Solids, liquids and gases:
Banana hammers and soft nails
Supports National Curriculum, Key Stages 1 and Units 2D, 3C, 4D, 5C, 5D, 6D
Suitable for years 4,5 and 6.
The time for whole session is about 1 hour. This can be varied by taking shorter paths
through the material. Choices may depend on the apparatus available or the particular
needs of the class.
Outline of content
Aims to
- establish what the children already know, through a class activity
- develop an appreciation of the difference in temperature between 0˚C and 30˚C
- extend their understanding of high and low temperatures so that they understand
the liquid nitrogen is very cold indeed
- find out about changes from liquids to gases
- find out that cooling materials can cause them to change
- find out about changes from liquids to solids
- discover that some changes are reversible changes
Points to note:
Liquid nitrogen is hazardous. When planning this visit, contact your local
source of liquid nitrogen and ask for a practice session with an experienced user.
Please read the notes about safety and agree the assessment with the teacher before
the session! Children should be at least two metres from the demonstrations and must
not sit on the floor for this presentation.
 Detailed instructions for the activities are provided.
 Apparatus details are listed and linked to the relevant sections.
 Pictures are provided for the rope activity.
 A temperature range summary is provided in a PowerPoint file for projection
or it can be drawn onto a black or white board.
 There is a video animation for use with activity 5.
 Vocabulary: The presentation uses mainly expressions included in the KS 1&2
strategies http://www.standards.dfes.gov.uk/schemes2/science/?view=get

Misconceptions to be corrected:
Liquids cannot be very cold
True gases (like air) cannot be liquefied.
Boiling can only occur at hot temperatures.
Temperatures can be lowered indefinitely.
Feedback from teachers after the trial visits:
“The material and methods used engaged the pupils in highly productive learning which
was vivid and relevant to the QCA schemes of work followed.”
“The pupils were challenged appropriately in this imaginative, effective and thought
provoking session.”
“The children were interested and engaged throughout the session.”
“The session enabled them to grasp the basics of the subject and develop a greater
understanding, thereby gaining a higher grade in the end of unit test.”
Solids, liquids and gases:
Banana hammers and soft nails
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ACTIVITY
Bring in a bunch of flowers and
fix them somewhere prominent.
They will be used later.
Then empty a heap of clothing
onto a table. (see apparatus list)
Select a large, very thick pair of
gloves and ask for a volunteer to
try them on.
Then choose a sunhat and ask for
a volunteer. Pick up other objects
from the heap and ask the class
to choose which should be worn
by which child.
DISCUSSION
(If appropriate, point out
that school uniform is
designed for about 20˚C.)
Ask the children when they
would wear the clothing
from the bag. Move the
discussion from winter and
summer to actual
temperatures. 0˚C for a
cold winter day in the UK
and 30˚C for a very hot
summer day in the UK.
Rope activity. Ask the two
Continue the discussion
children to hold the rope at the
emphasising that the length
knots which represent 0˚C and
of rope represents a very
30˚C and use the pegs to attach
large change in weather
the pictures for 0˚C and 30˚C to
conditions. That at 0˚C
the rope. (see apparatus list)
water freezes - ie changes
from a liquid to a solid so
ponds freeze. Discuss hot
weather conditions and
lead into the next section.
Ask where on Earth it is very
The children may suggest
hot. After a discussion, point out Spain for a very hot place.
that the hottest temperature
Lead the discussion by
recorded in the Sahara desert is
explaining that we are used
57˚C. Attach the desert picture at to lots of green trees and
the knot representing 57˚C.
plants around. Ask about
(Frequently over 40˚C)
hot places so that they
Ask where on Earth it is very
think of deserts.
cold. The South Pole is a usual
When cold places are
suggestion. Tell them the
discussed make sure they
average temperature in
understand negative
Antarctica is -57˚C. Attach the
temperatures are a way of
picture at the appropriate knot.
showing temperatures
AIM/FACTS
From our very
cold weather to
our very hot
weather the
temperature
varies by about 30
degrees Celsius.
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(Coldest recorded temperature at
the South Pole is -80˚C)
Show the picture of the boiling
kettle and ask what temperature
on the rope it should be placed.
They may know that water boils
at 100˚C. Ask a child to peg on
the picture and hold it.
