The chemical compounds

A chemical compound is a pure substance consisting of atoms of two or more chemical elements, whereby - in contrast to mixtures - the types of atoms are in a fixed ratio to each other. The numerical ratio of the atoms to each other is determined by chemical bonds between the atoms, and the ratio can be represented in a sum formula. However, a molecular formula is often ambiguous, since isomeric compounds have the same molecular formula but are different compounds. Each chemical compound is characterized by its unique chemical structure.

Among the more than 100 million chemical compounds known (as of June 2015), a distinction is made between roughly ionic or salt-like compounds and complex, metallic and molecular compounds. The subdivision inorganic or organic is also fundamental, whereby “organic” - with a few exceptions - refers to the carbon compounds. There are significantly more organic compounds than inorganic compounds. The more than 6 million organic compounds are contrasted by less than 100,000 compounds of an inorganic nature.

Basically, there are four types of chemical compounds with regard to the type of bond between the elements involved:

A more precise distinction between compounds and their assignment to one of these four types can be made with the help of the electronegativity difference of the elements involved in the bond. There are also transitional forms between the four ideal types mentioned above.

Alumina

Chemical properties Two modifications are known of alumina. The α form forms crystals with high hardness. This modification is present in mineral corundum and is required for the production of abrasives. It is neither soluble in water nor in acids or alkalis. The hardness according to Mohs is 9 to 9.5. The γ form forms a white powder that attracts water and is hygroscopic and dissolves in acids and alkalis. This form is also known as alumina, an important raw material for the production of ceramics and aluminium.

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Aluminium Chloride

Aluminium chloride is an inorganic chemical compound; it is the chloride of aluminium with the molecular formula AlCl3. Chemical properties The anhydrous aluminium chloride in particular likes to attract water and dissolve in the process; it is highly hygroscopic. It smokes in the air and decomposes to form hydrogen chloride. Due to contamination with ferric chloride, the colourless crystals are often light yellow. They react very violently when heated strongly with water to form hexahydrate.

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Aluminium sulphate

Aluminium sulphate, molecular formula Al2(SO4)3, is a chemical compound of aluminium from the group of sulphates. It forms a colourless powder with a density of 2.71 g/cm3. Chemical properties Octadecahydrate, which is more commonly used in the laboratory, forms white crystals that dissolve very well in water. They are insoluble in ethanol. The aqueous solution reacts acidically. Due to this reaction, an acid is formed during eye contact, which can damage the cornea.

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Ammonium carbonate

Chemical properties The salt forms a colourless powder with a faint smell of ammonia. It has a density around 1,6 g-cm-3 (at 20 °C). In aqueous solution it reacts weakly basic, a balance is established between ammonia NH3, ammonium ions NH4+, carbonate ions CO32-, hydrogen carbonate ions HCO3- and carbon dioxide CO2. The pH value of a 10% aqueous solution at 25 °C is 9.4. The salt thus reacts with water to form hydrogen carbonate and hydroxide ions.

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Ammonium chloride

Chemical properties Ammonium chloride forms colorless, octahedral crystals that smell of ammonia. When heated in a test tube, the ammonium chloride appears to sublimate. In reality, however, it decomposes at 338 °C. In the presence of air humidity above 350 °C, the substance decomposes completely into ammonia and hydrogen chloride. The ammonia molecules escape faster than the hydrogen chloride molecules due to their lower mass. Therefore the indicator paper turns blue-green (ammonia) in the upper area and red (hydrogen chloride) in the lower area.

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Ammonium dichromate

Chemical properties Ammonium dichromate forms odourless, orange crystals. When heated to 180°C, it decomposes, developing nitrogen. However, it can also explode due to strong friction. As a strong oxidant, it reacts explosively with metal powders, sulphur or phosphorus. The “volcano test” was formerly demonstrated in schools. A small pile of ammonium dichromate was ignited at the tip with a burner. The reaction continues on its own, the orange-coloured substance is transformed into a grey-green substance under lively spraying.

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ammonium iron(II) sulphate

Ammonium iron(II) sulphate (formerly also Mohr’s salt) is the sulphate of ammonium and iron, it forms water-soluble, light green monoclinic crystals. It is usually commercially available with six molecules of water of crystallization as ammonium iron(II) sulfate hexahydrate. It is a double salt from the group of Tutton salts. Chemical properties Ammonium iron(II) sulphate is mainly used as hexahydrate. The light green crystals are very soluble in water and form an acidic solution.

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Ammonium nitrat

Chemical properties Ammonium nitrate forms colourless hygroscopic crystals that melt at 169.6 °C. The solid can be in five different polymorphic crystal forms, with transformation temperatures at -16.9 °C, 32.3 °C, 84.2 °C and 125.2 °C. The tendency of ammonium nitrate crystals to cake together is mainly due to the first two phase transformations near room temperature. The phase transition between the polymorphs IV and III at 32.2 °C is relevant for the handling and storage of the substance.

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Ammonium sulphate

Chemical properties The clear ammonium sulphate crystals, which appear white when finely dispersed, dissolve well in water with a slightly acidic reaction. When heated to 235 °C, the ammonia gas is split off, forming ammonium hydrogen sulphate. On further heating above the decomposition point of 280 °C, the intermediate product also decomposes to ammonium disulfate (NH4)2S2O7. When mixed with nitrites, decomposition can be explosive. Production Ammonium sulphate is formed when ammonia is introduced into sulphuric acid in a highly exothermic reaction.

