Identification of ions and gases Revision Notes

Carbonate ions react with dilute acids such as hydrochloric acid to produce carbon dioxide gas. This reaction is a useful qualitative test for the presence of carbonate ions in a sample.

The procedure involves adding a few drops of dilute acid to the sample solution or solid. If carbonate ions are present, effervescence (bubbling) occurs due to the release of carbon dioxide gas.

To confirm that the gas produced is carbon dioxide, bubble it through limewater (a solution of calcium hydroxide). The limewater turns milky due to the formation of calcium carbonate, confirming CO₂ presence. This step is important to distinguish carbonate from other gases that might be evolved in different reactions.

• Add dilute acid (e.g., HCl) to the sample.
• Observe for bubbling indicating gas evolution.
• Bubble the gas through limewater to confirm carbon dioxide.
• A positive test confirms carbonate ions are present.

Halide ions are identified by their reaction with silver nitrate solution after acidification with dilute nitric acid. Acidification removes carbonate or hydroxide ions that could interfere by forming precipitates with silver ions.

The procedure is to first add dilute nitric acid to the sample to acidify it, then add aqueous silver nitrate. The silver ions react with halide ions to form silver halide precipitates, each with a characteristic colour.

Chloride ions produce a white precipitate of silver chloride (AgCl), bromide ions produce a cream precipitate of silver bromide (AgBr), and iodide ions produce a yellow precipitate of silver iodide (AgI). These colours help distinguish the halides from each other.

• Add dilute nitric acid to the sample to remove interfering ions.
• Add aqueous silver nitrate solution.
• Observe the precipitate colour: white (chloride), cream (bromide), yellow (iodide).
• Confirm identity by comparing precipitate colour and solubility if needed.

Sulfate ions are detected by adding barium nitrate solution to an acidified sample. Acidification with dilute nitric acid removes carbonate ions that could otherwise form barium carbonate precipitate, causing false positives.

When barium ions react with sulfate ions, a white precipitate of barium sulfate (BaSO₄) forms. This precipitate is insoluble in water and dilute acids, confirming the presence of sulfate ions.

The test is specific because barium sulfate is less soluble than other barium salts, making the white precipitate a reliable indicator.

• Acidify the sample with dilute nitric acid to remove carbonate ions.
• Add aqueous barium nitrate solution.
• Look for a white precipitate of barium sulfate.
• A positive test confirms sulfate ions are present.

Use dilute acids carefully to avoid excessive reaction or hazards. Wear safety goggles and gloves to protect from splashes.

Always add acid to the sample slowly to control the reaction rate and avoid splattering.

Repeat tests to confirm results and reduce errors caused by contamination or misinterpretation of precipitate colours or gas evolution.

• Wear appropriate personal protective equipment (PPE).
• Add acids slowly and carefully.
• Repeat tests for reliability.
• Use fresh reagents to avoid contamination.
• Dispose of chemical waste according to safety guidelines.

Each anion test follows a logical sequence: acidify if necessary to remove interfering ions, add a reagent that reacts specifically with the target ion, and observe a characteristic change (gas or precipitate).

Understanding the chemical basis of each test helps avoid common errors, such as mistaking carbonate gas bubbles for other gases or confusing precipitate colours.

Careful observation and confirmation tests (e.g., limewater test for CO₂) improve accuracy in identifying anions.

• Carbonate: dilute acid → CO₂ gas → limewater turns milky.
• Halides: acidify with nitric acid → silver nitrate → precipitate colour indicates ion.
• Sulfate: acidify with nitric acid → barium nitrate → white precipitate of BaSO₄.
• Confirm observations with additional tests if uncertain.

Aqueous sodium hydroxide (NaOH) and aqueous ammonia (NH₃) are common reagents used to identify metal cations in solution. When added to solutions containing metal ions, they often form precipitates of metal hydroxides with characteristic colours. Observing the colour and solubility of these precipitates helps distinguish between different cations.

The tests rely on the formation of insoluble metal hydroxides or the release of gases, and the behaviour of these precipitates in excess reagent provides further confirmation. These reactions are qualitative and widely used in inorganic analysis.

• NaOH and NH₃ react with metal cations to form hydroxide precipitates.
• Colour and solubility of precipitates are key identification features.
• Some cations release gases upon reaction with NaOH.

Ammonium ions do not form a precipitate with NaOH or NH₃. Instead, when aqueous sodium hydroxide is added to a solution containing ammonium ions and the mixture is gently warmed, ammonia gas (NH₃) is released.

This ammonia gas has a characteristic pungent smell and turns damp red litmus paper blue, confirming its alkaline nature.

The reaction can be represented as: NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

• Add NaOH to the solution and warm gently.
• Smell for ammonia gas or test with damp red litmus paper.
• Presence of ammonia confirms ammonium ions.

