Edexcel IGCSE Chemistry Chapter 4, page 32 ANSWERS
1. a)
Diamond
Graphite
b) Similarities: both are solid. They are covalent lattices (giant networks of atoms), so the atoms are
held in place by strong covalent bonds. These bonds take a lot of energy to break. This is why both have
high melting points (graphite melts at 3974°C, diamond above 3550°C).
Neither is soluble, since the covalent bonds require more energy to break than can be provided by any
solvent.
Differences: Because each carbon atom in diamond is bonded to four others, each atom is held rigidly in
place by the four covalent bonds. This makes diamond very hard. Because all electrons are involved in
making covalent bonds, they are not free to move through the lattice, so diamond in not a conductor.
In graphite, each carbon atom is bonded to three others, which forms two-dimensional lattices stacked
above one another. These are held in place by electrons which are not involved in forming covalent
bonds. These weak intramolecular forces (van der Waal’s forces) are easily broken by friction, so rubbing
a piece of graphite will separate the layers (pencil lead is graphite). The unbonded electrons are also
able to move through the lattice, so graphite conducts electricity.
2. a) Malleable means ‘able to be hammered into thin sheets’ (Latin ‘malleus’ = hammer). Ductile
means ‘able to be drawn into wire’. When subjected to a large stress, the atoms in a metal will
slide against one another, and then remain in their new position.
b) Metals are conductors. This is because metals have few electrons in their outer electron
shells. These are easily displaced and can be made to move beween atoms (i.e. through the
lattice).
Edexcel IGCSE Chemistry Chapter 4, page 32 ANSWERS
c) In a pure metal the atoms are all the same size and can easily slide against one another. In an
alloy the atoms are not the same size and will not slide against one another so easily, so the
alloy is harder than the original metal.
3. a) Sodium chloride has a high melting point ( 801°C) because the ionic bonds holding the sodium
and chloride ions together are quite strong. They require a large amount of energy to break,
although not as much as covalent bonds.
b) Sodium chloride is a crystal because the ions are regularly arranged in a 3-D lattice. It is brittle
because any deformation of the lattice will force ions with the same charge against each other,
causing repulsion and therefore fragmentation of the crystal.
c) Water is a polar solvent. The ions in sodium chloride are charged, so water molecules can
‘steal’ ions from the surface of the sodium chloride crystal, gradually dissolving it.
4. a) giant covalent
b) molecular
c) molecular (sulfur)
d) giant ionic (potassium dichromate)
e) giant metallic (copper)
f) molecular (an organic compound)
g) giant metallic
h) molecular (boron trioxide)
5. a) SiO2 is a solid, because it is a giant covalent structure where each atom is bonded to four
others. Four covalent bonds require a lot of energy to break, so it has a high melting point.
b) It has a tetrahedral arrangement of atoms similar to diamond. Its properties will therefore be
similar to diamond. It is a hard solid because the atoms are not easily displaced.
c) SiO2 should be insoluble in water, because the covalent bonds within the lattice are stronger
than the dipole forces exerted by water molecules on the silicon or oxygen atoms within the
lattice.
d) It is quartz, which is a naturally-occurring mineral formed in igneous rocks. It is harder than
most other minerals in igneous rocks, and persists after weathering as sand. (Hardness of
quartz = 7, diamond = 10, steel & glass = 5 or 6).