Ionic compounds and simple molecular substances behave differently because they are built from different particles held by different forces. An SPM explanation must name the particles, the force and the result.
This lesson is part of SPM Chemistry chemical bonding. It helps to have drawn ions first, as in drawing ionic bonding representations.
What is the three-part answer frame?
Use the same frame for any property question.
- Particles: ions, or molecules?
- Forces: which attraction must be overcome or is absent?
- Result: how does that explain the property asked about?
The frame stops the two most common failures, which are naming the property without a reason and naming a reason without the particles.
Worked example: two unknown substances
Here is an original data set. Substance P melts at 770 °C, does not conduct as a solid and conducts when molten.
Substance Q melts at −23 °C and does not conduct as a solid or as a liquid.
| Substance P | Substance Q | |
|---|---|---|
| Melting point | 770 °C | −23 °C |
| Solid conducts? | No | No |
| Liquid conducts? | Yes | No |
| Likely particles | Ions | Molecules |
For P, the answer runs: “P contains ions. There are strong electrostatic forces between the oppositely charged ions.”
It continues: “A lot of heat energy is needed to overcome them, so the melting point is high. The ions can move when molten, so it conducts.”
For Q: “Q contains neutral molecules. The forces between molecules are weak, so little energy is needed to separate them and the melting point is low.”
It continues: “There are no freely moving charged particles, so it does not conduct.”
The mistake that costs the explanation marks
A very common answer for Q says: “The covalent bonds are weak, so it melts easily.” This sounds reasonable and it is wrong.
| Wrong | Right |
|---|---|
| Covalent bonds are broken on melting | Only the weak forces between molecules are overcome |
| Covalent bonds are weak | Covalent bonds inside a molecule are strong, and they stay in place |
The molecules themselves survive melting. This distinction is developed further in distinguishing intermolecular forces from chemical bonds.
Check yourself
Substance R does not conduct as a solid, conducts when dissolved in water, and melts at 650 °C. State the likely type of particle and explain the conductivity.
Answer
R is most likely made of ions, because it has a high melting point and conducts once dissolved.
In the solid, the ions are held in fixed positions by strong electrostatic forces, so no charged particle can move. When R dissolves, the ions separate and move freely, so they can carry charge and the solution conducts.
What to study next
Practise the frame on new substances in the chemical bonding practice set. If calculations are your weak side instead, use the mole and stoichiometry steps tool.
For a teacher to check your explanations line by line, see online one-to-one Chemistry tuition.