Chemical Change – Irreversible Chemical Changes
Lessons 7 and 8
Irreversible Chemical Changes
Lesson Objectives
Choose from:
 Understand and explain the process of an irreversible change.
 Make a prediction that can be tested and use your data to support it.
Essential Knowledge/Science Explained - for teachers
1.
2.
3.
Soda Snake: The sodium hydrogen carbonate releases carbon dioxide when heated above 50 °C. Burning
the alcohol caramelises the sugar, giving the snakes their black appearance. The snakes grow because of
the pressure from the carbon dioxide gas being released. The reaction stops when all the alcohol has
burned.
Concrete is a mix of cement, sand, gravel and water. Pupils make some concrete and include various
additives in the mix. They investigate how changing the additives changes the properties of the resulting
composite.
A composite material is made by combining two or more materials – often ones that have very different
properties. The two materials work together to give the composite unique properties. However, within
the composite you can easily tell the different materials apart because they do not dissolve or blend into
each other.
Making concrete is an irreversible reaction because all the individual components have combined to form a
compound that cannot be broken down and returned to its original form, i.e. All the reactants have been
converted to form the product. In a reversible reaction the reactants and products are said to be in equilibrium at
any time.
E.g.,
 A reversible reaction is a chemical change in which the products can be converted back to the original
reactants under suitable conditions.
 In other words, you can change the position of the chemical

A reversible reaction is shown by the sign
,
o a half-arrow to the right (direction of forward reaction)
o a half-arrow to the left (direction of backward reaction)

Most reactions are not reversible (irreversible) and have the usual complete arrow
to the right.
http://www.docbrown.info/page04/4_74revNH3.htm
Common Misconceptions
1.
Reactants ‘equal’ products in chemical reactions - A chemical reaction involves atoms in
the reactants being rearranged to form the products. No material is lost or gained.
2. All chemical reactions are irreversible.
3. A substance is not a chemical if it doesn’t react with anything,
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Chemical Change – Irreversible Chemical Changes
Lessons 7 and 8
Termly Scientific Skills Development Focus: Recording and Analysis of Scientific Data and
Observations
 Collecting and presenting scientific observations in a way that can be analysed.
 Creating graphs and charts of the data
 Analysing data the data obtained from the experiment and determining whether or not it proves or
disproves the prediction.
Opportunities should be given throughout the lesson for children to use and develop their knowledge of planning
investigations, through questioning and discussions on questions to investigate, making predictions and suggesting
dependent and independent variables.
Cross curricular links
Literacy
Numeracy
Other subjects
 Write a newspaper report on the invention of a new type of
chocolate that doesn’t melt – possibly by the marvellous Willy
Wonka.
 Make or add to a class science dictionary of terms and definitions.
 Research into the work of the famous chocolatiers Teuscher, and
write a report.
 Calculating simple formulations using measurements of teaspoons
accurately and recording the data.
 Tabulating and/or graphing data recorded.
 Calculate averages using class data.
 Find out about the many different uses there are for concrete? What
additives are put into commercially made concrete and why? What
methods are used to strengthen it for its different applications?
 The development of concrete altered Roman architecture – find out
how and why?
 Make and decorate concrete leaves for the school garden area.
http://anotherjennifer.com/kid-friendly-urban-garden-art-projectconcrete-leaves/
 Alternatively, groups could make some concrete stepping stones for
an outdoor area. http://www.houselogic.com/homeadvice/landscaping-gardening/ideas-kids-projects/
Preparation
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For the investigation you will need lots of newspaper it maybe an idea to get the children to each bring in a
newspaper from home.
Photocopy moulds onto thin card you will need lots of these if each pair is going to make several different
composites.
Organise practical and delivery and set up of equipment in the classroom.
Children should work in pairs. Each pair needs two mould templates to cut out and stick together in
preparation for the concrete mix. (These ideally should have been photocopied onto thin card)
To make it easier for the children to differentiate between composites they could be use different coloured
moulds, so photocopy onto different coloured card. Once the concrete has set, the mould will need to be
removed but this can be left until just before the testing is carried out which will be at the start of the next
lesson.
Pupils need two lessons to complete the investigation. In the first lesson, they prepare the samples and
leave them to set. In the second lesson, they test their samples for strength.
