Dictionary, Defintion, Meaning, What is Depression of Freezing Point, Destructive Distillation, Density, Depillatories, Depolymerization, Depot Fat

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Density. (Symbol r) The amount of mass in a unit volume of the substance.
Density = mass/volume
The units are generally grams per cubic centimeter (g/cm³ or gcm^-3).

Depilatories. Materials used for hair removal. The most widely used is calcium thioglycolate in an alkaline medium at ph 12.3.

Depolymerization. The breaking down of polymers into their original monomers; the reverse of polyerization.

Depot fat. The reserve of fat in animals deposited in adipose tissue and which usually undergoes metabolism slowly.

Depression of freezing point. The reduction in the freezing point of a pure liquid when another substance is dissolved init. It is a colligative property—i.e. the lowering of the freezing point is proportional to the number of dissolved particles (molecules or ions), and does depend on their nature. It is given by Δ_t = K_fC_m, where C_m is the molar concentration of dissolved solute and K_f is a constant (the cryoscopic constant) for the solvent used. Measurements of freezing-point depression (using a Backmann thermometer) can be used for finding relative molecular masses of unknown substances.

Solvent	Molecular depression constant (0C/mole)
Water	1.86
Ethanoic acid	3.90
Benzene	4.90
Cyclohexane	20.5

Derivative. A compound derived from some other compound, usually keeping the general structure of the parent compound e.g. phenol is a derivative of benzene.

Desalination. The removal of salts from sea water or brine so as to make it fit for human use.

Desiccant. A materials used for drying. Some desiccants can react chemically with specimens being dried. Used in a desiccator. In order of effectiveness common desiccants are

CaCl₂<CaO<NaOH<MgO<CaSO₄<H₂SO₄<silica gel<Mg(ClO₄)₂<P₂O₅
Desiccator. A glass container used in the laboratory to keep the solids free of moisture. It generally contains calcium oxide or salica gel to absorb moisture.

Destructive distillation. The distillation of organic solids or liquids in which the substance being heated or wholly decomposes during distillation, leaving a solid or viscous liquid in the still. Valtatile liquid products are collected after condensation. Typical destructive distillation process involve the carbonization.

Desulphurization. The removal of sulphur compounds from petroleum fractions using hydrogen in the presence of a catalyst.

Detergent oil. Lubrication oil containing detergent of dispersant additives used in internal combustion engines in order to stabilize solid contaminants in a finely-divided state, and so prevent abrasion and accumulation of deposits.

Additives used include alkaline earth metal salts of petroleum sulphonic acids, alkyl phenol sulphides, etc.

Detergent. A substance added to water to improve its cleaning properties. Although water is a powerful solvent for many compounds, it will not dissolve grease and natural oils. Detergent are compounds that cause such non-polar substance to go into solution in water. Soap is the original example, owing its action to the presence of ions formed from long-chain fatty acids (e.g. the octadecanoate (stearate) ion, CH₃ (CH₂)₁₆ COO^-). These have two parts: a polar part (the hydrogen chain), which attaches to the grease; and a polar part (the hydrocarbon chain), which attaches to the grease; and a polar part (the —COO^- group), which is attracted to the water.

A disadvantage of soap is that it forms a scum with hard water(see hardness of water) and is relatively expensive to make. Various synthetic (‘soapless”) detergents have been developed from petrochemicals. The commonest, used in washing powders, is sodium Dodecyl-benzenesulphonate, which contains CH₃(CH₂)₁₁C₆H₄SO₂O^- ions. This, like soap, is an example of an anionic detergent, i.e. one in which the active part is a negative ion. Catlonic detergents have a long hydrocarbon chain connected to a positive ion. Usually they are amine salts., as in CH₃(CH₂)₁₅N(CH₃)₃+Br^-, in which the polar part is the ¾N(CH3)3+group. Nonionic detergents have nonionic polar groups of the type —C₂H₄—O—C₂H₄—OH, which form hydrogen bonds with the water. Synthetic detergent are also used as wetting agents, emulsifiers, and stabilizers for foam.
Dotinning. The recovery of metallic sin from scraption-plate by the action of chlorine that combines with tin to form volatile stannic chloride SaCl₂
Detonating gas.
A mixture of H₂ andO₂ (molar ratio 2: 1) produced by electrolysis of water and which explodes violently to reform H₂O on ignition.

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