Identifying Ions in Solution Revision Notes for HSC SSCE Chemistry

Identifying Ions in Solution

Why identify ions in solution?

Chemists need to identify inorganic substances that may have contaminated food, water or soil. When testing water quality, chemists must first identify which substances are present, then measure the levels of any substances that could be dangerous.

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Real-World Application: Sydney Mine Contamination

In August 2017, scientists discovered that water from an old mine site was escaping into Sydney's drinking water catchment. Testing revealed high levels of manganese, zinc, iron and sulfate ions. This case demonstrates why ion identification is so important for protecting public health.

Tests for identifying ions

Several qualitative tests can identify the presence of particular cations and anions in solution:

Flame tests - identify certain metal cations by their characteristic flame colours
Precipitation reactions - identify ions based on whether they form insoluble compounds
Gravimetric analysis - measure quantities by weighing precipitates
Precipitation titrations - determine concentrations through controlled precipitation
Instrumental techniques - use specialised equipment for detection and measurement

This note focuses on using precipitation reactions and solubility data for ion identification.

What are precipitation reactions?

A precipitation reaction occurs when a solid forms after mixing two solutions together. The solid that forms is called a precipitate.

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Why Precipitation Reactions Are Valuable

These reactions are valuable because they help us:

Identify whether a particular cation or anion is present in a sample
Test for specific ions like lead or barium in water supplies
Remove unwanted substances from water

The key to using precipitation reactions for identification is understanding which combinations of ions form precipitates and which remain dissolved.

Understanding solubility data

The solubility of ionic compounds determines whether they will precipitate when their solutions are mixed. Chemists classify compounds based on their solubility:

High solubility (aq): $\geq 0.1$ mol L $^{-1}$ - the compound dissolves readily and stays in solution
Low solubility (s): $< 0.1$ mol L $^{-1}$ - the compound forms a solid precipitate
Sparingly soluble: solubility range $1-10$ g L $^{-1}$ - may or may not precipitate depending on concentration

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The solubility table below shows which combinations of anions and cations produce soluble compounds (stay dissolved) versus insoluble compounds (form precipitates). Use this table to predict whether mixing two ionic solutions will produce a precipitate.

Anion	Cl $^-$ , Br $^-$ , I $^-$	S $^{2-}$	OH $^-$	SO $_4^{2-}$	CO $_3^{2-}$ , PO $_4^{3-}$ , SO $_3^{2-}$	CH $_3$ COO $^-$	NO $_3^-$
High solubility (aq) $\geq 0.1$ mol L $^{-1}$	Most	Group 1, Group 2, NH $_4^+$	Group 1, Sr $^{2+}$ , Ba $^{2+}$ , NH $_4^+$	Most	Group 1, NH $_4^+$	Most	All
Low solubility (s) $< 0.1$ mol L $^{-1}$	Ag $^+$ , Pb $^{2+}$ , Cu $^{2+}$	Most	Most	Ag $^+$ , Pb $^{2+}$ , Ca $^{2+}$ , Ba $^{2+}$ , Sr $^{2+}$	Most	Ag $^+$	None

Key solubility rules

These rules help you quickly determine whether a precipitate will form:

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Rule 1: Group 1 and Ammonium Compounds

All Group 1 compounds have high solubility in water
All ammonium (NH $_4^+$ ) compounds have high solubility in water
This means sodium chloride, potassium carbonate, ammonium sulfate etc. will always stay dissolved

Rule 2: Sparingly soluble compounds

The following compounds have intermediate solubility ( $1-10$ g L $^{-1}$ ). They may not precipitate if present in very low concentrations:

Silver acetate
Lead(II) chloride
Calcium sulfate
Silver sulfate
Calcium hydroxide

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Rule 3: Phosphate and pH

Pb $^{2+}$ , Ba $^{2+}$ , Cu $^{2+}$ and Ag $^+$ will not precipitate with PO $_4^{3-}$ in solutions where pH $< 2$ . This is because phosphate ion behaviour changes in very acidic conditions.

Detecting precipitation

When a precipitate forms, you will see a solid settling at the bottom of the container. However, if only a small amount of precipitate forms, it may stay suspended in the liquid, making the mixture appear cloudy rather than showing obvious solid particles.

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If mixing two clear solutions produces cloudiness, consider this evidence that a precipitate has formed.

Using precipitation reactions to identify ions

When you want to identify an unknown ion in solution, you can add reagents that will form precipitates with certain ions but not others. By observing which combinations produce precipitates, you can narrow down which ions must be present.

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Worked Example: Identifying Unknown Cations

Suppose you have a solution containing an unknown metal cation. You can test for specific ions:

Testing for silver, lead or copper(II) ions:

Add chloride ions (e.g., from sodium chloride solution)
If a precipitate forms → the solution contains Ag $^+$ , Pb $^{2+}$ or Cu $^{2+}$
If no precipitate forms → these ions are not present

Testing for Group 2 metal ions:

Add sulfate ions (e.g., from sodium sulfate solution)
If a precipitate forms → the solution may contain Ba $^{2+}$ , Sr $^{2+}$ or Ca $^{2+}$
Remember: Most other sulfates are soluble, so they won't precipitate

Testing with hydroxide ions:

Add hydroxide ions (e.g., from sodium hydroxide solution)
If a precipitate forms → the solution contains most metal cations (except Group 1)
If no precipitate forms → the solution likely contains a Group 1 metal or NH $_4^+$

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Key Points to Remember:

Precipitation reactions occur when a solid forms from mixing two solutions, providing a visual test for identifying ions in samples
Solubility rules predict which ion combinations will precipitate - Group 1 and ammonium compounds always stay dissolved, whilst most other combinations follow specific patterns
High solubility means $\geq 0.1$ mol L $^{-1}$ (stays dissolved), whilst low solubility means $< 0.1$ mol L $^{-1}$ (forms precipitate)
Cloudiness in a previously clear solution indicates precipitation has occurred, even if no solid is visible at the bottom
Real-world applications include testing water supplies for toxic ions like lead and barium, and removing unwanted substances through selective precipitation

Identifying Ions in Solution (HSC SSCE Chemistry): Revision Notes