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Solubility

  • Page ID
    2356
  • One of the general properties of ionic compounds is water solubility. The oceans are solutions of salt in water. In a mixture, two or more materials are mixed together but they remain essentially separate, like sand and water. The sand can be easily distinguished from the water, because even if a mixture of the two is shaken it will spontaneously separate over time.

    ICsaltwater1.gif

    Figure IC4.1. A mixture of an insoluble salt (orange and green ions) and water (blue molecules). The two components remain separate from each other.

    In a suspension, one or more materials is mixed into a liquid, and the mixture becomes somewhat homogeneous. Instead of having easily identifiable layers, the liquid has a uniform appearance throughout. However, suspensions are generally cloudy liquids. Milk is a suspension containing water, fats and proteins. They may settle out into separate layers eventually, but it takes time.

    In a solution, one or more materials is mixed into a liquid, and the mixture becomes a completely homogeneous liquid. Solutions are transparent, not cloudy, and can be colored or colorless. Saltwater, an example of a solution, is diagrammed in the figure below.

    Figure IC4.2. A solution of a salt (orange and green ions) in water (blue molecules). The ions of the salt are completely distributed throughout the water.

    Pieces of salt are not visible in the solution; the salt particles are too small. The salt is separated into individual ions, surrounded by water molecules. This change from Figure IC4.3 to Figure IC4.2 is not instantaneous upon adding salt to water; stirring is required to produce the solution.

    Figure IC4.3. A mixture of a salt (orange and green ions) and water (blue molecules). The salt is beginning to dissolve in the water.

    Eventually more of the salt dissolves in the water, as shown:

    Figure IC4.4. A mixture of a salt (orange and green ions) and water (blue molecules). The salt continues to dissolve in the water.

    If enough salt is added, the system might come to "equilibrium": the water has dissolved all of the salt that it can, so the rest of the salt remains solid. This equilibrium is "dynamic": ions are dissolved in the water at the same time that ions are deposited from solution into the solid state. However, the overall ratio of dissolved ions to water stays the same.

    Figure IC4.3. A mixture of a salt (orange and green ions) and water (blue molecules). The salt is partly dissolved in the water but has reached equilibrium.

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