Separating mixtures. Flashcards
Define filtration
Filtration is a technique used to separate an insoluble solid from a liquid.
Describe the process of filtration.
1) Pour the mixture into the filter paper
The liquid will pass through the tiny pores in the filter paper. However the solid material cannot pass through the filter paper, so it is trapped, and collects on the filter paper.
The solid has now been separated from the liquid.
How to separate sand from salt water.
1) Pour the mixture into the filter paper
The liquid will pass through the tiny pores in the filter paper. However the solid material cannot pass through the filter paper, so it is trapped, and collects on the filter paper.
The sand can then be washed with distilled water to remove any salt solution left on it.
The wet sand is then dried to evaporate any water off and leave the pure, dry sand.
Define crystallisation
Crystallisation is a technique used to separate a soluble solid from a liquid
Describe the process of crystallisation.
1) Pour the solution into an evaporating dish and gently heat the solution using a water bath. Some of the solvent will evaporate and the solution will get more concentrated. (Using a water bath is a gentler way of heating than heating the evaporating dish on a tripod and gauze).
2) Once some of the solvent has evaporated, or when crystals start to form (the point of crystallisation), remove the dish from the heat and leave the solution to cool. The rest of the water is left to evaporate off the saturated solution at room temperature
3) The salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution
4) Filter the crystals out of the solution, and leave them in a warm place to dry
A flat-bottomed crystallisation dish can be used to give a large surface area for the water to evaporate from.
Define simple distillation
Simple distillation is used to separate a liquid from a mixture.
Describe the process of distillation
1) A solution is heated and boiled to evaporate the solvent. The part of the solution that has the lowest boiling point evaporates first.
2) The vapour given off then enters a condenser.
3) The vapour is then cooled, condenses and is collected.
4) Any dissolved solids will remain in the heated flask
The rest of the solution (with a higher boiling point) will remain in the heated flask
Describe the process of separating salt from rock salt
1) Grind the mixture to make sure the salt crystals are small, so that the salt crystals will dissolve easily
2) Put the mixture in water and stir with a glass stirring rod. The salt will dissolve in the water but the sand will not.
3) Filter the mixture, the grains of sand won’t fit through the tiny pores in the filter paper, so they collect on the paper instead. The salt passes through the filter paper as it’s part of the solution.
4) Evaporate 1/3 of the total volume of water from the salt, with a Bunsen burner so that crystals start to form
5) Turn the Bunsen Burner off, remove the dish from the heat and leave the solution to cool. The rest of the water is left to evaporate off the saturated solution at room temperature. The salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution. Slow evaporation results in large crystal growth.
What is a condenser
A condenser is an outer glass tube with water flowing through it that acts as a cooling ‘jacket’ around the inner glass tube from the flask.
State a problem with simple distillation
Simple distillation can only be used to separate substances with very different boiling points. If the temperature goes higher than the boiling point of the substance with the highest boiling point, the different substances will mix again
What is fractional distillation used for
Fractional distillation can be used to separate a mixture of liquids that have similar boiling points
These liquids must have different boiling points
During distillation, how is the condenser kept cold
The condenser is kept cold as cold water is being circulated around it.
Apparatus for fractional distillation
Inside flask - mixture of different liquids
Flask (containing the mixture) is attached to a long column containing hundreds of glass beads.
This is called the fractionating column - this is not used for simple distillation
At the top of the fractionating column there is a thermometer
Condenser (contains circulating cold water)
Describe the process of fractional distillation
Gently heat the mixture using a Bunsen Burner.
Both of the liquids will start to evaporate. However, the liquid with the lower boiling point will evaporate more easily
Both of these liquids will start to make their way into the fractionating column.
We now have a mixture of two different vapours making their way into the fractionating column.
When the vapours hit the fractionating column, they condense and drip back into the flask where the liquids evaporate again.
This repeated evaporation and condensation, increases the amount of the lower boiling point chemical in the fractionating column.
As the warm vapours pass up the column, they reach the thermometer and temperature on the thermometer begins to rise.
When the temperature is rising it means that a mixture of two different vapours are passing over the thermometer. However the mixture will contain more of the chemical with the lower boiling point.
These vapours pass into the condenser and they turn back into a liquid.
However, this liquid is still a mixture of the two chemicals.
There comes a point when the temperature on the thermometer stops rising
This will be the lower of the two boiling points
At this point we now have mainly one chemical passing into the condenser
As the chemical condenses, we can collect it in a fresh beaker.
This is our first proper fraction (the chemical with the lowest boiling point)
After a while the temperature on the thermometer begins to rise again
This means that once a gain a mixture of vapours is passing into the condenser.
However this mixture mainly contains the chemical with the higher boiling point.
When the thermometer reaches a constant temperature, we are no collecting a relatively pure sample of the second chemical
We have now separated the two mixtures based on their boiling points
For fractional distillation, what would happen if the two liquids had very similar boiling points
If the two liquids have very similar boiling points then it is much harder to separate them.
We might need to carry out several rounds of fractional distillation.
