Design a cold pack
Your team’s job is to design a ‘cold pack’. These are
often used to treat injuries, for instance when footballers
strain a muscle during a match.
The cold pack has to be easy to carry around in a first aid
kit, safe to use and only get cold when it is needed.
Task 1 – which chemical should you use?
First of all you are going to decide which chemical should be in the pack by trying some
different chemicals.
Your team manager has decided that there are three different chemicals that would be
safe to use in the cold pack: sodium chloride (table salt), sodium nitrate and ammonium
nitrate.
You should try dissolving these and see how much colder they make the water when they
dissolve. Put your results into the table on the answer sheet.
Look at your results and decide which chemical your group thinks is best? How much of
the chemical works best?
Task 2 – how much water should you use?
Now you need to decide how much water to use. Use the same amount of the chemical
you have chosen for each experiment. Try dissolving the chemical in different amounts
of water to see what gives the best fall in temperature.
Discuss why you need to keep the chemical and the amount of chemical the same. Put
your results into the table on the answer sheet.
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Design a cold pack
Task 3 – how will you activate the cold pack?
Think about how the cold pack will be made so that it doesn’t get cold until it is meant
to. Draw and label a plan of your team’s cold pack on the answer sheet, showing how it
will work.
Task 4 – how do the chemicals go cold?
Here are some things that scientists know about salt (sodium chloride) and water:

salt is made of sodium particles and chlorine particles

the particles of sodium stick very strongly to the particles of chlorine

the particles of chlorine stick very strongly to the particles of sodium

it takes a lot of energy to pull the particles of sodium and chlorine apart

water particles stick well to sodium particles and chlorine particles

water particles form a ‘shell’ around the sodium and chlorine particles

water particles have energy that lets them move around

when water gives up some of its energy to another chemical it gets colder.
You will be shown how to make models of salt and water particles. Then you must
prepare a talk about how the water gets colder when the salt dissolves. You must use
your models as part of your talk.
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Design a cold pack
Team answer sheet
People in the team:
.............................................
............................................
.............................................
............................................
Task 1
Chemical
A
B
C
D
E
F
G
H
I
sodium
chloride
sodium
chloride
sodium
chloride
sodium
nitrate
sodium
nitrate
sodium
nitrate
ammonium
nitrate
ammonium
nitrate
ammonium
nitrate
Mass of
chemical (g)
Volume of
water
(cm3)
5
50
10
50
15
50
Temperature
of water
before
dissolving
(oC)
Temperature
of water
after
dissolving
(oC)
Change in
temperature
(oC)
5
10
15
5
10
15
Our team thought the best chemical to use would be ...........................................
We thought this one was the best because ........................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
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Design a cold pack
Team answer sheet cont.
Task 2
Chemical
Mass of
chemical
(g)
Volume of
water
(cm3)
Temperature
of water
before
dissolving
(oC)
Temperature
of water
after
dissolving
(oC)
Change in
temperature
(oC)
Our team thought the best volume of water to use would be ......................... cm3.
We thought this one was the best because ........................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
Task 3
Here is our design for the cold pack. Our plan shows how it will start to work when we
want it to.
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Design a cold pack
Teaching notes
This activity could be done over two lessons including the group presentations. It would
also lend itself to a science club investigation.
Students should work in groups of 3 or 4. To carry out the fourth task you will need to
show the students how to make models of the salt and water molecules using Molymod
atoms would be ideal or they could use playdough and cocktail sticks.
The groups could just be given Task 1 initially. When they have completed it give them
the next task.
Technician’s requirements
For each group:

sample bottles or similar with 5g, 10g and 15g of each of the following: sodium
chloride, sodium nitrate, ammonium nitrate. Label the bottles with the quantity
and colour code the different solutes e.g. by using red, green and yellow labels.

plastic 50 ml measuring cylinders

250 ml beakers

thermometers

stirring rods

one electronic balance per bench

one stock jar of the three solutes per bench

additional specimen bottles and spare coloured labels

sandwich bags.
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Design a cold pack
Mark sheet
Tasks 1 and 2
Group realises that the amount of water must be the same each time. ............ /1
Measures water accurately (bottom of meniscus on the line) (1 mark if +/- 3 cm3)
.................................................................................................. /3
Ensures that all solute is transferred to the beaker .................................... /1
Thinks about whether to stir or not ....................................................... /1
Measures temperature as accurately as possible, putting eye close to the
thermometer and level with the liquid ................................................... /2
Task 3
Group thinks about a workable solution to keeping solute and solvent apart before
activation. ..................................................................................... /2
Group can explain how their solution will work ......................................... /2
Pupils work together to come up with a solution ....................................... /2
Task 4
Using scientific terms accurately .......................................................... /3
Construction of accurate models of NaCl and H2O ...................................... /2
Using models to show what happens when salt dissolves/ ............................ /2
Using models to explain temperature drop .............................................. /2
Using advice form team members and adults well ..................................... /2
Total marks ......................................................................................... /25
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