C2.2 How structure influences the properties and uses of substances
Substances that have simple molecular, giant ionic and giant covalent structures have very different properties. Ionic, covalent and metallic bonds are strong. However, the forces between molecules are weaker, eg in carbon dioxide and iodine. Metals have many uses. When different metals are combined, alloys are formed. Shape memory alloys have a range of uses. There are different types of polymers with different uses. Nanomaterials have new properties because of their very small size.
Candidates should use their skills, knowledge and understanding to:
- relate the properties of substances to their uses
Candidates may be provided with information about the properties of substances that are not specified in this unit to enable them to relate these to their uses.
- suggest the type of structure of a substance given its properties
- evaluate developments and applications of new materials, eg nanomaterials, fullerenes and shape memory materials.
Candidates should be familiar with some examples of new materials but do not need to know the properties or names of specific new materials.
a) Substances that consist of simple molecules are gases, liquids or solids that have relatively low melting points and boiling points.
b) Higher tier only: Substances that consist of simple molecules have only weak forces between the molecules (intermolecular forces). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.
Candidates need to be able to explain that intermolecular forces are weak in comparison with covalent bonds.
c) Substances that consist of simple molecules do not conduct electricity because the molecules do not have an overall electric charge.
C2.2.2 Ionic compounds
a) Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces in all directions between oppositely charged ions. These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds.
Knowledge of the structures of specific ionic compounds other than sodium chloride is not required.
b) When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry the current.
C2.2.3 Covalent structures
a) Atoms that share electrons can also form giant structures or macromolecules. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms. All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high melting points.
Candidates should be able to recognise other giant structures or macromolecules from diagrams showing their bonding.
b) In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure, so diamond is very hard.
c) In graphite, each carbon atom bonds to three others, forming layers. The layers are free to slide over each other because there are no covalent bonds between the layers and so graphite is soft and slippery.
Additional guidance:Higher Tier candidates should be able to explain the properties of graphite in terms of weak intermolecular forces between the layers.
d) Higher tier only: In graphite, one electron from each carbon atom is delocalised. These delocalised electrons allow graphite to conduct heat and electricity.
Additional guidance:Candidates should realise that graphite is similar to metals in that it has delocalised electrons.
e) Higher tier only: Carbon can also form fullerenes with different numbers of carbon atoms. Fullerenes can be used for drug delivery into the body, in lubricants, as catalysts, and in nanotubes for reinforcing materials, eg in tennis rackets.
Additional guidance:Candidates' knowledge is limited to the fact that the structure of fullerenes is based on hexagonal rings of carbon atoms.
a)Highertieronly:Metals conduct heat and electricitybecause of the delocalised electrons in theirstructures.
Additional guidance:Candidates should know that conduction depends on the ability of electrons to move throughout the metal.
b) The layers of atoms in metals are able to slide over each other and so metals can be bent and shaped.
c) Alloys are usually made from two or more different metals. The different sized atoms of the metals distort the layers in the structure, making it more difficult for them to slide over each other and so make alloys harder than pure metals.
d) Shape memory alloys can return to their original shape after being deformed, eg Nitinol used in dental braces.
a) The properties of polymers depend on what they are made from and the conditions under which they are made. For example, low density (LD) and high density (HD) poly(ethene) are produced using different catalysts and reaction conditions.
b) Thermosoftening polymers consist of individual, tangled polymer chains. Thermosetting polymers consist of polymer chains with cross-links between them so that they do not melt when they are heated.
Additional guidance:Higher Tier candidates should be able to explain the properties of thermosoftening polymers in terms of intermolecular forces.
a) Nanoscience refers to structures that are 1–100nm in size, of the order of a few hundred atoms. Nanoparticles show different properties to the same materials in bulk and have a high surface area to volume ratio, which may lead to the development of new computers, new catalysts, new coatings, highly selective sensors, stronger and lighter construction materials, and new cosmetics such as sun tan creams and deodorants.
Candidates should know what is meant by nanoscience and nanoparticles and should consider some of the applications of these materials, but do not need to know specific examples or properties.
Questions may be set on information that is provided about these materials and their uses.
Suggested ideas for practical work to develop skills and understanding include the following:
- demonstration of heating sulfur and pouring it into cold water to produce plastic sulfur
- investigating the properties of ionic compounds, eg NaCl:
- melting point, conductivity, solubility, use of hand lens to study crystal structure
- investigating the properties of covalent compounds:
- simple molecules, eg wax, methane, hexane
- macromolecules, eg SiO2 (sand)
- investigating the properties of graphite
- demonstrations involving shape memory alloys
- investigating the properties of metals and alloys:
- melting point and conductivity, hardness, tensile strength, flexibility
- using models, for example using expanded polystyrene spheres or computer animations to show how layers of atoms slide
- making metal crystals by displacement reactions, eg copper wire in silver nitrate solution
- distinguishing between LD and HD poly(ethene) using 50:50 ethanol:water
- making slime using different concentrations of poly(ethenol) and borax solutions
- investigating the effect of heat on polymers to find which are thermosoftening and which are thermosetting.
Tuesday 16 May 2017 PM – Core Science & Triple Science (Biology) B1 – ALL
Thursday 18 May 2017 AM – Core Science & Triple Science (Chemistry) C1 – ALL
Wednesday 24 May 2017 PM – Core Science & Triple Science (Physics) P1 – ALL
Friday 9 June 2017 AM – Additional Science (Biology) & Triple Science (Biology) B2 – All Y11
Friday 9 June 2017 AM – Triple Science (Biology) B3 – Triple ONLY
Wednesday 14 June 2017 AM – Additional Science (Chemistry) & Triple Science (Chemistry) C2 – All Y11
Wednesday 14 June 2017 AM – Triple Science (Chemistry) B3 – Triple ONLY
Friday 16 June 2017 AM – Additional Science (Physics) & Triple Science (Physics) P2 – All Y11
Friday 16 June 2017 AM – Triple Science (Physics) B3 – Triple ONLY