Show the animation of water
splashing on a hot plate (video
animation)
Alternatively, if the risk
assessment has been agreed,
splash a few drops of water onto
a hot iron which has been
unplugged from the mains
socket. Emphasise that this is not
something to try at home! As
soon as this has been done, place
the iron on an insulating mat
where the children will be not
able to touch it.
(Read the safety notes)
colder than 0˚C
Ask what is happening
when water boils.
Encourage discussion of
the change from a liquid to
a gas. Explain that this
happens at different
temperatures for different
liquids.
Explain that because the
hotplate or iron is very
much hotter than 100˚C the
water boils off very
quickly. Droplets have
steam beneath them so they
skid around. (Some
children may know that
water vapour exists above
water at temperatures
below 100˚C but this
complication is best
avoided if possible.)
At 100˚C water
boils.
ie changes from
liquid to gas
(Read the safety notes)
Tell the children that you have
some liquid air which is very
cold. Ask what they think will
happen as it lands on the table
which is at room temperature.
Pour a small quantity (a few ml)
liquid nitrogen onto the bench or
table. The appearance is very
similar to that of the water on the
hotplate. Ask them to guess the
temperature of the liquid air
(nitrogen). (Nitrogen liquefies at
-196˚C.)
(see apparatus list for essential
kit)
Show the position on the rope
representing -196˚C. Ask a child
to hold the knot.
Explain that the table at
20˚C is very, very much
hotter than the boiling
temperature of the liquid
nitrogen so the nitrogen gas
boils off vigorously.
Spend time answering their
questions and emphasising
how very cold this is. Use
the rope to help to explain.
Nitrogen changes
from a gas to a
liquid at a
temperature very,
very much lower
than water. It will
therefore boil at a
much lower
temperature than
water.
Ask where the temperature is
extremely cold. Explain that the
temperature in outer space is
-269˚C. Show the knot on the
rope and emphasize the long
length of rope indicating how
Make it clear that you want
them to think of places that
are colder than the South
Pole. The might guess on
the outer planets.
Some will suggest places in
Different liquids
change from
liquids to gases at
different
temperatures.
Different liquids
boil at different
temperatures.
very cold this is. Ask a child to
peg on and hold the picture
which represents space.
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Ask if they know of even colder
places. Ask whether we can go
on finding colder and colder
temperatures. Have a vote.
Depending on the class, the next
section could be omitted and the
minimum temperature just stated
after the vote.
Following the discussion linking
atom vibration with temperature
- explain that at very low
temperatures the atoms are
hardly vibrating at all.
Eventually as they get extremely
cold the atoms stop moving and
then it is not possible to go
colder. This temperature is 273˚C. Show the end of the rope.
Ask whether they know of any
very, very hot places. The sun is
often suggested. Extend the hot
end of the rope and ask them to
guess where a knot should be put
to represent the centre of the
Sun. (15.6 million K) Show that
this would need an enormously
long rope. It would need to be
long enough to stretch out of the
building 150 km i.e. to a city that
is ~100 miles from the school.
space. Explain that away
from objects in space the
temperature is very, very
cold. Again use the rope to
help to explain.
Ask if they know what is
happening when substances
get colder. For some able
groups, this is an
opportunity to discuss
atoms moving more slowly
as the temperature drops.
There is a lowest
possible
temperature.
(-273˚C)
Spend time on this because
it helps to put the liquid
nitrogen into perspective as
not being magic but being
fully explainable.(Science
often explains effects that
appear to be magic.)
Spend time reinforcing the
ideas using the temperature
scale slide. Or by drawing
the chart on a black or
white board.
Emphasise:
The UK seasonal
temperatures are small
compared to other parts of
the Earth.
The very cold temperatures
in Space- nearly at the
minimum possible
temperature.
The very, very high
temperatures possible e.g.
in stars.
(Note: the outer layer of
the Sun has a temperature
of 6,000˚C.)
Ask what happens if we touch
Liquid nitrogen is very,
something very hot? Also what
very cold so it can hurt us
happens to mountaineers who get if we touch it so we must
very cold? Explain that you will be very careful.
do some experiments using the
liquid air (or nitrogen).
(See apparatus list for items used Explain that the water in
Very, very, very
high temperatures
are possible.
Activities so far
provide a contrast
between our
everyday
understanding of
hot and cold and
the huge range of
possible
temperatures.
Very hot and very
cold can hurt us.