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Ammonium thiocyanat

Chemical properties Ammonium thiocyanate forms colourless, monoclinic crystals which appear white in the crystalline powder. They are highly soluble in water and ethyl alcohol. The substance is strongly hygroscopic, the crystals dissolve in air. When heated to 70 to 120 °C thiourea is formed. When heated strongly to over 170 °C, decomposition takes place and hydrogen cyanide, ammonia and nitrogen oxides are formed. Mixing with barium hydroxide produces an efficient cooling mixture.

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Ammonium thiosulfat

Chemical properties Ammonium thiosulfate forms colorless, elongated crystals. The commercially available crystalline powder appears white. The hygroscopic salt is very soluble in water, it dissolves very little in ethyl alcohol and not at all in organic solvents. Ammonium thiosulfate is chemically more reactive than sodium thiosulfate. It decomposes already at heat and in warm, concentrated, aqueous solutions with yellowing and formation of sulphur. Strong heating to over 150 °C also produces sulphur dioxide, ammonia and nitrogen oxides.

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Ammoniumcer(IV)-nitrat

Chemical properties Ammonium cer(IV) nitrate is an orange, crystalline powder that dissolves well in water, the solution reacts acidically. The substance is a strong oxidizing agent, dangerous reactions are possible when mixed with flammable substances. Sudden decomposition can also occur when heated strongly. Adding a small amount of ammonium cerium(IV) nitrate to 20% nitric acid gives a light yellow solution. With alcohols the reagent turns orange-red. The reaction takes place with any substance from the group of alcohols.

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Ammoniumhydrogencarbonat

Chemical properties Ammonium hydrogen carbonate forms colourless crystals according to the orthorhombic crystal system. Compared to the similar ammonium carbonate, the white crystalline powder appears more coarse-grained. The salt dissolves well in water, the solubility increases with increasing temperature. The aqueous solution reacts slightly basic. Ammonium hydrogen carbonate which is not too old is odourless and relatively stable, decomposition takes place only very slowly at room temperature. When heated, it decomposes to ammonia, carbon dioxide and water.

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Ammoniummolybdat

Chemical properties Ammonium molybdate tetrahydrate is marketed as a white, crystalline powder. It is also known as ammonium heptamolybdate. The tetrahydrate forms clear, colourless or slightly yellow-green crystals as six-sided prisms. When heated to over 90 °C the crystals release their water of crystallization. The salt dissolves well in the water to form an acidic solution. Ammonium molybdate weathers in the air, releasing ammonia. With phosphate ions it forms a yellow precipitate after previous treatment of the sample with nitric acid in aqueous solution.

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antimony (III) oxide

Chemical properties Antimony(III) oxide occurs as a natural compound in the form of the minerals senarmontite and valentinite. Antimony(III) oxide is a white, crystalline powder that is insoluble in water. It dissolves in concentrated acids and alkalis. Antimony(III) oxide occurs in nature like arsenic(III) oxide in two mineral modifications: Cubic sénarmontite is transformed into orthorhombic valentinite when heated to 570 °C. This mineral is also known as “white-spit luster”. When heated openly in air, antimony (III) oxide oxidizes to antimony tetraoxide Sb2O4 under oxygen uptake.

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antimony (III) sulfide

Chemical properties Antimony(III) sulfide occurs in two modifications: The orange-red, amorphous form is not stable. When heated in the absence of oxygen, the dark grey, crystalline form is obtained. When heated vigorously in air or in an oxygen stream, antimony(III) sulfide oxidizes to antimony(III) oxide and sulfur dioxide. In boiling water or with water vapor it slowly decomposes to antimony(III) oxide and hydrogen sulfide. Explosive reactions may occur with oxidizing agents such as potassium chlorate or silver oxide.

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arsenic (III) oxide

Chemical properties Arsenic(III) oxide is an odourless white powder which is only slightly soluble in cold water and more soluble in hot water. The aqueous solution has a sweetish metallic taste and reacts acidic: it contains arsenic acid H3AsO3, which is only stable in the aqueous solution. The solubility is better in alkaline solutions because arsenites, the salts of arsenious acid, are formed. In acidic solutions the solubility is worse. In concentrated hydrochloric acid the solubility is good again.

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Barium chloride

Chemical properties Anhydrous barium chloride is marketed as a white, crystalline powder. It tastes bitter and is toxic when eaten or inhaled. When crystallized from an aqueous solution, the dihydrate is obtained in colorless, flat sheets. When heated above 120 °C, the dihydrate releases its water of crystallization. Barium chloride, like barium and all its salts, has a green flame coloration, is highly soluble in water and, like all soluble barium compounds, is toxic.

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Bariumnitrat

Chemical properties In nature, barium nitrate is found in the mineral nitrobarite, which forms clear, colourless crystals. A white, crystalline powder is commercially available, which is only moderately soluble in cold water. When heated to 100 °C, the solubility increases by about three times. Barium nitrate is hygroscopic, it attracts water from the air. When heated above the melting point, it decomposes with green glow to oxygen, barium peroxide, nitrogen and nitrogen monoxide.

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lead(II) nitrate

Chemical properties Lead(II) nitrate, a white crystalline solid, is the lead(II) salt of nitric acid. Lead(II) nitrate is a strong oxidizing agent. Unlike other lead salts, it is highly soluble in water. For this reason it is toxic. Unlike many other lead(II) salts such as lead(II) chloride or lead(II) sulphate, lead(II) nitrate is readily soluble in water. If lead(II) nitrate is heated up to its decomposition temperature of 470 °C, it decomposes to form nitrogen dioxide, oxygen and lead(II) oxide.

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