Chemistry (9202) | Identification of ions

🧪 Comprehensive Study Notes: Identification of Ions

Welcome to the exciting world of Chemical Analysis! In this chapter, we are going to become chemical detectives. Our mission is to identify unknown substances by performing simple tests on solutions containing dissolved ions.

Understanding these tests is crucial! Chemists use these exact methods in quality control, environmental monitoring, and forensic science to figure out exactly what is inside a sample. Don't worry if this seems like a lot of information; we will break down the tests step-by-step, focusing on the key observation—the color or reaction—that gives away the identity of the ion.

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🔬 Section 1: Identifying Cations (Positive Ions)

Cations are positive ions, usually formed from metals (like Iron or Copper). We often use aqueous sodium hydroxide ($\text{NaOH}$) and aqueous ammonia ($\text{NH}_3$) to identify these ions because they cause the formation of brightly coloured solids called precipitates.

Pre-Requisite: What is a Precipitate?

When you mix two solutions, and they react to form an insoluble solid (a solid that does not dissolve), that solid is called a precipitate (or ppt). It looks cloudy or like fine powder suspended in the liquid.

The General Test Procedure for Metal Ions

1. Take a small sample of the solution containing the unknown ion.
2. Add $\text{NaOH}$ solution drop by drop, noting the initial color and appearance of the precipitate.
3. Add excess $\text{NaOH}$ solution to see if the precipitate redissolves.
4. Repeat steps 1–3 using $\text{NH}_3$ solution instead of $\text{NaOH}$.

Key Cations and Their Characteristic Reactions

The identity of the ion is determined by the color of the precipitate and whether it dissolves in excess alkali.

Cations Identified by Colour: Iron and Copper

These ions are easy to spot because they produce distinctly colored precipitates with both $\text{NaOH}$ and $\text{NH}_3$ and do not dissolve in excess of either (with one important exception for Copper!).

• Iron (II) Ion (\(\text{Fe}^{2+}\)):
  • Result: Forms a green precipitate (Iron(II) Hydroxide).
  • Mnemonics: Think of the 'e' in $\text{Fe}^{2+}$ sounding like 'green'.
  • Common Mistake: After standing for a while, the green precipitate might turn reddish-brown as it reacts with oxygen in the air (oxidation).
• Iron (III) Ion (\(\text{Fe}^{3+}\)):
  • Result: Forms a distinctive red-brown (rusty) precipitate (Iron(III) Hydroxide).
  • Analogy: Think of old rusty metal – that reddish-brown colour is $\text{Fe}^{3+}$!
• Copper (II) Ion (\(\text{Cu}^{2+}\)):
  • Result with NaOH: Forms a light blue precipitate.
  • Result with excess \(\text{NH}_3\): The light blue precipitate dissolves to form a deep blue solution. This is a crucial confirmation test!

Cations Identified by Solubility: Aluminium, Calcium, and Zinc

These ions all produce white precipitates, so we must rely on how they behave when we add excess alkali.

• Aluminium Ion (\(\text{Al}^{3+}\)):
  • Result with NaOH (initial): White precipitate.
  • Result with excess NaOH: The white precipitate dissolves.
  • Result with \(\text{NH}_3\): White precipitate (which does not dissolve in excess $\text{NH}_3$).
• Zinc Ion (\(\text{Zn}^{2+}\)):
  • Result with NaOH (initial): White precipitate.
  • Result with excess NaOH: The white precipitate dissolves.
  • Result with \(\text{NH}_3\): White precipitate (which dissolves in excess $\text{NH}_3$).
• Calcium Ion (\(\text{Ca}^{2+}\)):
  • Result with NaOH: White precipitate.
  • Result with excess NaOH: The white precipitate does not dissolve.
  • Result with \(\text{NH}_3\): Either no precipitate, or a very faint precipitate, as $\text{Ca(OH)}_2$ is slightly soluble.

🔑 Quick Tip for White Precipitates:
The main way to tell $\text{Al}^{3+}$ and $\text{Zn}^{2+}$ apart from $\text{Ca}^{2+}$ is that both $\text{Al}^{3+}$ and $\text{Zn}^{2+}$ precipitates dissolve in excess $\text{NaOH}$. If it dissolves in excess $\text{NaOH}$, it is Aluminium or Zinc. If it stays white and cloudy, it is Calcium.

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🔥 Section 2: Identifying the Ammonium Ion (\(\text{NH}_4^+\))

The ammonium ion is unique because it is the only common positive ion that is not a metal ion and does not form a precipitate when reacted with $\text{NaOH}$ or $\text{NH}_3$.

The Test for Ammonium (\(\text{NH}_4^+\))

1. Add aqueous sodium hydroxide ($\text{NaOH}$) to the sample containing the suspected ammonium ion.
2. Gently warm the mixture.
3. The $\text{NH}_4^+$ ion reacts to produce ammonia gas ($\text{NH}_3$).

Confirmation Test

4. Test the gas being produced by holding a piece of damp red litmus paper near the mouth of the test tube. (Do not touch the solution.)
5. Positive Result: Ammonia gas is alkaline, so it will turn the damp red litmus paper blue. The gas also has a characteristic pungent smell.

