Qualitative analysis

Qualitative analysis

The process used to identify substances containing ions in solution, compounds, and atoms.

Reagents

Substances, such as NH4OH(aq) and NaOH(aq), used to identify or test for cations in a solution.

Metal cations

Positively charged metal ions that form insoluble metal hydroxides when reacting with aqueous sodium hydroxide.

Ca2+, Al3+, Pb2+, Zn2+

Cations that form white precipitates when NaOH(aq) is added.

Fe2+

Iron(II) cation that forms a green precipitate when NaOH(aq) is added.

NH4+

Ammonium cation that does not form a precipitate when NaOH(aq) is added, but emits ammonia gas and changes red litmus paper to blue.

Basic hydroxides

Hydroxides of certain metals (e.g., calcium hydroxide, copper(II) hydroxide) that do not react with sodium hydroxide and remain as precipitates when excess NaOH(aq) is added.

Amphoteric hydroxides

Hydroxides of aluminum, lead(II), and zinc that react with excess NaOH(aq) to form soluble salts, causing the precipitates to dissolve.

Cu2+

Copper(II) cation that forms a blue precipitate when NaOH(aq) is added.

Fe3+

Iron(III) cation that forms a red-brown precipitate when NaOH(aq) is added.

NH4OH(aq)

Ammonium hydroxide, a soluble compound that does not form a precipitate when NaOH(aq) is added.

Anions

Negatively charged ions that can be tested for using specific methods.

CO3 2-

Carbonate anion that produces CO2 gas and a white precipitate in lime water when heated.

KNO3, NaNO3

Nitrates that release oxygen gas when heated and cause a glowing splint to relight.

NO2 -

Nitrite anion that forms a blue solution and emits brown nitrogen dioxide gas when heated with concentrated sulfuric acid and copper turnings.

Dilute hydrochloric or nitric acid

Acids used to test for carbonates and sulfates by observing the gas given off.

Cl-

Chloride anion that emits hydrogen chloride gas and forms white fumes with ammonia gas when concentrated sulfuric acid is added.

Br-

Bromide anion that releases bromine gas and forms a red-brown gas when concentrated sulfuric acid is added.

I-

Iodide anion that produces iodine and a gray-black solid when sulfuric acid is added, which sublimes to form a purple vapor when heated.

AgNO3

Silver nitrate, which forms specific precipitates when added to chloride, bromide, and iodide ions.

Ba(NO3)2

Barium nitrate, used to test for carbonates and sulfates by observing the precipitates formed and their reactions with dilute acids.

O2

Oxygen gas, which supports combustion and causes a glowing splint to glow brighter or relight.

H2

Hydrogen gas, which reacts explosively with oxygen to form steam and produces a pop sound when ignited with a lighted splint.

HCl

Hydrogen chloride, a colorless gas with a sharp acid odor that forms white fumes when near ammonia gas.

NH3

Ammonia, a colorless gas with a pungent odor that turns red litmus paper blue and forms white fumes when near hydrogen chloride gas.

Cl2

Chlorine gas, yellow-green with a sharp odor that turns moistened blue litmus paper red and bleaches it white.

SO2

Sulfur dioxide gas, colorless with a pungent odor that turns acidified potassium manganate(VII) solution from purple to colorless and acidified potassium dichromate(VI) solution from orange to green.

NO2

Nitrogen dioxide gas, brown with a sharp irritating odor that turns moistened blue litmus paper red and reacts with water to form nitrous and nitric acids.

H2O

Water, colorless and odorless, turns dry cobalt(II) chloride paper from blue to pink and anhydrous copper(II) sulfate from white to blue.