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Pure Substances�and Mixtures

Grade 7 Science

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Separating Solutions

Lesson 6

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Separating Solutions

  • When a solute is dissolved in a solvent, the particles of the two mix together but the particles themselves aren’t changed
    • e.g. Salt dissolves in water but the salt particles remain unchanged (the water still tastes salty)
  • We can separate solutions in two ways.

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1) Evaporation

  • Evaporation is the change of matter from liquid to gas.
    • Works well when separating a liquid and a solid.
  • The solution is heated until the liquid component turns to a gas, leaving behind the solid component that had been dissolved in the solution.
    • e.g. To separate the salt-water solution mentioned earlier, we can heat the solution to 100° C
      • This will cause the water to change state from liquid to gas and therefore separate from the solution
      • Once the water evaporates, the salt will be�left behind

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1) Evaporation

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2) Distillation

  • Distillation is the process of separating liquids in a solution by heating the solution, trapping and cooling the gas and collecting the resulting pure liquid
    • Works well when separating solutions made of two liquids, as long as the liquid have different boiling points

Ethanol

Boiling Point: 78.4°C

Water

Boiling Point: 100°C

Water and Ethanol could be separated due to their different boiling points.

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2) Distillation

  • The solution is heated to a point at which one of the component liquids will boil off and the other will remain as a liquid.
    • The steam that boils off of the solution (the liquid with the lower boiling point) is collected and cooled so that it turns back into a liquid.
    • The result is two separate containers, each containing a pure form of the two�components.

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2) Distillation

Mixture

Bunsen Burner

Condenser

(cold water flows through it)

Collection Flask

Distilling Flask

Thermometer

Distillate

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2) Distillation

  • e.g. A solution of water (boiling point of 100° C) and ethanol (boiling point of 78° C) are separated by distillation
    • The water-ethanol solution is heated to just over 78° C, causing the ethanol to boil but allowing the water to remain as a liquid.
    • The ethanol steam is captured and made to flow through a cooling chamber.
    • The ethanol steam is condensed back into a liquid and this liquid is collected.
    • The result is pure water in the first container and�pure ethanol in the second container.

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