Properties of Ionic Compounds

This lesson explains the physical properties of ionic compounds and links them to the giant ionic lattice structure. Understanding why ionic compounds have high melting points, can conduct electricity under certain conditions and are often soluble in water is key to success in the Edexcel GCSE Chemistry (1CH0) exam.

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Recap: The Giant Ionic Lattice

Ionic compounds consist of a regular, repeating three-dimensional arrangement of positive and negative ions. This is called a giant ionic lattice. The ions are held together by strong electrostatic forces of attraction that act in all directions between oppositely charged ions.

The key properties of ionic compounds are all explained by this structure.

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High Melting and Boiling Points

Ionic compounds have high melting points and high boiling points.

Why?

• There are strong electrostatic forces between oppositely charged ions throughout the giant lattice.
• These forces act in all directions (not just between one pair of ions).
• A large amount of energy is needed to overcome these strong attractions and separate the ions.
• Therefore, high temperatures are required to melt or boil an ionic compound.

Ionic Compound	Formula	Melting Point (°C)	Boiling Point (°C)
Sodium chloride	NaCl	801	1413
Magnesium oxide	MgO	2852	3600
Calcium chloride	CaCl₂	772	1935
Potassium bromide	KBr	734	1435

Notice that magnesium oxide has a much higher melting point than sodium chloride. This is because Mg²⁺ and O²⁻ have greater charges than Na⁺ and Cl⁻, so the electrostatic forces between them are stronger.

Exam Tip: When explaining high melting points, always refer to the strong electrostatic forces between oppositely charged ions in the giant ionic lattice. Simply saying "strong bonds" is not enough — you must specify the type of force.

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Electrical Conductivity

Ionic compounds can conduct electricity, but only under certain conditions.

Solid State — Does NOT Conduct

• In the solid state, the ions are held in fixed positions in the lattice.
• They cannot move freely.
• Because electrical conduction requires the movement of charged particles, solid ionic compounds do not conduct electricity.

Molten (Liquid) State — DOES Conduct

• When an ionic compound is melted, the lattice structure breaks down.
• The ions are free to move throughout the liquid.
• These mobile ions can carry charge from one electrode to another.
• Therefore, molten ionic compounds conduct electricity.

Dissolved in Water — DOES Conduct

• When an ionic compound dissolves in water, the lattice breaks apart.
• The ions separate and become free to move in the solution.
• The mobile ions can carry charge, so the solution conducts electricity.

Condition	Conduct Electricity?	Reason
Solid	No	Ions are in fixed positions — cannot move
Molten (liquid)	Yes	Ions are free to move and carry charge
Dissolved in water	Yes	Ions are free to move and carry charge

Exam Tip: The key phrase is "ions are free to move." It is not enough to say "ions are present" — you must explain that they can move to carry charge. Solid ionic compounds contain ions, but the ions cannot move, so they don't conduct.

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Solubility in Water

Many ionic compounds are soluble in water.

Why?

• Water molecules are polar — they have a slight positive charge on the hydrogen atoms and a slight negative charge on the oxygen atom.
• The water molecules are attracted to the ions on the surface of the ionic lattice.
• They surround the individual ions and pull them away from the lattice.
• This process is called dissolving — the lattice breaks apart and the ions become dispersed throughout the solution.

However, not all ionic compounds dissolve in water. Some lattices have such strong forces between the ions that water molecules cannot separate them. For example, barium sulfate (BaSO₄) is insoluble in water.

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Brittleness

Ionic compounds are hard but brittle — they shatter when a force is applied.

Why?

• When a force is applied, layers of ions can be displaced (shifted).
• This brings ions of the same charge next to each other.
• The repulsive forces between like-charged ions cause the lattice to shatter (cleave apart).

This can be visualised as:

1. Before force: + − + − (alternating charges — attractive).
2. After displacement: + + and − − (same charges adjacent — repulsive — shatters).

Exam Tip: This is a popular higher-tier question. Make sure you explain that ions of the same charge end up next to each other, causing repulsion, which makes the crystal shatter.

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Worked Example: Linking Properties to Structure

Question: Sodium chloride has a melting point of 801 °C and conducts electricity when dissolved in water but not as a solid. Explain these properties in terms of its structure and bonding.

Answer:

• Sodium chloride has a giant ionic lattice structure.
• It is held together by strong electrostatic forces between Na⁺ and Cl⁻ ions, acting in all directions.
• A large amount of energy is needed to overcome these forces, which is why the melting point is high (801 °C).
• In the solid state, the ions are in fixed positions and cannot move, so it does not conduct electricity.
• When dissolved in water, the ions are free to move and can carry electrical charge, so the solution does conduct electricity.

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Worked Example: Comparing Ionic Compounds

Question: Explain why magnesium oxide (MgO) has a higher melting point than sodium chloride (NaCl).

Answer:

• Both are giant ionic lattice structures held by strong electrostatic forces between oppositely charged ions.
• In MgO, the ions are Mg²⁺ and O²⁻ — both have charges of ±2.
• In NaCl, the ions are Na⁺ and Cl⁻ — both have charges of ±1.
• The greater the charge on the ions, the stronger the electrostatic attraction.
• Therefore more energy is needed to overcome the forces in MgO, giving it a higher melting point.

Exam Tip: When comparing melting points of ionic compounds, always compare the charges on the ions. Greater charges = stronger forces = higher melting points.

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Summary

Property	Explanation
High melting/boiling point	Strong electrostatic forces between ions in all directions — lots of energy needed
Conducts when molten/dissolved	Ions are free to move and carry charge
Does not conduct as solid	Ions are in fixed positions — cannot move
Soluble in water (many)	Polar water molecules pull ions from the lattice
Brittle	Displacement brings like charges together — repulsion causes shattering

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Key Points

• Ionic compounds have a giant ionic lattice structure.
• High melting and boiling points because of strong electrostatic attraction between oppositely charged ions in all directions.
• Conduct electricity when molten or dissolved because ions are free to move and carry charge.
• Do not conduct as solids because ions are in fixed positions.
• Many are soluble in water because polar water molecules can separate the ions.
• They are brittle because displacement of layers brings like-charged ions together, causing repulsion.

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Practice Questions