Calcium ions react with aqueous sodium hydroxide to form a white precipitate of calcium hydroxide, Ca(OH)₂, which is insoluble in excess NaOH.

When aqueous ammonia is added, no precipitate forms because calcium hydroxide is not sufficiently insoluble in NH₃ solution to precipitate.

This difference helps distinguish calcium from other cations that form precipitates with both reagents.

• Add NaOH: white precipitate forms, insoluble in excess.
• Add NH₃: no precipitate forms.
• White precipitate is calcium hydroxide.

Copper(II) ions form a distinctive blue precipitate of copper(II) hydroxide, Cu(OH)₂, when aqueous sodium hydroxide is added.

This precipitate is insoluble in excess NaOH, remaining as a blue solid.

When aqueous ammonia is added, a blue precipitate initially forms but dissolves in excess ammonia to give a deep blue solution due to complex ion formation (complex formulas are not required). This solubility in excess NH₃ distinguishes copper(II) from other cations.

• Add NaOH: blue precipitate forms, insoluble in excess.
• Add NH₃: blue precipitate forms then dissolves in excess to deep blue solution.
• Solubility in excess NH₃ is diagnostic for Cu²⁺.

Iron(III) ions react with aqueous sodium hydroxide to produce a reddish-brown precipitate of iron(III) hydroxide, Fe(OH)₃.

This precipitate is insoluble in excess NaOH, remaining as a solid.

When aqueous ammonia is added, a similar reddish-brown precipitate forms, which is also insoluble in excess NH₃ solution, helping to confirm the presence of Fe³⁺ ions.

• Add NaOH: reddish-brown precipitate forms, insoluble in excess.
• Add NH₃: reddish-brown precipitate forms, insoluble in excess.
• Colour and insolubility confirm Fe³⁺ presence.

Iron(II) ions form a green precipitate of iron(II) hydroxide, Fe(OH)₂, when aqueous sodium hydroxide is added.

This precipitate is insoluble in excess NaOH and tends to oxidise slowly in air to iron(III) hydroxide, changing colour from green to reddish-brown.

With aqueous ammonia, a similar green precipitate forms, which is also insoluble in excess NH₃ solution.

• Add NaOH: green precipitate forms, insoluble in excess.
• Add NH₃: green precipitate forms, insoluble in excess.
• Green colour and oxidation behaviour help identify Fe²⁺.

Identifying gases accurately is essential in chemistry to confirm the presence of specific substances during experiments or chemical reactions. Each gas has unique properties that allow it to be distinguished using simple tests involving indicators or reactions. This note covers five important gases: ammonia (NH₃), carbon dioxide (CO₂), chlorine (Cl₂), hydrogen (H₂), and oxygen (O₂). Understanding these tests helps in practical work and answering examination questions effectively.

• Gas identification relies on characteristic reactions or effects on indicators.
• Tests are simple, quick, and use readily available materials.
• Observations such as colour changes, sounds, or physical effects confirm the gas identity.

Ammonia is a colourless gas with a distinctive pungent smell. It is alkaline and soluble in water, forming ammonium hydroxide. To identify ammonia, damp red litmus paper is used as an indicator.

When ammonia gas comes into contact with damp red litmus paper, the paper turns blue. This colour change occurs because ammonia is basic and reacts with water on the paper to produce hydroxide ions (OH⁻), which cause the red litmus to change colour.

This test is simple and reliable for detecting ammonia gas in the laboratory.

• Use damp red litmus paper to detect ammonia.
• Ammonia turns red litmus paper blue due to its alkaline nature.
• The test confirms the presence of ammonia by a clear colour change.

Carbon dioxide is a colourless, odourless gas produced by respiration and combustion. It is slightly acidic when dissolved in water, forming carbonic acid.

To test for carbon dioxide, the gas is bubbled through limewater, which is a dilute solution of calcium hydroxide (Ca(OH)₂). When carbon dioxide passes through limewater, it reacts to form calcium carbonate (CaCO₃), a white precipitate, causing the limewater to turn milky or cloudy.

This reaction is reversible if excess carbon dioxide is added, but the initial milky appearance is a reliable indicator of carbon dioxide presence.

• Bubble gas through limewater to test for CO₂.
• Limewater turns milky due to calcium carbonate formation.
• The milky appearance confirms carbon dioxide gas.

Chlorine is a greenish-yellow gas with a strong, choking smell. It is a powerful oxidising agent and bleaches coloured substances.

To identify chlorine gas, damp blue or red litmus paper is exposed to the gas. Chlorine bleaches the colour from the litmus paper, turning it white or colourless.

