Chemistry · Year 11 · Module 1 · Lesson 6
HSC Exam Practice
Chemical Bonding Overview
Short answer
1.Short answer
Define electrical conductivity and state the two types of mobile charge carrier that enable conductivity in chemical substances.
Identify the structural type of a substance that conducts electricity as a solid, has a melting point of 1085 °C, and is malleable. Name a specific substance with these properties.
Explain why an ionic compound does not conduct electricity as a solid but does conduct when molten. In your answer, refer to the arrangement of particles in each state.
Distinguish between hardness and malleability as physical properties of substances. In your answer, identify one structural type associated with each property and explain the bonding reason for each.
Outline why a student who classifies graphite as “does not conduct electricity and is extremely hard” is incorrect. State the correct properties of graphite and the structural features responsible for each exception.
Describe the “diagnostic trio” of properties used to classify an unknown substance. For each property in the trio, state what information it provides about structural type.
Data response
2.Data response — comparing melting points and conductivity of four substances
A researcher measures the melting points and electrical conductivity (as solid and as liquid) of four substances (A–D) and records the results below.
(a) Using the data in Figure 2.1, classify each substance (A–D) as ionic compound, covalent molecular, covalent network, or metallic element. Justify each classification with specific reference to the graph data. (4 marks)
(b) Explain why substance B and substance C both conduct electricity when molten, but for different reasons. (3 marks)
(c) A student proposes that substance D could be either a covalent network solid or an ionic compound because both have high melting points. Evaluate this claim and explain which additional measurement from the table is most useful for distinguishing them. (2 marks)
Extended response
3.Extended response
Analyse the relationship between bonding type and physical properties for three of the four structural categories (ionic compound, covalent molecular, covalent network, metallic element). In your response, discuss how the bonding in each selected type accounts for its characteristic melting point, electrical conductivity, and mechanical behaviour, and evaluate which property is the most useful single discriminator when classifying an unknown substance.
Chemistry · Year 11 · Module 1 · Lesson 6
Answer Key & Marking Guidelines
Section 1 · Short answer · 3 marks · Band 3
Sample response. Electrical conductivity is the ability of a substance to allow electric current (a flow of charge) to pass through it. The two types of mobile charge carrier are: (1) free (delocalised) electrons — found in metals and graphite; (2) free ions — found in ionic compounds when molten or dissolved in water.
Marking notes. 1 mark for a correct definition (flow of charge / current allowed to pass through); 1 mark for delocalised/free electrons; 1 mark for free/mobile ions.
Section 1 · Short answer · 2 marks · Band 3
Sample response. Metallic element [1 mark]. A specific example is copper (Cu), which has a melting point of 1085°C, conducts electricity in all states due to its delocalised electrons, and is malleable [1 mark].
Marking notes. 1 mark for “metallic element”; 1 mark for naming copper (or any metal with MP ~1085°C — copper is the exact match).
Section 1 · Short answer · 3 marks · Band 3–4
Sample response. As a solid, an ionic compound consists of positive and negative ions arranged in a rigid, ordered 3D lattice held together by strong electrostatic attractions [1]. The ions are fixed in their positions and cannot move to carry electric current, so the solid does not conduct [1]. When the ionic compound is melted, the lattice structure breaks down and the ions become free to move throughout the liquid; these mobile ions can now carry charge and allow electrical conduction [1].
Marking notes. 1 mark for describing the solid lattice (ions fixed); 1 mark for linking non-conductivity to ion immobility; 1 mark for stating that melting releases mobile ions that then carry charge.
Section 1 · Short answer · 4 marks · Band 3–4
Sample response. Hardness is the resistance of a substance to scratching or indentation; malleability is the ability to be deformed (hammered into sheets) without fracturing [1 mark — both defined or clearly distinguished]. Hardness is characteristic of covalent network solids (e.g. diamond) because every atom is covalently bonded to its neighbours in a continuous 3D network; any attempt to scratch must break strong covalent bonds, requiring a large force [1 mark — structural reason for hardness]. Malleability is characteristic of metallic elements (e.g. copper) because the non-directional metallic bonding allows layers of metal cations to slide past each other while the delocalised electron sea redistributes to maintain bonding, enabling deformation without fracture [1 mark — structural reason for malleability]. Note: ionic solids are hard (rigid lattice) but brittle, not malleable — the contrast between hard/brittle ionic and malleable metal is important [1 mark — this contrast stated].
Marking notes. 1 mark per bullet point as above. Accept any structurally appropriate example for each type.
