Chemical Bonding part 2 Flashcards
Recrystallisation
a technique used to purify certain crystalline substances
what concept is recrystallisation based on
the fact that some substances are more soluble in hot solvents than in cold solvents
what are benzoic acid crystals soluble in?
hot water but only slightly in cold water
Benzoic acid crystals experiment; soluble impurity
NaCl
Benzoic acid insoluble impurity
Charcoal
why is the minimum amount of water used in the benzoic acid crystals experiment?
to get the maximum yield of benzoic acid crystals
what stays behind in the benzoic acid crystals experiment after the first filtration?
charcoal
first thing you do in the benzoic acid experiment
dissolve impure crystals in the hot water
what type of solution with impure benzoic acid crystals do you want?
saturated solution
what stays behind in the second filtration in the benzoic acid experiment?
benzoic acid crystals
what goes through the second filtration in the benzoic acid crystals experiment
NaCl and water
why would you rinse the benzoic acid crystals with water after the second filtration?
if they are damp with water still, that water could still contain the soluble impurity i.e NaCl
where do you finally dry the benzoic acid crystals?
in the air or a hot oven
why must the first filtration of benzoic acid crystals be done quickly?
So that recrystallisation doesn’t occur on the filter paper
what can help speed up the recrystallisation?
scraping the inside with a glass rod
what is the flask used in the cold filtration called
buchner funnel
why is a buchner funnel used?
it uses air suction to speed up filtration, causes a partial vacuum
what is connected to the buchner funnel?
a t junction connected to a pipe of flowing water from a tap
what does benzoic acid look like
white crystals
what is benzoic acid used for?
food preservative
4 pieces of apparatus used in melting point determination
a hot plate
a melting point block
a thermometer
capillary tubes
what has to be special about he thermometer in melting point determination
it has to read up to about 150 degrees celsius
what are capillary tubes
very light and narrow glass tubes
what has to be special about the capillary tubes used in melting point determination
sealed at one end
how do you know when the crystals have reached their melting point?
they break down and turn to a colourless liquid
probably impure if…
- they melt over a large range eg. 6 degrees
2. they melt under the normal melting poingt
anions
negative ions
where are anions found
in salts with positive ions