Tests for anions in aqueous solutions

Procedure of to detect the presence of chloride ions in aqueous solution

• Fill a test tube to about ¼ of its capacity with deionised water

• Add a small amount of a soluble chloride salt to the water, then shake the test tube to help the salt dissolve

• Using a clean dropped add a few drops of silver nitrate solution

• Add about ¼ of a test tube of dilute ammonia solution and shake gently once again to test that the white precipitate is silver chloride.

Observations of to detect the presence of chloride ions in aqueous solution

• A clear solution is obtained after adding chloride salt to the deionised water

• The solution turns cloudy and a white material settles out of the solution when silver nitrate is added

• The cloudiness disappears when dilute ammonia solution is added due to silver chloride being soluble in ammonia solution

Chemical equation(s) of to detect the presence of chloride ions in aqueous solution

Ag⁺ + Cl^- →  AgCl ↓

Explanation of to detect the presence of chloride ions in aqueous solution

In the experiment the precipitate formed was silver chloride. It formed when the silver ions from the silver nitrate combined with the chloride ions from the chloride salt that dissolved in the water.

Procedure of detecting the presence of sulfate ions (SO42-) and sulfite ions (SO32-) in an aqueous solution.

• Fill two test tubes to about ¼ of their capacities with deionised water

• Add a small amount of the sulfate salt into one test tube, shake the test tube to encourage the salt to dissolve.

• Repeat the previous step with the other test tube where sulfite salt is used instead. Label each test tube.

• Using a clean dropper, add a few drops of barium chloride solution to each test tube.

• TO distinguish between barium sulfate and barium sulfite, add approximately ¼ of a test tube of dilute hydrochloric acid to each test tube, shake each one gently.

Observations of detecting the presence of sulfate ions (SO42-) and sulfite ions (SO32-) in an aqueous solution.

• When barium chloride solution is added, the solution turns cloudy and a white solid material settles out of the solutions in each test tube

• When dilute hydrochloric acid is added to each test tube, the cloudy precipitate remains in the test tube which contains sulfate ions. The test tube with sulfite ions has its cloudiness disappear

Chemical equation(s) of detecting the presence of sulfate ions (SO42-) and sulfite ions (SO32-) in an aqueous solution.

• Barium Sulfate formation: Ba²⁺ + SO₃^2- → BaSO₄ ↓

• Barium Sulfite formation: Ba²⁺ + SO₃^2- → BaSO₃ ↓

• Distinguishing Test: BaSO₄ + HCl → no reaction

• Distinguishing Test: BaSO₃ + 2HCl → BaCl₂ + SO₂ + H₂O

Explanation of detecting the presence of sulfate ions (SO42-) and sulfite ions (SO32-) in an aqueous solution.

• The insoluble material formed after adding barium chloride is either barium sulfate or barium sulfite. These compounds formed when barium ions combined with the dissolved sulfate/sulfite ions

• Barium sulfate is insoluble in dilute hydrochloric acid, the cloudiness remains in the test tube with sulfate ions.

• Barium sulfite reacts with dilute hydrochloric acid, making the cloudiness disappear in the test tube with sulfite ions.

Procedure of detecting the presence of carbonate ions (CO32-) and hydrocarbonate ions (HCO3-) in an aqueous solution.

• Place a small amount of carbonate salt in a boiling tube to a depth of about 1cm

• Prepare a second test tube with limewater.

• Insert a one-holed rubber stopper into the boiling tube and set up the apparatus such that one end of a delivery tube is connected to the one-holed rubber stopper and the other is inside of the test tube with limewater.

• Remove the one-holed stopper and add about ¼ of a test-tube of dilute hydrochloric acid to the carbonate salt in the boiling tube. Once done, quickly replace the rubber stopper to prevent the gas from escaping.

• Repeat the previous steps using hydrogencarbonate salt instead of the carbonate salt.

