Tests for anions for IGCSE and O Level Chemistry

Summary

In this chemistry practical, various anions are tested using distinct reagents. Carbonates are identified by their reaction with acid, producing CO2 gas that turns lime water milky. Sulfates are confirmed with barium nitrate, differentiating from carbonates using nitric acid. Halides (chlorides, bromides, iodides) are identified through silver nitrate, producing different color precipitates. Nitrates are detected by the ammonia gas produced when reacting with sodium hydroxide and aluminum foil. Finally, sulfites are tested by generating sulfur dioxide gas, which decolorizes potassium manganate, indicating the presence of sulfite ions.

Takeaways

• 😀 Carbonates react with dilute acid to produce CO₂ gas, which is captured in lime water, turning it milky.
• 😀 Sulfate ions can be identified by adding barium nitrate, which forms a white precipitate of barium sulfate.
• 😀 To distinguish between carbonate and sulfate ions, nitric acid is added to remove carbonates before adding barium nitrate.
• 😀 Halides (chlorides, bromides, iodides) are tested using silver nitrate. Chloride forms a white precipitate, bromide forms cream, and iodide forms yellow.
• 😀 A subtle difference in the color of the silver bromide (green) and silver iodide (yellow) precipitates can be observed.
• 😀 The nitrate test involves adding sodium hydroxide and aluminum foil, followed by gentle heating to produce ammonia gas.
• 😀 Ammonia gas presence is confirmed when red litmus paper turns blue, indicating the presence of nitrate ions.
• 😀 Sulfur dioxide gas is produced by reacting a sulfite salt with hydrochloric acid, and it decolorizes acidified potassium permanganate.
• 😀 The purple color of potassium permanganate is decolorized to colorless when it reacts with sulfur dioxide.
• 😀 A damp red litmus paper is used to confirm the presence of ammonia gas during the nitrate test, turning blue in the process.

Q & A

What is the purpose of the experiment?

The purpose of the experiment is to test for the presence of various anions, including carbonate, sulfate, halides, nitrate, and sulfite ions, using specific chemical reactions and reagents.

How is the presence of carbonate ions tested?

Carbonate ions are tested by adding a dilute acid to the sample. The carbon dioxide gas produced is captured and passed into lime water, which turns milky, indicating the presence of carbonate ions.

Why does the addition of nitric acid help differentiate between carbonate and sulfate ions?

Nitric acid is added to eliminate carbonate ions, as carbonates react with acids to release carbon dioxide. After this step, the presence of sulfate ions can be confirmed by adding barium nitrate, which forms a white precipitate only if sulfate ions are present.

What is the role of barium nitrate in the sulfate test?

Barium nitrate is used in the sulfate test because barium ions react with sulfate ions to form a white precipitate of barium sulfate, confirming the presence of sulfate ions.

How are halide ions (chloride, bromide, iodide) tested for?

Halide ions are tested by adding nitric acid to remove any carbonate ions, followed by the addition of silver nitrate. Chloride ions form a white precipitate, bromide ions form a cream precipitate, and iodide ions form a yellow precipitate.

What is the difference between silver bromide and silver iodide precipitates?

The silver bromide precipitate is cream-colored, while silver iodide is yellow. Although they are similar in appearance, iodide forms a more distinct yellow precipitate compared to the greener cream color of bromide.

How is the presence of nitrate ions detected?

Nitrate ions are detected by adding sodium hydroxide and aluminum foil to the sample. Heating this mixture releases ammonia gas, which can be detected by its ability to turn damp red litmus paper blue.

What chemical reaction produces ammonia gas in the nitrate test?

The reaction between sodium hydroxide, aluminum foil, and nitrate ions produces ammonia gas, which is detected by the color change of damp red litmus paper turning blue.

How is sulfur dioxide gas produced in this experiment?

Sulfur dioxide gas is produced by reacting a sulfite salt, such as sodium sulfite, with hydrochloric acid.

What is the role of potassium manganate in the sulfite test?

Potassium manganate (VII) is used to test for sulfur dioxide. The purple color of the manganate ion is decolorized in the presence of sulfur dioxide, indicating its presence.