Tests for ions and gases

g) Tests for ions and gases

2.38 describe simple tests for the cations:

1. Li⁺, Na⁺, K⁺, Ca²⁺using flame tests
2. NH₄⁺ using sodium hydroxide solution and identifying the ammonia evolved
3. Cu²⁺, Fe²⁺ and Fe³⁺using sodium hydroxide solution

1. Flame tests:

• Li⁺ (Lithium)à Red flame
• Na⁺ (Sodium)à Orange/Yellow flame
• K⁺ (Potassium)à Lilac
• Ca²⁺ (Calcium)à Brick red flame

2. NH₄⁺(Ammonium ion)à Add sodium hydroxide solution (aqueous NaOH) and do NH₃ (ammonia) test on the fumes evolved. You use damp red litmus paper, and the NH₃ evolved will turn it blue.

3. When you add sodium hydroxide solution to the following:

• Cu²⁺ à Light blue precipitate formed
• Fe²⁺ à Green precipitate formed
• Fe³⁺ à Orange/brown precipitate formed

2.39 describe simple tests for the anions:

1. Cl^-, Br^- and I^-, using dilute nitric acid and silver nitrate solution
2. SO₄^2-, using dilute hydrochloric acid and barium chloride solution
3. CO₃^2-, using dilute hydrochloric acid and identifying the carbon dioxide evolved

1. To test for the halide ions. When you add dilute nitric acid (aqueous HNO₃) followed by silver nitrate solution (aqueous AgNO₃) to the following:

• Cl^-  à white precipitate formed – insoluble AgCl
• Br^- à cream precipitate formed – insoluble AgBr
• I^-     à yellow precipitate formed – insoluble AgI

2. To test for sulphate ions (SO₄^2-).  When you add dilute hydrochloric acid followed by barium chloride solution (aqueous BaCl₂): a white precipitate of insoluble BaSO₄ is formed

3. To test for carbonate ions (CO₃^2-). You can:

• Add an acid (e.g. hydrochloric acid) and test any gas evolved with limewater. You should observe effervescence (fizzing), and the gas will turn limewater milky white as it is CO₂ that is given off. Remember that acid + metal carbonate à salt + water + carbon dioxide
• Heat it strongly, and bubble gas into limewater. Here, thermal decomposition is occurring and CO₂ is given off, hence the limewater will turn milky white too. The most common example is with calcium carbonate, calcium carbonate à calcium oxide + carbon dioxide + water

2.40 describe simple tests for the gases:

1. hydrogen
2. oxygen
3. carbon dioxide
4. ammonia
5. chlorine.

• Hydrogen: apply a litsplint and you will hear a squeaky pop sound
• Oxygen: apply a glowing splint and the splint relights (there’s also the option of applying a lit splint, and the flame just gets brighter but relighting the glowing splint one is much better)
• Carbon dioxide: bubble it into limewater and it goes milky white
• Ammonia: use a damp red litmus paper and it turns blue

• You can also hold it near fumes of concentrated HCl and you  will observe cloudy fumes evolving, which is NH₄Cl (ammonium chloride) and this gas has a characteristic pungent smell

• Chlorine: use damp blue litmus paper and it goes red (then bleaches it white)

To remember which ones uses blue litmus paper and which ones use red litmus paper, remember the periodic table. The metal elements on the left form alkaline substances so they must turn red litmus paper blue – like the universal indicator turns blue in alkaline solutions. And vice versa with the non-metal elements on the right side of the periodic table, they form acidic substances so use blue litmus paper, and observe that it turns red. With the gases, the litmus paper must be damp so that the gases dissolve then act on it. For instance with the chlorine gas.

As an extra, I think you all should know the test for pure water too. You must heat it to boiling point and it boils at exactly  100°C, but you should also cool it till freezing point and it should freeze at 0°C. The reason for adding the freezing bit too is to make it fool-proof, as pure water is the only substance that has these exact boiling and freezing points whilst other substances may boil at 100°C too etc.

For testing the mere presence of water, you just need to add a few drops to anhydrous copper sulphate crystals, which are white, and they turn blue to become hydrated copper sulphate (formula: CuSO₄.H₂O the dot ‘.’ shows that the water is part of the structure, it is what makes it turn blue--called the water of crystallisation)

Water also turns blue cobalt chloride (CoCl₂) pink.