Section 1 · Short answer · 4 marks · Band 4
Sample response. The student is incorrect on both counts. Graphite is a covalent network solid that does conduct electricity and is soft, not hard — both properties are exceptions to the general rules [1 mark — identifies two errors]. Graphite conducts because each carbon atom in a layer forms only three covalent bonds, leaving one electron per carbon atom delocalised and free to move within the layer; these mobile electrons carry charge [1 mark — conductivity reason: delocalised electrons from sp2 bonding]. Graphite is soft because the hexagonal carbon layers are held together by only weak van der Waals (dispersion) forces, which are easily overcome; layers slide over each other with minimal force [1 mark — softness reason: weak interlayer forces]. In contrast, diamond — also a covalent network solid — is a non-conductor (all bonds are used in tetrahedral covalent bonding, no delocalised electrons) and is the hardest known natural substance. Students must treat diamond and graphite as separate cases [1 mark — diamond contrast stated].
Section 1 · Short answer · 3 marks · Band 4
Sample response. The diagnostic trio consists of: (1) Melting point — distinguishes low-MP covalent molecular substances (weak intermolecular forces) from high-MP ionic and metallic substances and very-high-MP covalent network solids; helps narrow the category [1]. (2) Electrical conductivity (solid and molten/dissolved) — determines whether free electrons (always conduct = metallic), mobile ions only when liquid/dissolved (= ionic), or no mobile charge carriers at all (= covalent) are present [1]. (3) Hardness/malleability — distinguishes malleable metallic elements from hard-but-brittle ionic solids and extremely-hard covalent network solids [1].
Marking notes. 1 mark per property in the trio with an adequate explanation of what information it provides.
Section 2 · Data response · 4 marks · Band 4–5
Sample response. A (MP −78°C, no conductivity): Covalent molecular — very low melting point indicates weak intermolecular forces between discrete molecules; no conductivity in any state confirms no free electrons or ions. Likely dry ice (CO2). B (MP 801°C, no solid conductivity, conducts molten): Ionic compound — moderately high MP; conducts only when molten (mobile ions); consistent with ionic lattice. Likely NaCl. C (MP 1085°C, conducts solid and molten): Metallic element — conducts in solid state (delocalised electrons); malleable behaviour expected; MP consistent with copper. D (MP 1713°C, no conductivity in any state): Covalent network solid — very high MP (strong covalent bonds throughout); no conductivity in any state. Likely SiO2.
Marking notes. 1 mark per substance: correct structural type + at least one justifying reference to the data.
Section 2 · Data response · 3 marks · Band 4–5
Sample response. Substance B (ionic compound, e.g. NaCl) conducts when molten because the ionic lattice breaks down on melting, releasing free ions (Na+ and Cl−) that are then mobile and can carry electric charge [1]. Substance C (metallic element, e.g. copper) conducts when solid because it contains delocalised electrons that are always free to move throughout the metallic structure, carrying charge regardless of physical state [1]. The key difference: B's charge carriers are ions (only mobile when lattice is broken); C's charge carriers are electrons (always mobile in the electron sea) [1 mark — explicit comparison of charge carrier type and state-dependence].
Section 2 · Data response · 2 marks · Band 4–5
Sample response. The student’s claim is partially correct — both ionic compounds and covalent network solids can have high melting points [1]. However, the conductivity data distinguishes them: substance D does NOT conduct when molten, ruling out ionic classification (ionic compounds always conduct when molten due to mobile ions). The most useful additional measurement is molten electrical conductivity: if a substance has a very high MP and does not conduct when molten, it is a covalent network solid; if it does conduct when molten, it is ionic [1].
Section 3 · Extended response · 8 marks · Band 5–6
Marking criteria (1 mark each, total 8):
- Correctly identifies the bonding type of each of the three selected structural categories and describes the nature of the attractive force.
- Links the melting point of each structural type to the strength of the attractive forces that must be overcome, with comparison (e.g. ionic > covalent molecular).
- Explains the conductivity of each selected type in terms of the presence or absence of mobile charge carriers and specifies the carrier type (electrons or ions) and the state(s) in which they are mobile.
- Explains the mechanical behaviour (malleability, hardness, or brittleness) of each selected type in terms of the directionality (or non-directionality) of the bonding.
- Gives a specific named example for each of the three structural types discussed and correctly assigns each property to the example substance.
- Identifies electrical conductivity (particularly the combination of solid AND molten/dissolved conductivity states) as the most useful single discriminator, with a clear justification — e.g. conductivity uniquely distinguishes metallic from ionic from covalent types in ways that MP and hardness alone cannot (because ionic and covalent network both have high MPs and are hard).
- Provides a counter-example or limitation of the chosen discriminator (e.g. graphite as a covalent network solid that conducts, which would be misclassified by conductivity alone as metallic without additional information).
- Uses precise scientific terminology throughout: structural type names, charge carrier types, bond types, and relevant property terms (MP, conductivity, malleability, hardness, brittle).
Notes. Any three structural types are acceptable. Answers need not discuss all four. The extended response should be coherent, structured (not a bullet list), and demonstrate causal reasoning linking bonding → property at each step. Band 5 responses will correctly analyse two types with limited comparative evaluation. Band 6 responses will correctly and precisely analyse all three selected types, compare them, and evaluate the discriminator with a counter-example or nuance.