Mixture
A mixture (impure) is made from different
substances that are not chemically joined.
It does not have a fixed composition.
As it does not have a fixed composition all
the components melt at different tempera-
tures. This leads to a ranges melting point
as different substances require different
amounts of energy to overcome the attrac-
tive forces.
Risk assessment
In a risk assessment, the hazards (something that can cause harm) of doing an experi-
ment are identified. Then the ways of reducing the risk (chance) of a hazard causing
harm are considered.
During crystallisation, the risks from spitting can be reduced by wearing eye protection,
removing the Bunsen burner before the solution is completely dry and /or using steam to
heat the evaporating basin gently.
Filtration and Crystallisation
Filtration
This technique is used to separate an insoluble solid from a liquid. It can be used to ob-
tain a product that is free from unreacted chemicals, by-products or solvent. The solid
particles are
too large to
pass through
the holes in the
filter paper, the
filtrate (solvent
and solute) pass
through the fil-
ter paper.
Crystallisation
When a product
is made as a so-
lution, one way
to separate it from the solvent is to make crystals. This involves evaporating the solu-
tion to a much smaller volume and then leaving it to cool. As the solution cools, crystals
form, and these can be obtained by filtration..
Pure substance
In a pure substance the composition cannot
be changed and the composition is the same
in all the parts of a substance. As it has a
fixed composition we can’t separate it into
other substances using physical methods.
