Properties of ionic compoundsIB MYP Chemistry: Revision notes
Section 1
Structure explains properties
Ionic compounds are giant ionic lattices: huge numbers of positive and negative ions in a regular pattern, held together by strong electrostatic attraction. This one idea explains all their properties.
To answer an exam question, always link a property to the structure and the bonding: say what the particles are, what forces act, and what happens to them.
Section 2
High melting and boiling points
Ionic compounds are solids at room temperature with high melting points. For example, sodium chloride melts at 801 °C.
To melt a solid, the forces holding the particles in place must be overcome. In an ionic lattice the electrostatic attraction between oppositely charged ions is strong and acts in all directions, so a lot of energy is needed to break the lattice apart.
Do not write that covalent bonds break when an ionic compound melts. The electrostatic attractions between ions are overcome.
Section 3
Conductivity
To conduct electricity, a substance needs charged particles that are free to move.
- Solid: the ions are held in fixed positions in the lattice, so they cannot move. Solid ionic compounds do not conduct.
- Molten: the lattice breaks down and the ions are free to move, so the liquid does conduct.
- Dissolved in water: the ions are free to move in the solution, so it does conduct.
In ionic compounds, it is the ions that carry the charge, not electrons.
Saying "it has no free electrons" is not enough. Say that the ions are in fixed positions in the solid but free to move when molten or dissolved.
Section 4
Solubility
Many ionic compounds dissolve in water. Water molecules attract the ions at the surface of the lattice and pull them away, so the lattice breaks up and the ions spread through the water.
Not every ionic compound is soluble. Some, such as calcium carbonate (in limestone and chalk), are insoluble because the attraction between their ions is too strong for water to overcome.
Section 5
Testing and comparing: a worked example
A student tests a white solid. It has a high melting point, does not conduct as a solid, conducts when dissolved in water and when molten. These results fit an ionic compound.
Compare with other types of structure: simple molecular compounds have low melting points and do not conduct in any state; metals conduct when solid.
When you evaluate an investigation into conductivity, remember to keep the electrodes the same distance apart, use the same volume of water, and repeat readings to find a mean.
Must know
- Ionic compounds have high melting points because strong electrostatic attraction needs a lot of energy to overcome.
- They conduct when molten or dissolved, because the ions are free to move, but not when solid.
- Many are soluble in water because water molecules pull the ions from the lattice.
- Always link the property to the structure and the bonding.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Properties of ionic compounds
- A student in Cairo heats a few crystals of sodium chloride in a crucible over a Bunsen burner. The crystals do not melt, even though the flame is very hot. A furnace is needed to melt sodium chloride, at 801 °C.Explain why sodium chloride has such a high melting point.2 marks
- A teacher in Auckland gives students a white solid labelled X. They find that X does not conduct electricity when solid, melts only above 700 °C, and conducts electricity well when molten. X also dissolves in water, and the solution conducts electricity.Explain why X does not conduct electricity when solid but does when molten.2 marks
- A student in Kuala Lumpur investigates how well salt solutions conduct electricity. She dissolves 0 g, 2 g, 4 g, 6 g and 8 g of sodium chloride in separate 100 cm³ portions of distilled water and measures the current in each solution, using the same circuit and electrodes every time. The ammeter readings were 0.00 A, 0.18 A, 0.35 A, 0.52 A and 0.69 A. She predicted that the more sodium chloride dissolved, the greater the current.Describe the trend in her results and explain it.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).