• We now know if these salts either contain hydrogencarbonate ions or carbonate ions

• To distinguish between carbonate and hydrogencarbonate ions, add a small amount of the carbonate salt to about ¼ of a test tube of deionised water. Add about ¼ of a test tube of magnesium sulfate solution to the solution of the carbonate.

• Repeat the previous step with the hydrogencarbonate salt.

Observations of detecting the presence of carbonate ions (CO32-) and hydrocarbonate ions (HCO3-) in an aqueous solution.

• When dilute hydrochloric acid is added to a salt containing carbonate or hydrogencarbonate ions, a brisk effervescence or fizzing is observed as the acid comes in contact with the salt. The gas given off is seen bubbling through the limewater and the limewater turns a milky colour as a result.

• When magnesium sulfate solution is added to the solution of the salt, a white precipitate is formed in the solution containing the carbonate while no precipitate is formed in the solution containing the hydrogencarbonate.

Chemical equation(s) of detecting the presence of carbonate ions (CO32-) and hydrocarbonate ions (HCO3-) in an aqueous solution.

• Presence test: CO₃^2- + 2H⁺ →  CO₂ + H₂O

• Presence test: HCO₃^- + H⁺ → CO₂ + H₂O

• Distinguishing test: Mg²⁺ + CO₃^2- → MgCO₃ ↓

• Distinguishing test: Mg²⁺ + 2HCO₃^- → Mg(HCO₃)₂

Explanation of detecting the presence of carbonate ions (CO32-) and hydrocarbonate ions (HCO3-) in an aqueous solution.

• The fizzing seen is caused by carbonate/hydrogencarbonate ions reacting with the acid to create a gas. We know that the gas is carbon dioxide from the water turning milky.

• In the solution of the carbonate, a white precipitate of insoluble magnesium carbonate is formed.

• In the solution of the hydrogencarbonate salt, no precipitate is formed, as magnesium hydrogencarbonate is soluble in water.

Procedure of detecting the presence of nitrate ions (NO3-) in an aqueous solution.

• Fill a test tube to about ¼ of its capacity with deionised water

• Add a small amount of the nitrate salt to the water then shake the test tube to help the salt to dissolve

• Add the same amount of freshly prepared iron(ii) sulfate solution as the nitrate salt

• Using a dropper, add about 3cm cubed of concentrated sulfuric acid down the inside of the test tube while it’s slightly slanted (Be careful concentrated sulfuric acid is highly corrosive)

Observations of detecting the presence of nitrate ions (NO3-) in an aqueous solution.

• When nitrate salt is added it dissolves in water, giving a clear solution

• When concentrated sulfuric acid is added, a brown ring is formed at the boundary of the two layers

Explanation of detecting the presence of nitrate ions (NO3-) in an aqueous solution.

If nitrate ions are present then a brown ring will form at the junction of the concentrated sulfuric acid and solution of nitrate ions and iron(ii) sulfate. The brown ring is due to the formation of the substance FeSO₄NO

Procedure of detecting the presence of phosphate ions in an aqueous solution.

• Fill a test tube to about 1/10th of its capacity with deionised water

• Using a spatula, add a small amount of the phosphate salt to the water, then shake the test tube to help the salt dissolve

• Pour in ammonium molybdate solution to ½ of the test tube’s capacity

• Using a dropper, add 5 drops of concentrated nitric acid to the test tube

• Place the test tube in water at roughly 60 C

Observations of detecting the presence of phosphate ions in an aqueous solution.

• When phosphate salt is added to the water, a clear solution is obtained

• When the test tube is placed in water at roughly 60 C, a yellow precipitate is formed

Chemical equation(s) of detecting the presence of phosphate ions in an aqueous solution.

The chemical formula of ammonium molybdate is (NH₄)₂MoO_4-

Explanation of detecting the presence of phosphate ions in an aqueous solution.

The yellow precipitate is a substance called ammonium phosphomolybdate