How can we work out the type of bonding in a substance from its properties?
Linking the type of structure and bonding to the properties of a substance, and using properties to identify the bonding type.
A focused answer to the WJEC GCSE Science Double Award Unit 5 topic linking structure and bonding to properties, covering how melting point, conductivity and hardness reveal whether a substance is ionic, simple molecular, giant covalent or metallic.
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What this dot point is asking
WJEC Double Award Unit 5 wants you to link the type of structure and bonding to the properties of a substance, and to use properties to identify the bonding type.
Linking structure to properties
Each property comes from the structure: strong forces in giant structures give high melting points; free-moving charges (ions or electrons) allow conduction.
Using melting point as a clue
A high melting point points to a giant structure (ionic, giant covalent or metallic), because many strong bonds or forces must be broken. A low melting point points to a simple molecular substance, because only weak forces between molecules are overcome. So the first thing to check is whether the melting point is high or low.
Using conductivity as a clue
Conductivity then narrows it down:
- Conducts in all states to metallic (free electrons).
- Conducts only when molten or dissolved to ionic (ions free to move only when not solid).
- Never conducts to simple molecular or most giant covalent (no free charges).
- The special case: graphite is giant covalent but conducts because of its spare electrons.
Putting the clues together
A summary table of the four types
It is worth memorising a summary that links each structure to its properties, because comparison questions are common. Ionic: giant lattice, high melting point, conducts only when molten or dissolved, brittle. Simple molecular: small molecules, low melting point, never conducts. Giant covalent: huge network, very high melting point, very hard, usually non-conducting (graphite conducts). Metallic: ions in a sea of electrons, high melting point, conducts in all states, malleable. Having this table clear in your mind lets you both predict properties from the structure and deduce the structure from the properties.
Why the same data can be ambiguous
Sometimes one property alone is not enough to decide the bonding, which is why you should use more than one clue. For example, both ionic and metallic substances have high melting points, so the melting point alone cannot tell them apart - but their conductivity differs (metals conduct when solid, ionic compounds do not). Likewise, a non-conducting solid could be simple molecular or giant covalent, so you also check the melting point and hardness. Using two or three properties together, rather than just one, gives a reliable answer and is what examiners expect.
Try this
Q1. A substance conducts electricity when solid. Which type of bonding is it likely to have? [1 mark]
- Cue. Metallic.
Q2. A substance has a low melting point and never conducts. What type of structure is it? [1 mark]
- Cue. Simple molecular.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksA substance has a high melting point, does not conduct when solid but does when molten. Identify the type of bonding and justify your answer.Show worked answer →
A Unit 5 deduction question worth 4 marks. Reward: the bonding is ionic (1); the high melting point shows strong forces in a giant lattice (1); conducting only when molten (not solid) shows the charge is carried by ions that become free to move when melted (2). Markers credit identifying ionic and using both the melting point and the conductivity pattern. A common error is to choose metallic (metals conduct when solid too).
WJEC style3 marksA substance has a low melting point and does not conduct electricity in any state. What type of structure does it have, and why?Show worked answer →
A Unit 5 deduction question. Reward: it is a simple molecular substance (1); the low melting point shows only weak forces between molecules need to be overcome (1); it does not conduct because it has no free electrons or ions to carry charge (1). Markers credit simple molecular, the weak intermolecular forces, and the lack of charged particles. A common error is to say ionic (ionic compounds have high melting points).
Related dot points
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