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🛡️ Section 3: Identifying Anions (Negative Ions)

Anions are negative ions (like Chloride or Sulfate). Unlike the cation tests, we use a different specific reagent for each group of anions.

1. The Carbonate Ion (\(\text{CO}_3^{2-}\))

This is usually the first anion test performed because carbonates react immediately with any acid, which could interfere with later tests.

The Test for Carbonates

1. Add a small amount of dilute acid (like $\text{HCl}$ or $\text{HNO}_3$) to the sample.
2. Positive Result: The mixture will show effervescence (fizzing or bubbling) as carbon dioxide gas ($\text{CO}_2$) is produced.

Confirmation Test

3. Bubble the gas produced through limewater (aqueous calcium hydroxide, $\text{Ca(OH)}_2$).
4. Confirmation Result: If $\text{CO}_2$ is present, the limewater will turn cloudy or milky due to the formation of insoluble calcium carbonate.

Did you know? Carbonates are important in geology! Limestone caves are formed by the reaction of acid rain (a weak acid) with calcium carbonate rock.

2. The Halide Ions (\(\text{Cl}^{-}, \text{Br}^{-}, \text{I}^{-}\))

The halides (Chloride, Bromide, and Iodide) are identified using silver nitrate ($\text{AgNO}_3$) solution.

Step-by-Step Halide Test

1. Pre-Test Step: Add a few drops of dilute nitric acid ($\text{HNO}_3$).

Why acid first? We add acid to remove any carbonates that might be present. Carbonates also react with silver nitrate to produce a white precipitate, which would give a false positive result for chloride!

2. Now add a few drops of aqueous silver nitrate ($\text{AgNO}_3$).
3. Positive Results (Observe Color):

• Chloride Ion (\(\text{Cl}^{-}\)): Forms a white precipitate (Silver Chloride).
• Bromide Ion (\(\text{Br}^{-}\)): Forms a cream (or off-white) precipitate (Silver Bromide).
• Iodide Ion (\(\text{I}^{-}\)): Forms a yellow precipitate (Silver Iodide).

🔑 Quick Aid for Halides (The Colour Sequence):
$\text{Cl}$, $\text{Br}$, $\text{I}$ -> White, Cream, Yellow (The colors get darker as you go down the group in the Periodic Table).

3. The Sulfate Ion (\(\text{SO}_4^{2-}\))

Sulfate ions are identified using a barium salt, usually barium nitrate ($\text{Ba(NO}_3\text{)}_2$).

Step-by-Step Sulfate Test

1. Pre-Test Step: Add a few drops of dilute nitric acid ($\text{HNO}_3$).

Remember: We add acid here for the same reason as the halide test—to remove any carbonate ions that might be present and cause interference.

2. Now add a few drops of aqueous barium nitrate ($\text{Ba(NO}_3\text{)}_2$).
3. Positive Result: A thick white precipitate is formed. This solid is Barium Sulfate ($\text{BaSO}_4$), which is highly insoluble.

Key Takeaway for Anions: Always add dilute nitric acid first when testing for sulfates and halides to ensure the results are accurate and not confused with a carbonate reaction.

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📝 Summary of Ion Identification Tests

Knowing this summary table is essential for the exam!

Cations (Positive Ions)

Ion	Reagent	Observation
\(\text{Fe}^{2+}\)	\(\text{NaOH}\) or \(\text{NH}_3\)	Green precipitate
\(\text{Fe}^{3+}\)	\(\text{NaOH}\) or \(\text{NH}_3\)	Red-brown precipitate
\(\text{Cu}^{2+}\)	\(\text{NaOH}\)	Light blue precipitate
\(\text{Cu}^{2+}\)	Excess \(\text{NH}_3\)	Precipitate dissolves to form deep blue solution
\(\text{Al}^{3+}\)	Excess \(\text{NaOH}\)	White precipitate dissolves
\(\text{Zn}^{2+}\)	Excess \(\text{NaOH}\) or Excess \(\text{NH}_3\)	White precipitate dissolves
\(\text{Ca}^{2+}\)	Excess \(\text{NaOH}\)	White precipitate does not dissolve
\(\text{NH}_4^+\)	Warm with \(\text{NaOH}\)	Pungent gas produced (turns damp red litmus blue)

Anions (Negative Ions)

Ion	Test Reagent	Observation
\(\text{CO}_3^{2-}\)	Dilute acid (e.g., \(\text{HNO}_3\))	Effervescence; gas turns limewater cloudy
\(\text{SO}_4^{2-}\)	Acid + Barium Nitrate (\(\text{Ba(NO}_3\text{)}_2\))	White precipitate
\(\text{Cl}^{-}\)	Acid + Silver Nitrate (\(\text{AgNO}_3\))	White precipitate
\(\text{Br}^{-}\)	Acid + Silver Nitrate (\(\text{AgNO}_3\))	Cream precipitate
\(\text{I}^{-}\)	Acid + Silver Nitrate (\(\text{AgNO}_3\))	Yellow precipitate

You’ve covered all the required identification tests! Remember that practice makes perfect. Keep reviewing the colors and solubility rules until they become second nature. Great job!