topic 2- separating mixtures Flashcards
How can we tell if a substance is pure?
- pure substances have a horizontal line at the melting and boiling points
- pure substances have 1 melting point not a range
define an ‘element’
a substance that only consists of atoms with the same atomic number (number of protons in their nucleus)
define a ‘compound’
a substance that consists of atoms of two or more different elements chemically joined together
pure substances contain…
only one element or compund
define ‘mixtures’
substances containing different elements and/or compounds. They are impure substances. The components of a mixture are NOT chemically joined together.
How can we tell if a substance is impure?
-it has a range of temperatures instead of a single temperature
What does simple distillation separate?
A solvent from a solution
How does simple distillation work?
-simple distillation works because the solute in the solution has a much higher boiling point than the solvent
-when the solution is heated:
>the solvent boils
>solvent vapour passes into the condenser
>the vapour is cooled and condensed back to the liquid state
What does fractional distillation separate?
-a liquid from a mixture of miscible liquids (liquids that mix completely with each other)
How does fractional distillation work?
It works because the liquids in the mixture have different boiling points
-when the mixture is heated:
>the mixture boils
>hot vapour rises up the fractionating column
>vapour condenses when it hits the cool surface of the column and drips back
>the fraction with the lowest boiling point reaches the top of the column first
>it’s vapour passes into the condenser
What does filtration separate?
an insoluble substance from a liquid or a solution
when would you use filtration?
-to purify a liquid or a solution by removing solid impurities from it
»_space; i.e. sand from sea water
-to separate the solid you want from the liquid it is mixed in with
»_space; i.e. to separate crystals from a solution after crystallisation
How does filtration work?
Filtration works because the filter paper has tiny pores. These are
- large enough to let water molecules and dissolved substances through
- small enough to stop insoluble solid particles going through
What does crystallisation produce?
-solid crystals from a solution
Method of crystallisation:
- the solution is heated to remove enough solvent to produce a saturated solution (one that cannot hold any more solute)
- the saturated solution is allowed to cool
- crystals form in the solution
- the crystals are separated from the liquid and dried.
How does crystallisation work?
-the solubility of the solute decreases as the saturated solution cools
-crystals form from the excess solute
solubility- is the mass of solute that dissolves in a given volume of solvent at a given temperature
stationary phase
a substance that does not move
mobile phase
a substance that moves through the stationary phase- the solvent
How many spots will a pure substance produce in paper chromatography?
One
Equation for the Rf value:
Rf= distance travelled by spot/ distance travelled by solvent
apparatus for paper chromatography:
- eye protection
- chromatography paper
- boiling tube with bung
- dropping pipette
- pencil and ruler
- solvent for mobile phase
Method for simple distillation (of ink):
- add some ink to a boiling tube, then fit a delivery tube and bung
- hold the boiling tube with a test tube holder
- heat the ink using a roaring Bunsen burner flame until it’s dry
- collect the vapour in a test tube
Potable drinking water must have:
- low levels of contaminating substances
- low levels of microbes
Fresh water from lakes, rivers and reservoirs is likely to contain:
- objects such as leaves and twigs
- insoluble solids such as particles of soil
- microbes, which may cause disease
Tap water is potable but not a pure substance it contains:
- dissolved salts
- dissolved chlorine
Stages in water treatment:
- sedimentation»large insoluble particles sink to the bottom of a tank
- filtration»small insoluble particles are removed by filtering through beds of sand
- chlorination»chlorine gas is bubbled from the water to kill microbes
Explain why water used for chemical analysis must not contain any dissolved salts? ( 3 marks )
Dissolved salts could react with the substances used in the analysis. A product formed in the reaction could interfere with the analysis, giving a false result. If the water does not contain any dissolved salts, this will not happen.
The fractionating column used in fractional distillation has a…
temperature gradient
