6.2.5 Hardness and Brittleness

Hardness:

- Lattice stiffness makes ionic compounds hard.

Brittleness:

- However, when a voltage is applied, like-charged ions can be pushed together, causing repulsion and brittleness — a tendency to break.

Key point: Ionic compounds are hard but brittle because of the lattice structure.

Examples of Ionic Compounds

Examples of ionic compounds are:

• Sodium chloride (NaCl)
• Potassium bromide (KBr)
• Calcium carbonate (CaCO₃)
• Magnesium sulphate (MgSO₄)

Each of these compounds forms a clear crystal lattice that determines their structure and many of their physical properties.

Summary Table

Intermediate Level

Physical state:

• Salts are solids at STP due to strong electrostatic forces.
• They form a crystal lattice.

Electrical conductivity:

• Solid salts: do not conduct (ions fixed in lattice).
• Molten salts: conduct (ions free to move).
• Aqueous solutions: conduct (ions free to move).

Melting and boiling points:

- High, because strong electrostatic forces must be overcome.

Hardness and brittleness:

• Hard because of lattice stiffness.
• Brittle because like-charged ions repel when a voltage is applied.

Key point: The properties of salts are determined by the strong electrostatic forces in their lattice structure.

Important Points

• - At STP, salts are usually solids.
• - This is due to the strong electrostatic forces of attraction between oppositely charged ions.
• - These forces form a tightly bound lattice structure.
• - These forces require significant energy to overcome, resulting in high melting points.
• - Lattice structure: Crystal lattice that maximizes attraction and minimizes repulsion.
• - Electrical conductivity:
• - Solid salts: do not conduct.
• - Molten salts: conduct.
• - Aqueous solutions: conduct.
• - High melting and boiling points due to strong electrostatic forces.
• - Hardness due to lattice stiffness.
• - Brittleness due to repulsion between like-charged ions when voltage is applied.
• - Examples: NaCl, KBr, CaCO₃, MgSO₄.

Remember

• - Salts are solids at STP.
• - Strong electrostatic forces hold ions in a lattice.
• - High melting points due to strong forces.
• - Solid salts do not conduct electricity.
• - Molten and aqueous salts conduct electricity.
• - Salts are hard but brittle.
• - Examples: NaCl, KBr, CaCO₃, MgSO₄.
• - Lattice maximizes attraction, minimizes repulsion.
• - Free-moving ions enable conductivity.

Common Mistakes

Quick Revision

Physical State and Melting Points:

6.2.1 Solids at STP: At Standard Temperature and Pressure (STP), salts are usually solids. This is due to the strong electrostatic forces of attraction between the oppositely charged ions, which form a tightly bound lattice structure. These forces require significant energy to overcome, resulting in high melting points.

6.2.2 Lattice Structure of Ionic Compounds: Under normal conditions, ionic compounds are usually solids with a well-defined lattice structure. This property is due to the nature of the ionic bonds that hold the ions together. Ionic bonds are formed by electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). These ions are arranged in a repeating three-dimensional pattern called a crystal lattice. This arrangement maximizes the electrostatic attractions and minimizes the repulsions between ions, leading to a stable structure.

6.2.3 Electrical Conductivity: Molten and aqueous solutions of salts are good conductors of electricity because of the presence of free-moving ions. When salts are in their solid state, their ions are held together in a rigid lattice structure by strong ionic bonds. In this state, the ions are not free to move, so solid salts do not conduct electricity. Upon melting, the rigid lattice breaks down due to the high temperature. The ions become free to move in the molten state. These free-moving positive (cations) and negative (anions) ions can carry electrical charge, allowing the molten salt to conduct electricity. Similarly, when salts dissolve in water, they dissociate into their constituent ions due to the polar nature of water molecules. For example, sodium chloride (NaCl) dissociates into Na⁺ and Cl^- ions. These free ions are now surrounded by water molecules but can move freely within the solution. When an electric potential is applied, cations migrate toward the cathode (negative electrode), and anions migrate toward the anode (positive electrode), completing the circuit and allowing electrical conductivity. In both molten and aqueous states, the ionic lattice is disrupted, liberating the ions. Free-moving ions under the influence of an electric field enable the conduction of electricity.

6.2.4 High Melting Points and Boiling Points: The strong electrostatic forces of the lattice require significant energy to break, resulting in high melting points and boiling points. Ionic compounds are solid at STP with high melting points because of the strong electrostatic forces of attraction between their ions, which hold these ions in fixed positions.

6.2.5 Hardness and Brittleness: Lattice stiffness makes ionic compounds hard. However, when a voltage is applied, like-charged ions can be pushed together, causing repulsion and brittleness and a tendency to break. Examples of ionic compounds are sodium chloride (NaCl), potassium bromide (KBr), calcium carbonate (CaCO₃), magnesium sulphate (MgSO₄), etc. Each of these compounds forms a clear crystal lattice that determines their structure and many of their physical properties.

60-Second Revision

• **Salts** = solids at STP.
• Strong electrostatic forces hold ions in a lattice.
• High melting points due to strong forces.
• Solid salts = do not conduct electricity.
• Molten salts = conduct electricity.
• Aqueous salts = conduct electricity.
• Hard but brittle.
• Examples: NaCl, KBr, CaCO₃, MgSO₄.
• Lattice maximizes attraction, minimizes repulsion.
• Free-moving ions enable conductivity.
• ⚠️ Warning: solid salts do not conduct electricity because their ions are not free to move — they conduct only when molten or dissolved in water.