Testing for Ions and Gases
Identify unknown ions by flame colour and by the precipitate they form, test the common gases at the bench, and say why an instrument beats these tests on a tiny sample.
What a learner can do afterwards
- Matches flame colours to the group 1 and group 2 metal ions that produce them
- Predicts the precipitate formed when a halide or a sulfate solution meets its test reagent, and writes the ionic equation
- Describes the bench tests for hydrogen, oxygen, carbon dioxide and chlorine and the result each gives
- Explains a flame colour as electrons dropping back between shells, and gives one reason instruments are used instead
1 · Read
Titration tells how much acid, not which ion. Flame tests name the metal ion. Lithium burns crimson, sodium burns yellow and potassium burns lilac. Calcium burns orange-red, strontium burns red and barium burns green. The colour appears because electrons jump to higher shells in the heat, then drop back and release light. Colours can look alike, so analysts confirm with instruments such as flame photometers.
Anions are named by precipitates. Add acidified silver nitrate to test halides: chloride gives a white precipitate, bromide gives cream and iodide gives yellow. Add acidified barium chloride to test sulfates: a white precipitate of barium sulfate appears. Each ionic equation shows only the ions that join into the solid, such as Ag+ + Cl- -> AgCl, leaving out the spectators.
To show a solution is sodium chloride rather than potassium sulfate, run two tests. A yellow flame points to sodium, while potassium would burn lilac. A white precipitate with acidified silver nitrate points to chloride, while sulfate would stay clear. Both results together name the salt.
Gases each have one bench test. Hydrogen burns with a squeaky pop when you hold a lit splint to it. Oxygen relights a glowing splint. Carbon dioxide turns limewater from clear to milky. Chlorine bleaches damp litmus paper white. Match each result to its gas and never swap the splints.
Flames name the metal, precipitates name the anion, and splints, limewater and litmus name the gas.
The rewritten gas-testing reading was written for this dataset. Three retained readings come from CK-12 Introductory Chemistry through the library collection: 7.3: Cations, 7.4: Anions, and 16.19: Predicting Precipitates Using Solubility Rules. Their publisher license terms have not been verified.
More to read
7.3: Cations
A cation is a positive ion, formed when a metal atom loses electrons. Group 1 and group 2 metals each give a set flame colour because their electrons jump to higher shells in the heat and then drop back, releasing light. Lithium burns crimson, sodium burns yellow, potassium burns lilac, calcium burns orange red, strontium burns red, and barium burns green. Because colours can look alike and mix, analysts confirm results with instruments such as flame photometers, which measure the exact wavelengths.
7.4: Anions
An anion is a negative ion, formed when a nonmetal atom gains electrons. Halides are tested with acidified silver nitrate solution: chloride gives a white precipitate of silver chloride, bromide gives cream silver bromide, and iodide gives yellow silver iodide. Sulfates are tested with acidified barium chloride, which gives a white precipitate of barium sulfate. In each case the ionic equation shows only the ions that join to make the solid, leaving out the spectator ions that stay in solution.
16.19: Predicting Precipitates Using Solubility Rules
Whether a precipitate forms comes down to solubility rules. Silver halides and barium sulfate are insoluble, so they appear as solids the moment their ions meet. Predicting this means checking each pairing of ions in turn: if any pairing is insoluble, a precipitate of that compound forms. This is exactly the logic behind the halide and sulfate bench tests. Learning which common salts dissolve and which do not lets you predict the test result before you mix anything.
Recognising gases in the laboratory
Electrolysis splits water, producing hydrogen at the negatively charged electrode and oxygen at the positively charged electrode. A burning splint gives a squeaky pop with hydrogen; a glowing one reignites in oxygen. Carbon dioxide from exhaled breath or an acid reacting with a carbonate makes clear limewater milky. Chlorine turns damp litmus paper white. These are useful identification tests, but their results are not exclusive to these gases.
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2 · Watch
Take it off screen
Where it sits
Learn first
Optional connections
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.