Why are polar substances soluble in water

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Properties of substances made up of molecules 19 All substances that are gaseous at room temperature consist of small molecules (exception: noble gases, which only consist of individual atoms). Substances made up of larger molecules are liquid or solid. Normally, molecular substances can be melted and evaporated. They are therefore also called volatile substances because they evaporate when heated, that is to say "escape". Molecules are not electrically charged and therefore do not conduct electricity. Movable electrical charge carriers are necessary to transport electricity. Substances made from molecules are therefore used as insulators. Both air and plastics are the most important insulators in electrical engineering. The non-metal atoms are not all equally strong in their attraction to electrons. Fluorine, oxygen, nitrogen and chlorine are the nonmetallic atoms with the strongest, hydrogen is the nonmetal with the weakest attraction for electrons. So if hydrogen is combined with oxygen in water, the hydrogen does not completely lose its valence electrons, as would a metal atom, but the electrons of the binding electron pair are closer to oxygen than to hydrogen. This creates electrical charges in the molecule, albeit weaker than in the case of ionic bonds. The water molecule has negative partial charges at the oxygen end and positive partial charges at the hydrogen end (Fig. 19.2). Such molecules with differently charged ends are called dipole molecules (di = two, ie a molecule with two different sides = poles). Substances with dipole molecules are called polar substances. Dipole molecules attract each other more strongly than molecules without these properties, which are called nonpolar. Water-soluble and water-insoluble substances Polar molecules adhere tightly to one another. Therefore polar substances have high melting and boiling points, are often water-soluble (hydrophilic; Greek: hydro = water, phil = loving) and not fat-soluble. Non-polar molecules adhere only weakly to one another. Therefore, non-polar substances have low melting and boiling points, are not water-soluble (hydrophobic; Greek: phobos = hostile) but are often fat-soluble. Polar substances (eg salt, sugar) dissolve in polar solvents (eg water), non-polar substances (eg fat) in non-polar solvents (eg gasoline). 2.6 Properties of substances from molecules H O H Fig. 19.2: In the water molecule, the oxygen attracts the electrons. Fig. 19.1: Common electrons are not always evenly distributed between the atoms involved. Common electrons H H H Cl ä ä Water Ethanol Water Gasoline Polar substance Non-polar substance Polar substance Polar substance Fill each 4 sample tubes approx. 1/3 with water or gasoline. Now 4 substances are examined for their solubility in water and gasoline. Add a) some common salt crystals, b) some sugar crystals, c) some iodine crystals and d) a few drops of cooking oil into a test tube filled with water or gasoline. Seal each with a stopper and shake vigorously. Make a note of your observations in the illustration on the right. Experiment 19.1 Miscible and immiscible liquids Fig. 19.4: Polar substances mix. A non-polar substance remains separate from a polar one. Fig. 19.3: forces of attraction between non-polar molecules and between polar molecules non-polar substance polar substance low forces of attraction high forces of attraction butane lighter gas water common salt water petrol sugar iodine cooking oil common salt sugar iodine cooking oil atomic lattice For testing purposes only - property of the publisher öbv

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