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Reading Selection, Lesson 15

Separating Solids From Liquids


When separating substances, it is important to choose the correct separation technique, which is a method used to separate the components of a mixture from each other. For example, insoluble solids can be separated from liquids in several different ways. The technique used depends on how well substances are mixed together.

To separate insoluble impurities from salt, you used a process called filtration. The filter paper allowed the soluble solute (salt) and the solvent (water) to pass through, but it trapped the larger pieces of insoluble impurities as a residue. The substances that pass through the filter paper are called the filtrate.

A tabletop centrifuge with its cover open.

A tabletop centrifuge.

Large pieces of insoluble substances will often settle out of a mixture of a solid and a liquid. This process is called sedimentation, because the solid forms a sediment on the bottom of the container. If the solid is very fine, this process can be speeded up with a machine called a centrifuge .   Centrifuges spin a test tube very fast, and the solid moves quickly to the bottom of the test tube.

Potassium ferricyanide crystals

These potassium ferricyanide crystals were produced when the water in a saturated solution of potassium ferricyanide was slowly evaporated.

To separate a solid solute from a solvent (like salt from water), you used evaporation. At room temperature, water evaporates from a solution very slowly. But the rate of evaporation can be accelerated by heating the solution. As the water evaporated from your salt and water solution, the solution became more concentrated. Eventually, a saturated solution of salt formed. As more water evaporated, the salt crystallized into white crystals. When crystallization happens slowly, big crystals form. Small crystals form when crystallization happens quickly. Crystalline solids have unique crystal shapes. Therefore, crystal shape is a characteristic property of a substance.

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