Crystal size and cooling
rate: fast and slow
cooling of lead iodide:
teachers’ notes
Level
This activity is intended for students aged 11-14.
Topic
The activity relates to the process of igneous rock
formation by the cooling of magma. It can be used
to illustrate how the rate at which molten rock
cools affects the size of the crystals that form
within the solid rock – rapid cooling producing
small crystals, slower cooling producing larger
ones.
Description
Hot, saturated solutions of lead iodide are cooled
at different rates. The solution that cools faster
produces smaller crystals.
Context
Students need to be aware that igneous rock
forms when magma cools and forms crystals. They
should know from examining rock samples and / or
photographs that igneous rock such as granite
contains crystals and that different types of
igneous rock contain crystals of different sizes.
Teaching points
The link that the students should be encouraged to
make is that the intrusive igneous rock (the
granite) has cooled slowly from magma, and the
rhyolite lava (extrusive igneous rock) has cooled
very quickly. This leads to a fuller explanation of
the terms intrusive and extrusive – the intrusive
rock has cooled slowly, at depth, where the
overlying rocks have had an insulating effect. The
extrusive rock has cooled quickly on the surface of
the Earth, on land or on the ocean floor, and so
crystals have little time to form and are therefore
small. The time taken for cooling has had a direct
observable effect on the physical appearance of
the rock.
It is important to point out to students that they
are looking at an analogy or a model – granite
forms by crystallisation from a melt rather than
from a saturated solution.
It is likely that the first tube (the one that cools at
room temperature) will contain some undissolved
lead iodide and students may object that this
experiment is not a fair test. In fact the
undissolved solute may tend to promote
crystallisation by acting as seed crystals. Students
could be encouraged to design (and, if time allows,
carry out) a better experimental procedure in
which they pour a little of the saturated solution
into each of two boiling-tubes, leaving all the
undissolved solid in the original tube to be
discarded.
Timing
The activity takes about half an hour including
about 15 minutes waiting for the solutions to cool.
The activity
Ask the students to compare a sample of granite
with a sample of rhyolite. Alternatively, they can
look at photographs, such as Figure 1 and Figure 2,
if good samples are not available. Granite contains
large crystals of the minerals feldspar, quartz and
mica. The bulk of the rhyolite contains no obvious
crystals, when seen in hand specimen. Challenge
the students to speculate on the reasons for the
obvious difference in crystal size.
Figure 1 A sample of granite, note the large
crystals
Figure 2 A sample of rhyolite - the crystals are too
small to see. (The colour banding was produced as
the sticky lava flowed over the ground)
Students carry out the following activity. Half fill a
boiling-tube with water. Add a small spatula
measure of lead iodide. Heat over a Bunsen flame,
until the liquid starts to boil, taking care as the
mixture can ‘bump’ very easily, spraying hot liquid
out of the tube. Continue to boil for a further
minute, then quickly tip half of the contents into
another clean boiling-tube. Cool this second tube
and contents immediately under a stream of cold
water from the tap. Leave the original tube to cool
down slowly.
Leave both boiling-tubes and contents for about
15 minutes, then inspect the contents. This allows
the students some ‘writing up’ time. Both tubes
need to be at the same temperature before they
are compared.
The yellow lead iodide powder gradually goes into
solution on heating. On rapid cooling, lead iodide
falls out of solution quickly, with tiny sparkly flecks
of crystals being formed. The solution that cools
slowly produces distinctly larger crystals.
Apparatus
Each student or group will need
eye protection
2 boiling-tubes
boiling-tube rack
Bunsen burner
heatproof mat
boiling-tube holder
spatula
thermometer (0-100 °C)
Chemicals
a spatula measure of lead iodide (harmful by
ingestion and inhalation of dust)
samples of granite and of rhyolite (or
photographs, see Figures 1 and 2, if samples are
not available) - suitable specimens can be
obtained from a geological supplier. Click here for
details of some suppliers .
Safety notes
Wear eye protection.
Lead iodide is harmful.
It is the responsibility of the teacher to carry out
an appropriate risk assessment.
Answers to questions
Q 1. The granite has the larger crystals.
Q 2. The crystals are larger in the tube that
cooled more slowly, and smaller in the tube that
cooled more quickly.
Q 3. The tube with the larger crystals cooled
more slowly.
Q 4. Rhyolite cooled faster.
Q 5. Rock that cools underground (intrusive
rock) will tend to cool more slowly than rock that
cools on the surface (extrusive rock) because of
the insulating effect of the rocks above it. Other
sensible suggestions should be given credit.
Extension
An alternative practical activity that may be used
to illustrate the effect of rate of cooling on crystal
size is to cool molten salol (phenyl 2-
hydroxybenzoate, phenyl salicylate) on microscope
slides that are at different temperatures. (Safety
note; salol presents a minimal hazard.)
In this experiment, salol is melted in a boiling-tube
in a hot water bath. A few drops of the melt are
placed, using a glass rod, onto two slides – one
that has been cooled in a freezer and one that has
been warmed in a water bath (and then dried).
Each sample should immediately be covered by
another slide at the same temperature. Once they
have formed, the crystals can be observed using a
hand lens, microscope or on an OHP, projection
microscope or video microscope. The crystals that
form on the cooled slide should be smaller than
those on the warmed slide, see Figures 3 and 4.
This activity is a better analogy with crystallisation
of magma than the lead iodide experiment,
because it involves crystallisation from a melt. The
activity is not, however, 100% reliable, partly
because of supercooling of the salol which can
delay crystallisation, and teachers may wish to try
it out several times before doing it with a class.
Some suggested tips for success include:
Avoid overheating the salol – heat it only until it
has just melted (42 °C).
Once the slides have been removed from the
freezer and water bath respectively, speed is
essential to ensure that there is a significant
difference in temperature between them once the
salol is placed on them.
Use another slide at the same temperature to
cover the salol on the slides – this makes the rate
of cooling more uniform and prevents crystals
growing vertically upwards towards the observer
thus appearing smaller in cross section than they
actually are.
Figure 3 Small crystals of salol formed by rapid
cooling
Figure 4 Large crystals of salol formed by slow
cooling
teacher’s notes
student’s notes