qualitive analysis: ions test Flashcards
define precipitate
an insoluble solid formed when two solutions are mixed together
why does a precipitate form?
because the compound produced by a combination of 2 oppositely charged ions is insoluble in the solvent used
describe how you carry out a test for carbonate ions
-add dilute aqueous NH3 (aq)
-if see bubbles of gas, CO2 is produced
-further identify by bubbling through limewater, if white precipitate then CO2 gas present
whats the ionic equation for testing carbonate ions?
CO3^2- (aq) + 2H+(aq) = H2O(l) + CO2(g)
describe how to carry out the test for sulfate (VI) ions
add dilute nitric acid HNO3(aq)
followed by barium nitrate solution Ba(NO3)2 (aq)
if white precipitate forms barium sulfate is present
to test for sulfate ions why do you need to add dilute nitric acid first?
to get rid of any CO3^2- ions that may be present as carbonate ions give a white precipitate aswell
may result in a false positive test for barium sulfate
what is the ionic equation for sulfate ions?
Ba2+ (aq) + SO4^2- (aq) = BaSO4 (s)
describe how you would carry out the test for halide ions
add dilute nitric acid until no further bubbling followed by aqueous silver nitrate solution, AgNO3 (aq)
positive result shows insoluble silver halide
white preicipitate = chloride ion
cream precipitate = bromide ion
yellow precipitate = ioidide ion (never dissolves)
how might ammonia NH3(aq) help in distinguishing a more accurate positive in identifying the halides?
-uses basis of solubility
-dilute ammonia dissolves a white precipitate
-a yellow precipitate will never dissolve
whats the ionic equation for testing for halides?
Ag+(aq) + X-(aq) = AgX(s)
why should ammonia be used in a flame cupboard?
concentrated ammonia is corrosive and must not be inhaled
describe how you would carry out the test for cations (+)
add warm dilute aqueous sodium hydroxide, NaOH(aq)
ammonia gas is produced when solution is warmed
if the moist litmus paper (pH indicator paper) turns from red to blue then ammonia gas is given off
whats the ionic equation for the cations (+) test?
NH4+ (aq) + OH- = NH3 (g) + H2O (l)
what is effervesence?
bubbles in a liquid