A change in
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in the demonstrations.)
(Read the safety notes and apply
them for all the remaining
activities.)
Tap a hand bell or other metal
object with a banana and let
them hear the dull tap that is
made. Then dip one end of the
banana into liquid nitrogen for a
few minutes. Holding the
unfrozen end with a gloved hand,
remove it and again tap the bell.
The banana rings the bell.
Hold an elastic band up and
show that it stretches. Then using
insulated tongs dip the band into
liquid nitrogen and then quickly
hold it up and show that it is no
longer stretchy but that its elastic
property returns as it warms up.
Dip a length of bicycle inner
tube into the liquid nitrogen for a
moment the hold it up to show
that it looks stiff. (Be careful to
put it in with the upper end
folded over so that it is not
possible for a liquid spurt to
form.) Use it to ring the bell.
Then lay it on the table and hit
the frozen end with a hammer.
The rubber shatters into small
fragments with jagged edges.
Hold up a couple of small soft
toys and ask them what will
happen if they are dipped into
the liquid nitrogen. Get them to
vote which is dunked. Use the
tongs to lower the toy into the
liquid nitrogen and then lift it
out. Allow it to drain over the
Dewar taking care not to get
liquid nitrogen onto your hand
and to avoid any surplus liquid
nitrogen falling on the floor.
Sit the toy on the table and hit it
with the hammer or try to ring
the bell with it. It is soon clear
that the toy is unchanged and not
damaged by the hammer.
A variation on this is to ask the
the banana has frozen very
hard.
The unfrozen banana
contains liquids which have
changed to solids. The
banana is very hard
because it is so very cold.
temperature can
change the
properties of
substances.
Properties of
materials change
at very low
temperatures.
The frozen rubber is brittle
and shatters like glass when
hit.
After a short discussion
hold up the fragments of
rubber and show that they
are flexible again. Once the
fragments have warmed up
for several minutes, pass
them round for the class to
see the edges and feel that
the rubber is flexible again.
After seeing the rubber
tube become brittle the
children expect the toy or
tie to be damaged, when
hit, and are surprised at the
result.
Discuss change of
properties of materials at
low temperatures.
This provides an
example of a
reversible change.
The properties of
the fibrous
materials do not
change as they are
cooled.
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teacher for a tie or scarf (usually
these are undamaged but rarely
colours are affected so it is a
good idea to bring one for this
purpose).
Lower a small inflated balloon,
which has been tied, into the
liquid nitrogen and as it shrinks
feed it down slowly using an
insulating glove. Using insulated
tongs lift the balloon out with
your gloved hand so they can see
that the balloon has shrunk to be
very small.
Place the balloon on the table
and at once it begins to expand
until it is soon fully inflated.
Shape a small lump of blu-tack
to look like a nail. Then dip it
into the liquid nitrogen for about
2 minutes holding it with the
insulated tongs in your gloved
hand. Hold the blu-tack carefully
with the tongs and use a frozen
banana as a hammer to hit it into
a block of expanded polystyrene.
Bounce a ball of bouncing putty.
Then dip it into the liquid
nitrogen and remove using a
plastic spoon. Pick the ball up
with a gloved hand and show
that it is no longer bouncy it may
be so brittle that it shatters.
Cool a frying pan by pouring
some liquid nitrogen into it.
Then crack an egg into the pan
and add some more liquid
nitrogen. The albumen turns
white and the egg appears to
‘cook’.
Air in the balloon has
cooled and the volume has
decreased – some of the air
will have liquefied and it
may be possible for the
children to see the liquid.
Discuss changes of gases to
liquids as they cool.
Encourage the children to
explain what is happening
as the balloon reflates.
Gases change into
liquids when they
are cooled. The
liquid changes
back into a gas as
it warms up.
Discuss this and allow it to
warm up. Then slice
through the polystyrene
with a sharp knife to show
the blu-tack ‘nail’ inside
the block. Once warmed
up, they can pass this round
to feel the soft ‘nail’.
(This is fun but could be
omitted.)
The children will be
puzzled and ask whether
the egg is actually cooking.
Get them to think what is
happening. However it is
important that they do not
touch the cold egg. Follow
this by the next activity and
then return to the egg to let
them see that the change is
reversible so the egg did
not ‘cook’.
