How can we work out the bonding in a substance from its melting point and conductivity?
Use the properties of a substance (melting point, conductivity, state) to deduce its type of bonding and structure.
A focused answer to WJEC GCSE Chemistry topic 2.1, bringing ionic, simple molecular, giant covalent and metallic structures together and showing how to deduce the bonding in a substance from its melting point, electrical conductivity and state.
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What this topic is asking
WJEC topic 2.1 brings the four structure types together. You must be able to work backwards from a substance's properties - its melting point, electrical conductivity and state - to deduce its bonding and structure. This is a very common exam skill because it tests whether you really understand each model.
The four structure types side by side
Each type of bonding gives a characteristic set of properties:
- Ionic (giant ionic lattice): high melting and boiling points; conducts only when molten or dissolved; often soluble in water; brittle.
- Simple molecular (small covalent molecules): low melting and boiling points; does not conduct at all; often gases or liquids at room temperature.
- Giant covalent (macromolecular network): very high melting points; very hard; usually does not conduct (except graphite, which conducts).
- Metallic (positive ions in a sea of electrons): high melting points; conducts as a solid and when molten; malleable and ductile.
Using melting point
So the first split is low (simple molecular) versus high (one of the three giant structures).
Using electrical conductivity
Conductivity is the most powerful clue for telling the three giant structures apart:
Putting the two together: first use the melting point to decide simple molecular or giant, then use conductivity to decide which giant structure.
Solubility and other clues
Other properties can support a deduction. Ionic compounds are often soluble in water; simple molecular substances are often insoluble in water but may dissolve in other solvents; giant covalent substances are usually insoluble and metals are insoluble. State at room temperature also helps: only simple molecular substances are commonly gases.
A useful way to organise these clues in the exam is to work through them in order. First, is the melting point low or high? Low means simple molecular and you can stop there. If high, ask: does it conduct as a solid? If yes, it is a metal. If it conducts only when molten or dissolved, it is ionic. If it does not conduct at all and is hard, it is giant covalent (unless it is graphite, which conducts). Following the same order every time stops you jumping to a wrong answer from a single clue.
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 sample4 marksA substance has a high melting point, does not conduct as a solid but does conduct when molten, and dissolves in water. Identify its type of bonding and justify your answer.Show worked answer →
A Unit 2.1 deduction question. Reward: this is an ionic substance. The high melting point shows strong forces in a giant structure; not conducting as a solid but conducting when molten is the key clue, because the ions are fixed in the solid but free to move when molten; dissolving in water is also typical of ionic compounds. Markers credit the conclusion (ionic) and the use of at least two pieces of evidence, especially the conduction only when molten, linked to free-moving ions. A common error is to confuse this with a metal, but a metal would conduct as a solid too.
WJEC sample3 marksA substance has a very high melting point, is very hard, and does not conduct electricity. Suggest its structure and explain your reasoning.Show worked answer →
A Unit 2.1 deduction question. Reward: this is a giant covalent structure (such as diamond or silicon dioxide). The very high melting point and hardness show a giant network of strong covalent bonds throughout; not conducting shows there are no free electrons or ions to carry charge (all electrons are used in bonding). Markers credit giant covalent and the reasoning that strong covalent bonds throughout give a high melting point and hardness while leaving no free charged particles. A common slip is to call it ionic, but an ionic solid would conduct when molten.
Related dot points
- Describe ionic bonding as electron transfer, draw dot-and-cross diagrams for simple ionic compounds, and relate the giant ionic lattice to the properties of ionic compounds.
A focused answer to WJEC GCSE Chemistry topic 2.1, covering ionic bonding as the transfer of electrons between metals and non-metals, drawing dot-and-cross diagrams, the giant ionic lattice, and how the structure explains high melting points and conduction when molten or dissolved.
- Describe covalent bonding as shared pairs of electrons, draw dot-and-cross diagrams for simple molecules, and relate simple molecular structure to low melting points and poor conduction.
A focused answer to WJEC GCSE Chemistry topic 2.1, covering covalent bonding as the sharing of electron pairs between non-metal atoms, drawing dot-and-cross diagrams for simple molecules, and explaining why simple molecular substances have low melting points and do not conduct.
- Describe giant covalent structures including diamond and graphite, and relate their bonding and structure to their very different properties.
A focused answer to WJEC GCSE Chemistry topic 2.1, covering giant covalent structures, the bonding and structure of diamond and graphite, and how these explain their hardness, melting points and electrical conductivity.
- Describe metallic bonding as positive ions in a sea of delocalised electrons, relate it to metal properties, and explain why alloys are harder than pure metals.
A focused answer to WJEC GCSE Chemistry topic 2.1, covering metallic bonding as positive ions in a sea of delocalised electrons, how this explains conductivity and malleability, and why alloys are harder than pure metals.
- Describe the arrangement of electrons into shells for the first twenty elements and relate electronic structure to group and period in the Periodic Table.
A focused answer to WJEC GCSE Chemistry topic 1.2, covering how electrons fill shells, writing the electronic structures of the first twenty elements, and relating the number of outer electrons to group number and the number of shells to period number.
Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)