Gas tests
What is the positive result when a lit splint is held in a test tube of hydrogen gas?
A "squeaky pop" sound.
What flame colour does sodium (Na⁺) produce?
Yellow.
Sodium hydroxide is added to a solution and a light blue precipitate forms. Which cation is present?
Copper(II), Cu²⁺.
Dilute nitric acid and silver nitrate are added to a halide solution and a white precipitate forms. Which halide ion is present?
Chloride (Cl⁻).
What colour does litmus turn in an acid?
Red
Complete the golden rule: all sodium, potassium and ammonium salts are ______.
Soluble. (And all nitrates are soluble too — the two no-exception rules.)
A glowing (not burning) splint is held in a gas and it relights. Which gas is present?
Oxygen.
Which metal ion gives a lilac flame?
Potassium (K⁺).
A pale green precipitate forms when NaOH is added to a solution. Which cation is present?
Iron(II), Fe²⁺.
State the precipitate colours for chloride, bromide and iodide in the silver nitrate test.
Chloride = white, bromide = cream, iodide = yellow.
Anhydrous copper(II) sulfate is used to test for water. What colour change shows water is present?
White → blue.
Most chlorides are soluble. Name the two common chlorides that are insoluble exceptions.
Silver chloride (AgCl) and lead(II) chloride (PbCl₂).
Name the substance used to test for carbon dioxide, and state the positive result.
Limewater (calcium hydroxide solution) — it turns cloudy/milky.
Describe how the metal rod is cleaned before a flame test, and explain why.
Dip the (platinum/nichrome) wire into dilute hydrochloric acid — to remove any ions already on it so they don't affect the result.
Describe the test and result used to identify iron(III) ions (Fe³⁺).
Add a few drops of sodium hydroxide solution; an orange/brown precipitate forms.
Describe the test for carbonate ions, including the two positive observations.
Add dilute (hydrochloric) acid → fizzing/effervescence and a gas is released; bubble the gas through limewater → it turns cloudy, confirming CO₂ (so carbonate is present).
Describe a physical test to show whether a sample of water is pure.
Measure its boiling point — pure water boils at exactly 100°C; impurities raise the boiling point.
When preparing a soluble salt by reacting an acid with an insoluble base, why is the base added in excess?
To ensure all the acid has reacted, so no unreacted acid is left to contaminate the salt (and the excess solid is simply filtered off).
Damp red litmus paper held at the mouth of a test tube turns blue. Name the gas, and state one situation in cation testing where this gas is produced.
Ammonia (NH₃). It is released when sodium hydroxide is added to (and warmed with) an ammonium (NH₄⁺) salt.
One sample gives an orange-red flame and another gives a blue-green flame. Identify both metal ions.
Orange-red = calcium (Ca²⁺); blue-green = copper (Cu²⁺).
Describe how you would test a solution for ammonium ions (NH₄⁺), including the result.
Add sodium hydroxide solution and warm the mixture; ammonia gas is released, which turns damp red litmus paper blue.
Describe the test for sulfate ions, including the reagent added first and why.
Add dilute hydrochloric acid first (to remove any carbonate ions, which would also give a precipitate), then add barium chloride solution; a white precipitate (barium sulfate) confirms sulfate ions.
State the colour of methyl orange and the colour of phenolphthalein, each in an alkali.
Methyl orange = yellow (in alkali); phenolphthalein = pink (in alkali).
A student needs to prepare a pure sample of lead(II) sulfate. State which method they should use and explain why.
Precipitation — because lead(II) sulfate is insoluble, so it can't be made by acid + base (the salt won't dissolve). Mix two soluble solutions (e.g. lead(II) nitrate + a soluble sulfate) so the insoluble salt crashes out, then filter, wash and dry it.
Chlorine is tested with damp blue litmus paper. Describe the full colour change seen and explain why it happens.
Blue litmus turns red, then is bleached white. Chlorine is acidic (turns the litmus red) and acts as a bleach (removes the colour, turning it white).
A student records a flame colour as "red." State the two specification ions that could give a red/red-ish flame, and explain why the result must be observed carefully.
Lithium (Li⁺) gives red and calcium (Ca²⁺) gives orange-red. The colours are similar, so careful observation is needed to distinguish pure red (lithium) from the orange-tinged red (calcium).
Write the ionic equation, with state symbols, for the reaction that forms the iron(III) precipitate.
Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s).
In the halide test, the silver nitrate is acidified with dilute nitric acid. Explain why nitric acid is used rather than dilute hydrochloric acid.
Hydrochloric acid contains chloride ions, which would react with silver nitrate to form a white precipitate (silver chloride) and give a false positive. Nitric acid introduces no halide ions, so the result stays valid.
A colourless solution turns anhydrous copper sulfate blue and boils at 102°C. Explain what both results tell you about the sample.
The copper sulfate turning blue shows water is present. But boiling at 102°C (above 100°C) shows it is not pure — dissolved impurities have raised the boiling point. So it is an impure aqueous solution, not pure water.
Write the balanced symbol equation, with state symbols, for making lead(II) chloride by precipitation from lead(II) nitrate and sodium chloride.
Pb(NO₃)₂(aq) + 2NaCl(aq) → PbCl₂(s) + 2NaNO₃(aq).