tests for ions Flashcards
Calcium (Ca2+) flame test, excess OH-
Flame test: brick red
Excess OH-: slight white precipitate
Strontium (Sr2+) flame test, excess OH-
Flame test: red
Excess OH-: slight white precipitate
Barium (Ba2+) flame test, excess OH-, sulfate ions
Flame test: pale green
Excess OH-: no change
Sulfate ions: white precipitate
Magnesium (Mg2+) excess OH-
Excess OH-: white precipitate
Flame test process
Dip a nichrome wire loop in concentrated hydrochloric acid, then into the unknown compound
Hold loop in the clear blue part of a Bunsen burner flame and observe colour change
Test for Ammonium ions (NH4+)
Add dilute sodium hydroxide (NaOH) to unknown solution and gently heat.
Ammonia gas (NH3) is alkaline so you can test for it using a damp piece of red litmus paper (damp so that ammonia gas can dissolve)
Positive result = blue
Test for sulfate ions (SO4 2-)
Add some dilute hydrochloric acid to unknown solution
Then add barium chloride solution (BaCl2)
If white precipitate of barium sulfate forms, positive result
Test for hydroxide ions
Use pH indicator
Hydroxide ions make solutions alkaline
Eg red litmus paper turn blue
Test for halides (Cl-) (Br-) (I-)
Add dilute nitric acid, followed by silver nitrate solution
Chloride gives white precipitate of silver chloride
Bromide gives cream precipitate of silver bromide
Iodide gives yellow precipitate of silver iodide
Test for carbonates
When you add dilute hydrochloric acid, solution containing carbonate ions will fizz as hydrogen ions react with carbonate ions to give carbon dioxide.
Test for carbon dioxide using limewater, it will turn limewater cloudy
Displacement of less reactive halide ions from a solution
A halogen will replace a halide from a solution if the halide is below it in the periodic table
eg
Bromide displaced by Cl2, forms orange solution
Iodide displaced by Cl2 and Br2, brown solution is formed.
Solubility of silver halide precipitates in ammonia
Chloride precipitate dissolves in dilute NH3
Bromide precipitate dissolves in concentrated NH3
Iodide precipitate insoluble in concentrated NH3
Halide ions reaction with sulfuric acid
NaF/NaCl - Hydrogen fluoride or hydrogen chloride gas is formed. Misty fumes as gas comes into contact with moisture in the air. Not a redox reaction.
NaBr - Misty fumes of hydrogen bromide gas. HBr is stronger reducing agent so redox reaction occurs which produces choking fumes of SO2 and orange fumes of Br2
NaI - Misty fumes of hydrogen iodide gas. Hydrogen iodide is a very strong reducing agent that reduces the SO2 to H2S (toxic gas). Solid iodine is also formed by this reaction