This bleaching effect is due to chlorine reacting with water in the damp paper to form hydrochloric acid and hypochlorous acid, which oxidise the dye molecules in the litmus paper, destroying their colour.

• Expose damp litmus paper to chlorine gas.
• Chlorine bleaches the litmus paper, turning it white.
• Bleaching confirms the presence of chlorine gas.

Hydrogen is a colourless, odourless gas that is highly flammable. It is less dense than air and burns with a pale blue flame.

To test for hydrogen, a lighted splint is brought near a test tube containing the gas. If hydrogen is present, it ignites with a distinctive 'pop' sound due to rapid combustion forming water.

This test is very sensitive and provides a clear audible confirmation of hydrogen gas.

• Bring a lighted splint near hydrogen gas.
• Hydrogen burns with a 'pop' sound.
• The 'pop' confirms hydrogen presence.

Oxygen is a colourless, odourless gas essential for combustion and respiration. It supports burning but does not burn itself.

To test for oxygen, a glowing splint (a wooden splint with a glowing ember but no flame) is inserted into a container of the gas. If oxygen is present, the glowing splint reignites and bursts into flame due to oxygen's support of combustion.

This test is a classic and reliable method to confirm oxygen gas.

• Insert a glowing splint into oxygen gas.
• The glowing splint reignites in oxygen.
• Reignition confirms oxygen presence.

When metal ions are heated in a flame, their electrons absorb energy and become excited to higher energy levels. As the electrons return to their original energy levels, they emit light at specific wavelengths characteristic of the metal ion. This emitted light appears as a distinct flame colour, which can be used to identify the cation present in a sample.

The flame test is a qualitative method because it indicates the presence of certain metal ions based on flame colour, but it does not provide quantitative concentration information. It is especially useful for detecting alkali and some transition metal ions.

• Excited electrons emit light of characteristic wavelengths.
• Flame colour depends on the metal ion present.
• Useful for quick identification of certain metal cations.

To perform a flame test, you need the following: a clean platinum or nichrome wire loop (platinum is preferred but nichrome is commonly used), a Bunsen burner, samples of the metal salts to be tested, and hydrochloric acid (dilute) for cleaning the wire.

The wire loop must be thoroughly cleaned before each test to avoid contamination and false colours. This is done by dipping the loop in dilute hydrochloric acid and then heating it in the flame until no colour is observed.

• Clean wire loop (platinum or nichrome).
• Bunsen burner with a clean, non-luminous flame.
• Samples of metal salts (e.g., lithium chloride, sodium chloride).
• Dilute hydrochloric acid for cleaning the wire loop.

1. Clean the wire loop by dipping it in dilute hydrochloric acid and heating it in the flame until it burns without colour.

2. Dip the clean wire loop into the solid sample or a solution of the metal salt to pick up a small amount of the cation.

3. Place the wire loop with the sample into the hottest part of the Bunsen burner flame (usually the blue cone). Observe the colour produced carefully against a white background for clarity and note the flame colour produced by the sample.

• Clean wire loop thoroughly before each test.
• Use a small sample to avoid excessive contamination.
• Observe flame colour carefully and compare to known standards.

Each of the four cations produces a distinctive flame colour:

Lithium ions (Li⁺) produce a crimson red flame, which is a deep red colour.

Sodium ions (Na⁺) give a bright yellow flame, often very intense and can mask other colours if present together with other ions in a mixture, due to the strong emission at about 589 nm wavelength (the sodium D-line). This can make detecting other ions difficult if sodium contamination is present. Potassium ions (K⁺) produce a lilac or light purple flame, which can be faint and sometimes difficult to see clearly without a blue glass filter to cut out yellow sodium light interference. Copper(II) ions (Cu²⁺) produce a blue-green flame, which is quite distinctive and different from the alkali metal colours.

• Li⁺: crimson red flame.
• Na⁺: bright yellow flame, very intense.
• K⁺: lilac (light purple) flame, sometimes faint.
• Cu²⁺: blue-green flame.

When identifying unknown samples, compare the observed flame colour to the known standards above. If the flame colour matches one of the characteristic colours, the corresponding cation is likely present.

However, some limitations exist: sodium’s intense yellow flame can mask other colours if sodium contamination is present. Potassium’s lilac flame can be faint and hard to distinguish without a filter. Flame tests are not suitable for mixtures of metal ions because overlapping colours can confuse identification. For more precise identification, instrumental methods like atomic absorption spectroscopy are used.

Repeat tests to confirm results and ensure the wire loop is clean to avoid contamination.

• Compare observed flame colour to known standards.
• Beware of sodium’s intense yellow masking other colours.
• Potassium flame may require a blue glass filter for clarity.
• Not suitable for mixtures of metal ions.
• Repeat tests to confirm results and avoid contamination.