Pieces of orange frozen in the
Talk about reversible
liquid nitrogen shatter when hit
change and explain that if a
with the hammer. Rubbery celery finger or other living object
works well also.
is frozen cells are burst and
so changes are not
Properties change
when substances
are very, very
cold.
Liquids change to
solids when
cooled and then
back to liquids
when warmed.
Changes from
liquid to solid are
reversible.
Freezing
vegetable matter
can cause changes
that cannot be
reversed.
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Plenary session.
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Dip a bunch of flowers into the
liquid nitrogen: they appear
unchanged. Then crush them
using a gloved hand. The sound
is like crunching paper and tiny
fragments will be scattered.
It is fun to repeat this.
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Encourage the children to ask
questions.
reversible.
Take time to recap to
ensure that each child has
grasped at least the basics.
Discuss that the appearance
is unchanged. The colour
stays. After the petals have
warmed for a few minutes
they will still feel cool. At
this stage they can be
passed round and the
children will notice that
they are cool. Ask them to
make sure all the class has
a few. Then as they watch
the petals turn brownish
because the cells have been
burst.
They may talk about
freezing embryos and body
parts- explain that they
must not be so cold that the
cells are ruptured. This is a
good area to allow them to
ask more questions.
The changes to
the petals are not
reversible. Cells
have been burst
so irreversible
changes have
happened.
Physics is
everywhere.
I would like to thank Sian Owen and Dr Dominic Dickson, of the University of
Liverpool, for their support with the liquid nitrogen activities.
______________________________________________
APPARATUS NEEDED
On arrival
A bunch of flowers - chrysanthemums work well.
Activity 2
A bag of brightly coloured clothes – some for very cold weather (0˚C) and some
for very hot weather (30˚C).
Eg:
thick gloves, ski jacket, thick scarf,
+ sunhat, thin tee shirt, sun glasses
These should all be easy to either put on or hang against volunteers. Items for
the upper body are best because they can be seen by the class.
Activities 3 to 10 - Rope temperature scale
 A length of brightly coloured thin rope about 10 metres long.
 6 pieces of thin A4 card
 6 clothes pegs
 Pictures
Choose the neatest end of the rope to represent absolute zero. Starting from
this end, which represents absolute zero, tie knots at intervals using a scale of
100 Celsius degrees ≡ 1 metre (~ 100K ≡ 1 metre) as follows:
77 cm (liquid nitrogen liquefies), 216 cm (Antarctic), 273 cm (0˚C ), 330 cm
(Sahara), 373 cm (water boils) and leave a long length of rope in a coil. Note
that the positions of the knots do not have to be very accurate.
Print the pictures in the PowerPoint file and then glue them to cards or laminate
them. (Alternatives are provided for 0˚C and 30˚C.)
Activity 5 - Video of water on hotplate
or Electric Iron demonstration (Please read the safety notes)
 Electric iron
 Insulating mat
 Small quantity of water in a plastic bottle.
 Safety glasses
 Gloves.
( As for liquid nitrogen)
Before the session, discuss this demonstration with the teacher. Agree which mains
electric socket is to be used for the iron. If possible, this should be located behind your
demonstration table. The iron should be placed on the insulating mat in a place where it
cannot be reached by the children. A few minutes before the iron is needed, switch it on.
When you perform the demonstration, unplug the iron and show the children that it is
unplugged. Explain clearly that this is not an experiment for them to try at home. Hold
the iron so that its base is uppermost and slash a few drops of water from a plastic
beaker or bottle held in the other hand. Once the demonstration is over, replace the iron
on the insulating mat ensuring that there is no risk of it falling over. Safety goggles
should be worn for this demonstration. Gloves should be worn in case hot water
splashes onto your hands and you drop the iron.
Activity 6 and 12 to 21
Essential kit for use with liquid nitrogen: (Please read the safety notes.)
 Dewar containing liquid nitrogen with venting lid – rated for nitrogen
 Thick, loose fitting leather gloves
 Safety goggles (not spectacles)
 Small Dewar flask – with a lid. (Special expanded-polystyrene flasks are
available which eliminate the hazard of broken glass.)
 Tongs
Activity 12 to 22
Items needed for liquid nitrogen demonstrations:
 Small hand bell
 Hammer
 Banana,
 Elastic band – preferably brightly coloured and fairly thick
 Length of cycle inner tube (Scrap tubes available free from some cycle shops.)
