OCR GCSE Chemistry A C2.3 Properties of Materials Exam Questions

About OCR GCSE Chemistry A C2.3 Properties of Materials

C2.3 links the structure and bonding of substances to their physical properties. On the OCR Gateway Chemistry A course it completes Topic C2 Elements, compounds and mixtures, assessed in Paper 1 (Foundation) or Paper 3 (Higher). You study the allotropes of carbon, explain melting points, conductivity and hardness using bonding models, and consider nanoparticles. It depends heavily on bonding from C2.2 and leads into materials and processes in C6.1 and polymers in C6.2. Examiners frequently ask why diamond is hard but graphite conducts, why ionic compounds conduct only when molten or dissolved, and why nanoparticles have different properties from bulk materials.

What you need to know

• Diamond and graphite In diamond each carbon atom forms four strong covalent bonds in a giant lattice, so it is very hard with a very high melting point and does not conduct. In graphite each carbon forms three bonds in layers with weak forces between them, and one delocalised electron per atom lets it conduct electricity.
• Graphene and fullerenes Graphene is a single layer of graphite, very strong and an excellent conductor. Fullerenes such as buckminsterfullerene (C60) are hollow molecules of carbon, and carbon nanotubes are cylindrical fullerenes with high tensile strength.
• Ionic and simple molecular substances Giant ionic lattices have high melting points because many strong electrostatic forces must be overcome; they conduct when molten or in solution because ions can move. Simple molecular substances have low melting points because only weak intermolecular forces are overcome, and they do not conduct.
• Metals and polymers Metals have high melting points, conduct electricity and heat through delocalised electrons, and are malleable because layers of ions can slide. Polymers are long chains of repeating units; their properties depend on chain length and forces between chains.
• Nanoparticles Nanoparticles are 1 to 100 nm across and have a very high surface area to volume ratio, so they can behave differently from bulk material. Uses include catalysts, sun creams and medicine; risks include unknown effects on health and the environment.

Common mistakes in C2.3 Properties of Materials

• Saying ionic compounds conduct when solid because they contain ions. The ions are fixed in the lattice when solid; they must be free to move.
• Explaining graphite's softness by weak covalent bonds. The covalent bonds within layers are strong; the forces between layers are weak.
• Writing that simple molecular substances have low melting points because covalent bonds are weak. Covalent bonds are strong; the intermolecular forces are weak.
• Stating that graphite conducts because of free ions. It conducts because of delocalised electrons.
• Forgetting that the surface area to volume ratio increases as particle size decreases.

Try three C2.3 Properties of Materials exam-style questions