teacher’s notes
student’s notes
lead to further discussion, especially with older students.) After
evaporation, show the basins to students – the one that contained
distilled or deionised water should be still clean, the one that contained
tap water may be clean (this depends on the hardness of the water in
your area), but the mineral water basin should have some mineral
deposits present, indicating that the deposits were, in fact, dissolved
by the water when passing through the filtering rocks.
Activity 3
Apparatus
Each student or group will need
Eye protection
Coffee jar with screw top
Dropping pipette
100 cm
3
beaker
Access to a freezer
Chemicals
Each student or group will need
A sample of porous rock, typically a shale, siltstone, chalk or
sandstone. Suitable material can be obtained from a geological
supplier if none is available locally. Click here for details of
some suppliers
http://www.earthscienceeducation.com/suppliers.
Safety notes
Wear eye protection
There may be sharp fragments of glass from the broken coffee
jar
It is the responsibility of the teacher to carry out an appropriate
risk assessment
The activity
It is very difficult to show freeze-thaw damage in one lesson, so it is
likely that this activity will need to be set up and left to be inspected
until next lesson.
Fill a coffee jar completely with water, making sure that there are no air
bubbles, and screw the lid firmly onto the jar (care is needed,
remember water cannot be compressed!) and place it in a container
such as a plastic bowl in the freezer.
Take a small sample of porous rock (it is sometimes useful to have
previously dried the rock slowly in an oven for a few days), place in a
beaker and, using a dropping pipette, carefully drip water onto the
rock. Add the water slowly, stopping when no more water appears to
have been absorbed. Alternatively, use a sample that has previously
been soaked in water overnight. Place the sample in a plastic bowl in
the freezer.
In the following lesson, remove the samples from the freezer. The
expansion of water on freezing will have cracked the glass jar. As the
rock sample thaws out, particles of rock may fall from the main
sample. Be warned, this does not always work and some trial and
error prior to the lesson may be needed to select a suitable sample of
rock that does show this effect – porosity is the main factor. The best
types of rock to try are shale, siltstone, chalk and sandstone – porous
red sandstones have been found to be most effective.
It may be necessary to carry out more than one freeze-thaw cycle to
produce fragmentation. This can be done by the teacher or lab.
technician between the lesson in which the activity is set up and the
one in which the results are examined. Note that natural freeze-thaw
weathering requires many cycles of freezing and thawing, rocks are
not broken by freezing alone.
The expansion of water on freezing is unusual. It occurs because in ice
the water molecules form an open hexagonal structure held together
by hydrogen bonds. On average the water molecules are further apart
in ice than in liquid water. So ice has a lower density than water and
floats on top of it. This effect means that ice forms an insulating
blanket on top of liquid water and is important for the survival of
aquatic life in cold climates.
Activity 4
Apparatus
Each student or group will need
Eye protection
Shallow tray (a baking tray is ideal)
Sand
100 cm
3
beaker
Dropping pipette
Washing up bowl or similar
Access to sink
Safety notes
Wear eye protection
It is the responsibility of the teacher to carry out an appropriate
risk assessment.
The activity
Set up the tray as described in Activity 1. Place a small mound of dry
sand on the tray. Carefully, drip water onto the sand using the
dropping pipette, see Figure 2. Students should be able to see grains
of sand being washed down the mound. Repeat the process, this time,
pouring water directly from the beaker (to simulate a torrential
downpour of rain). A washing up bowl placed in the sink is used to
catch the sandy water and prevent sink blockages.
Activity 3
Q 1. It increases (ie the water expands).
Q 2. Water can soak into porous rock and, when it freezes, it
expands and this can break off bits of the rock.
Activity 4
Q 1. It contains sand that has been washed away from the pile.
Q 2. Sand has been removed from it.
Q 3. Sediments in the estuaries of rivers or deltas, pebbles and
sand below sea cliffs etc.
Q 4. They may be worn away more quickly.