 Small soft toys and/or a tie or a ‘silk’ scarf.
 Small balloon- a long thin one works well
 Blu-tack - small amount
 Block of expanded polystyrene packing large enough to hammer a blu-tack ‘nail’
into it.
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If available- bouncing putty. (obtainable from toy shops)
Frying pan
Egg
An orange or stick of limp celery
Finish with the flowers
______________________________________________
Safety Notes
Electric iron demonstration.
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There is danger of hot steam splashing on a child or on the demonstrator.
Safety goggles should be worn and the children should be at least two metres
from the demonstration.
An electric iron is heavy and should not be placed so that it can fall on a child.
The flex should be in a position so that it cannot be reached or pulled by a child.
After the demonstration, the hot iron should be placed on an insulating mat
placed so that the children cannot reach it.
Safety Notes for Liquid Nitrogen Activities
When planning to use liquid nitrogen during a school visit contact your
local source of nitrogen and ask for a practice session with an
experienced user.
Please read the CLEAPSS advice on handling liquid nitrogen in schools. This is an
extract from Section 11 of the CLEAPSS Laboratory Handbook which is supplied to all member
secondary schools and colleges (but not to primary schools) on the CLEAPSS Science
Publications CD-ROM and on paper.
Essential Kit list:
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Dewar containing liquid nitrogen with venting lid – rated for nitrogen
Thick, loose fitting leather gloves
Safety goggles (not spectacles)
Small Dewar flask – with a lid. (Special expanded-polystyrene flasks are
available which eliminate the hazard of broken glass.)
Tongs
The risks to be controlled are:
- asphyxiation in oxygen-deficient atmospheres;
- fire in oxygen-enriched atmospheres;
- cold burns, frost bite & hypothermia from the intense cold;
- over-pressurisation from the large volume expansion of the liquid;
- manual-handling accidents if using large (25 litre) containers;
- those associated with the transport of liquid nitrogen particularly in private cars.
Safety notes

Thick gloves and goggles (not safety glasses) should be worn at all times when
handling liquid nitrogen because liquid nitrogen can cause severe burns. (Do
not wear metal watches or jewellery, which could be in contact with the liquid
nitrogen.)
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Guard against build up of pressure* by using a venting lid for all containers.
Plan ahead how much liquid nitrogen will be needed and do not take excess to
the school i.e. transport a small amount (not exceeding 2 litres).
Transport the liquid nitrogen in a Dewar rated for liquid nitrogen with a venting
lid to guard against the build up of pressure. Do not use an ordinary vacuum
flask.
On leaving the supplier, place the Dewar in the boot of a car – and ensure that
the lid can vent. Ensure that the Dewar cannot fall over or spill by standing it in a
large deep cardboard box filled with packing such as crumpled newspaper.
On arrival, place the Dewar used for transport in a place that the children cannot
access.
Inform the school about class seating arrangements before your visit.
Arrange that the children in the class are seated at least 2 metres away from the
demonstration bench and do not ask for volunteers during the liquid nitrogen
activities. Explain that the liquid is dangerous and so the children should not
approach the bench. (Good behaviour is essential – if the children misbehave
stop the presentation at once.)
Before a presentation decant some liquid nitrogen into a small Dewar flask and
keep the lid on the flask at all times except when accessing the liquid. This
prevents the build up of liquid oxygen in the flask** and the possibility of a
serious fire.
Perform the demonstrations in a well ventilated room or hall to ensure that the
percentage of oxygen*** in the air is not reduced dangerously. (If anyone suffers
breathing difficulties they should be taken to fresh air at once.)
Do not touch any objects that have been dipped into the liquid nitrogen with bare
hands. Such objects remain cold for considerable lengths of time so do not pass
them to the children. (One exception is petals which can be passed round about
5mins after the flowers have been taken out of the liquid nitrogen. They will still
be cold to the touch and as they warm, in the hands, they turn brown.)
Avoid dipping tubes into liquid nitrogen because a spurt of liquid can be caused.
*As the nitrogen vapourises the pressure build up would cause an explosion.
**Nitrogen liquefies at 77K, Oxygen liquefies at 90K
***Nitrogen is not poisonous but can asphyxiate. Nitrogen makes up 78% of air, Oxygen
only 21%. A small drop in the concentration of oxygen is very noticeable.