IGCSE Chemistry 0620: Linking Melting Point, Conductivity and Solubility to Bonding

A materials science teacher sets up a "mystery substances" identification activity for her class. She gives each group data for two unlabelled solid samples, P and Q, without revealing their names:

Sample P: melting point $800\text{ °C}$ . Does not conduct electricity as a solid, but conducts well when melted or dissolved in water. Dissolves readily in water.

Sample Q: melting point $-95\text{ °C}$ (it is a gas at room temperature). Does not conduct electricity as a solid, liquid or gas. Does not dissolve appreciably in water.

(a) State the type of structure and bonding present in sample P and in sample Q. [2]

(b) Use the structure and bonding of each sample to explain the large difference between their melting points. [3]

(c) Explain, in terms of structure and bonding, why sample P conducts electricity only when it is molten or dissolved in water, but not when it is a solid. [2]

(d) State whether you would expect sample P or sample Q to be more soluble in water, and give a reason for your answer based on structure and bonding. [2]

Part (a): Identifying the structure and bonding

Sample P’s very high melting point and its pattern of conductivity (none as a solid, but good conductivity when molten or dissolved) are typical of a giant ionic lattice held together by ionic bonding.

Sample Q’s very low melting point and its complete lack of conductivity in any state are typical of a simple molecular substance held together by covalent bonding.

Part (b): Explaining the difference in melting points

Sample P is a giant ionic lattice: a regular arrangement of oppositely charged ions held together throughout the structure by strong electrostatic forces of attraction. Melting the lattice means overcoming these strong forces across the whole structure, which requires a large amount of energy, giving sample P a high melting point.

Sample Q consists of small, separate molecules. The covalent bonds within each molecule are strong, but the forces of attraction between separate molecules are much weaker. Melting sample Q only requires overcoming these weak intermolecular forces, not breaking any covalent bonds, so its melting point is far lower than sample P’s. Low enough that sample Q is a gas at room temperature.

Part (c): Explaining sample P’s conductivity

In the solid state, the ions in sample P are held rigidly in fixed positions in the lattice by strong electrostatic forces. Since the ions cannot move from these positions, they cannot carry a charge from place to place, so solid sample P does not conduct electricity.

When sample P is melted, or dissolved in water, the rigid lattice structure breaks down and the ions become free to move. Since these charged ions can now move and carry charge, molten or dissolved sample P does conduct electricity.

Part (d): Comparing solubility

Sample P would be expected to be more soluble in water. Ionic compounds such as sample P are typically soluble in water, while simple molecular covalent substances such as sample Q are typically not very soluble in water, consistent with sample Q’s description as not dissolving appreciably.

Final answers

(a) P: giant ionic lattice, ionic bonding. Q: simple molecular, covalent bonding.
(b) P’s strong electrostatic forces throughout the lattice need a lot of energy to overcome (high melting point); Q’s weak forces between molecules need little energy to overcome (low melting point), even though the covalent bonds within each Q molecule are strong.
(c) Solid P: ions fixed, cannot move, no conduction. Molten/dissolved P: ions free to move, conducts electricity.
(d) Sample P is more soluble in water; ionic compounds are typically soluble, simple molecular covalent substances typically are not.