a catalyst in the original polymerization reaction). An improved form, known as Buna-S was developed by copolymerizing butadiene with styrene. In 1934, Buna-N was …
أكمل القراءة »Butler-Volmer equation
An equation for the rate of an electrochemical reaction; it describes the current density at an electrode in terms of the overpotential. The Butler-Volmer equation is given by: j = ja – jc = je[exp(1 – a)Fη/RT- exp(- aF/RT)] where ja and jc are the individual cathode and anode currents …
أكمل القراءة »(B-Z reaction (Belousov-Zhabotinskii reaction
A chemical reaction that shows a periodic colour change between magenta and blue with a period of about one minute. It occurs with a mixture of sulphuric acid, potassium bromate(V), cerium sulphate, and propanedioic acid. The colour change is caused by alternating oxidation-reductions in which cerium changes its oxidation state …
أكمل القراءة »anabolic steroids
Steroid hormones related to the male sex hormone testosterone. They promote the development of masculine characteristics and increase muscle growth. Anabolic steroids have been used medically for various conditions but are also used illegally by sportsmen and women and by bodybuilders. They have a number of deleterious side effects and …
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