Q 5. Softer rock and more soluble rock may be worn away more
quickly.
Q 6. The sand represents insoluble rock and the sugar soluble rock.
Activity 5
Q 1. They were knocked off by impacts from grains of sand in the air
stream.
Weathering and erosion: simulating rock attack
in the lab: teacher's notes
Level
This activity is designed for students aged 11-14.
Topic
This topic examines the ways in which water can weather rocks, by solution and by
freeze-thaw. Simulations of erosion by running water and by wind are also
provided.
Description
This material consists of a circus of class practical and demonstration activities that
simulate some of the processes occurring in weathering and erosion.
Activity 1 looks at the effect of pouring water onto brown sugar to simulate the effect
of water on soluble rock.
Activity 2 looks at the residue left after water samples have evaporated to show that
water can dissolve minerals.
Activity 3 shows that water expands on freezing by placing a sealed jar of water in
the freezer overnight and looks at the effect of freezing on a water-soaked sample of
porous rock.
Activity 4 simulates the effect of water impact on rocks by pouring water on sand.
Activity 5 uses a hair dryer to simulate wind erosion by ‘sandblasting’ copper sulfate
crystals from a cylinder of Blu-Tack®.
Context
These activities introduce the ideas of weathering and erosion and lead onto work
about the formation of sedimentary rocks.
Teaching points
Start with a discussion on how rocks can be broken down into smaller pieces (for
example how a sandy beach forms from a cliff), before leading on to a series of
activities to illustrate the various different types of weathering and erosion caused by
water and wind. The activities can be arranged as some combination of
demonstrations, class practicals or as a circus depending on resources
Points to bring out
Stress that although these simulations take a few minutes, the processes that they
model usually take place over a much longer timescale.
Encourage correct use of the terms weathering, erosion and transport.
Weathering can take place in three ways – physical weathering, eg water collecting
in cracks in rocks and breaking them when it expands on freezing; chemical
weathering, eg reaction (dissolution) of rock such as limestone in slightly acidic
rainwater; biological weathering, eg the roots of plants opening up cracks in rocks.
Erosion is the wearing away of exposed surfaces by agents such as wind, moving
water and ice. These agents usually contain weathered rock debris. Rock fall under
gravity is also erosion.
Transport begins as erosion and is the movement of rocks under gravity, or by
moving water, by wind or by ice (in glaciers, for example).
Figure 3 The setup for Activity 5
Switch on the hair drier and gently pour a stream of sand into the turbulent air flow,
so that the sand is blown onto the cylinder. (This part of the activity does need to be
practised beforehand to get the correct distances etc.) After a few minutes of this
bombardment, stop the sand and air flow and examine the contents of the bin liner.
There should be some blue crystals of copper(II) sulfate which have been
‘sandblasted’ from the coated Blu-Tack®
Answers to questions
The answers given here are for the guidance of the teacher. Other answers may be
equally acceptable.
Activity 1
Q 1. It has dissolved some of the sugar.
Q 2. It has been dissolved away.
Q 3. Most water has things dissolved in it as shown by the presence of deposits
when the water evaporates away. Other acceptable answers might include
the formation of stalagmites / stalactites, the formation of caves in limestone
and the formation of mineral water.
Q 4. Acceptable answers include that it may reappear if the water evaporates (as
in the formation of stalagmites / stalactites), or it is washed into the sea.
Q 5. Mineral water and tap water.
Activity 2
Q 1. Tap water has fallen as rain and soaked through parts of the Earth’s surface
before being collected for use, so it may contain dissolved material
depending on the type of rock it has soaked through. Distilled water has been
boiled and the steam condensed so that it contains no dissolved material.
Mineral water has soaked through rock and dissolved some of its mineral
content. Many students will probably give less-detailed answers.
Q 2. The deposits on the evaporating basins were originally dissolved in the water
/ the solids on the dish must have come from the water.
Q 3. Depending on the brand of water, there may be a description of its origin and
/ or an analysis of its mineral content.