Separation and purification Flashcards
what does a pure substance consist of?
one element or one compound
what does a mixture consist of?
different substances, not chemically joined together
what does an element contain?
just one type of atom
what does a compound contain?
two or more types of atom joined together
how could you distinguish between pure substances and mixtures? (2)
Pure substances have a sharp melting point
mixtures melt over a range of temperatures
what would an impure substance produce in relation to temperature?
gradual decrease over a range of temperatures as it freezes.
what does the horizontal part of a temperature-time graph show?
that the substance has a sharp melting point
what is the purpose of filtration?
to separate an insoluble solid from a liquid
what is filtration particularly useful for?
separating excess reactant from a solution.
why does filtration work? (2)
filter paper has tiny holes, or pores, in it.
large enough to let small molecules and dissolved ions through, but not the much larger particles of undissolved solid.
what is the purpose of crystallisation?
what happens during the process?
- to produce solid crystals from a solution.
- when the solution is warmed, some of the solvent evaporates leaving behind a more concentrated solution.
describe the process used to obtain large, regularly shaped crystals: (4)
1-put the solution in an evaporating basin
2-warm the solution by placing the evaporating basin over a boiling water bath
3-stop heating before all the solvent has evaporated
-After the remaining solution has cooled down, pour the excess liquid away (or filter it). Dry the crystals using a warm oven or in air.
what is the purpose of simple distillation?
to separate a solvent from a solution
what is simple distillation particularly useful for?
producing water from salt solution.
why does simple distillation work?
the dissolved solute has a much higher boiling point than the solvent
describe the process of simple distillation: (3)
- When the solution is heated, solvent vapour evaporates from the solution.
- The gas moves away and is cooled and condensed.
- The remaining solution becomes more concentrated in solute as the amount of solvent in it decreases.
what is the purpose of fractional distillation?
to separate different liquids from a mixture of liquids.
what is fractional distillation particularly useful for?
separating ethanol from a mixture of ethanol and water, and for separating different fractions from crude oil.
why does fractional distillation work?
different liquids have different boiling points
what happens when a mixture is heated in fractional distillation? (3)
- vapours rise through a column which is hot at the bottom, and cold at the top
- vapours condense when they reach a part of the column that is below the temperature of their boiling point
- the liquid flows out of the column
describe the two ways of obtaining different liquids from the column in fractional distillation: (2)
- By collecting different liquids from different parts of the column. The substance with the lowest boiling point is collected at the top of the column.
- By continuing to heat the mixture to increase the temperatures in the column. The substance with the lowest boiling point is collected first.
what is the purpose of chromatography?
to separate mixtures of soluble substances
describe the chromatography process:
- Water and ethanol solution is heated
- As the paper is lowered into the solvent, some of the dye spreads up the paper
- The paper has absorbed the solvent, and the dye has spread further up the paper
what does chromatography rely on? and what are they called? (2)
two different phases
- stationary and mobile phase
what is the stationary phase?
very uniform, absorbent paper
what is the mobile phase?
the solvent that moves through the paper, carrying different substances with it
describe the attractions of the different substances in a mixture? (2)
- the different dissolved substances in a mixture are attracted to the two phases in different proportions.
- this causes them to move at different rates through the paper.
what does Separation by chromatography produce?
chromatogram
what can a paper chromatogram be used to distinguish between?
pure and impure substances
results of pure and impure substances on a chromatogram: (2)
a pure substance produces one spot on the chromatogram
an impure substance produces two or more spots