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Chemical Change – Irreversible Chemical Changes
Lessons 7 and 8
Prior Discussion
Explain that you are about to carry out a demonstration which has links to one they may have seen before. Ask
children to try to remember where they have seen something similar, and the theory and vocabulary used
previously. (Children may suggest Elephant’s Toothpaste).
Teacher Demonstration
Demonstration – Soda Snake Firework
HEALTH & SAFETY: The teacher should wear eye protection.
Apparatus and chemicals
 Heatproof mat
 Sand
 Four teaspoons of icing sugar
 1 teaspoon of sodium hydrogen carbonate
 20ml Propan-2-ol (alcohol)
 Matches
 Teaspoon
 Safety glasses
Procedure
Take the heatproof mat and make a mound in the middle with the sand. In the centre of the mound make a dent
with your finger.
Pour some of the alcohol into the dent taking care not to collapse the mound.
Mix the icing sugar and sodium hydrogen carbonate together in the plastic cup and fill the dent.
Light the mixture with a match and stand back as the alcohol starts to burn.
Little black balls appear and the snake starts to grow from inside the mound.
Eventually the snake will emerge and it is an amazing sight to behold.
Warning: Do not under any circumstances pour any more alcohol on the mound whilst the reaction is taking
place, as isopropyl alcohol is highly flammable and it could flare up and cause severe burns.
This experiment can get very hot, so it is important that it is performed on a heatproof mat or heatproof surface.
The reaction should be performed in a ventilated room. No smoke is produced, but the alcohol vapour has a
tendency to smell.
Make sure that the sugar and baking soda fills the indentation evenly, otherwise your snake may appear with
mutant legs and eyes! Making a narrow, deep indent in the sand with your little finger or a pencil will help to make
your snake tall and slim.
http://www.sciencemuseum.org.uk/~/media/85A2FB9FF588413C81F2E94757C35547.ashx
Possible Questions/ Suggestions for discussion
What did you observe?
What was actually happening?
Who can explain it in terms of a chemical change?
Children’s Investigation
Class activity – Experiment : Making and testing concrete
Pupils make some concrete and include various additives in the mix. They investigate how changing the additives
changes the properties of the resulting composite. Concrete is a mix of cement, sand, gravel and water.
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Chemical Change – Irreversible Chemical Changes
Lessons 7 and 8
Apparatus and chemicals per pair
 3 table spoons of Cement
 2 cups of fine sand
 1 cup of gravel
 12 paper clips
 2 lollipop sticks
 1 plastic cups
 4 splints
 3 moulds (template below, prior to lesson photocopy onto card)
 1 pritt stick (to use in mould assembly)
 2 plastic tea spoons
 1 mass hook
 1m string
 500g mass
 Safety specs
 Disposable gloves
Part 1 – Making Concrete
Procedure
Health and safety:
Cement is corrosive. Eye protection to be worn by all and try to avoid raising cement dust. Avoid skin contact
with the cement. Disposable gloves to be worn by all.
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Cover your bench in newspaper. Collect a plastic cup, put two teaspoons of cement powder, four teaspoons
of sand and six teaspoons of gravel in the cup and mix them together thoroughly.
Add water a drop or two at a time. It is easier to add water than to remove it from the mixture and not much
is required so add it carefully. Keep mixing until a hole made in the mixture will remain but it can also be
poured.
Pour the mixture into a mould. Label the mould with your name and ‘Sample 1’ and then leave the mixture to
set.
Make other samples and include different additives or vary the mixture in other ways. Include some or all of
the following: no gravel; wooden splints placed in the mould and the cement mixture added to it; opened
paper clips or wire in the mould and the cement added to it; Plaster of Paris powder added to the mix. You
could also change the size of the gravel pieces or the ratio of cement, sand and gravel in the mixture.
For each sample you make, record what it contains in a table like the one below:
Sample number
1
Composite mixture
2 teaspoons cement,
4 teaspoons sand,
6 teaspoons water
Mass required to break it
2
3
When the pupils have finished making their samples, get them to place them all in a safe place to set. Wrap up the
newspaper on the desks, making sure they collect all the dust and bits. Put this into the rubbish bin, along with
your plastic mixing cup, stirrer and gloves.
Do not wash up anything that has had cement powder on it. Make sure the children wash their hands.
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Chemical Change – Irreversible Chemical Changes
Lessons 7 and 8
Part 2 - Testing concrete
Pupils are going to test the concrete bars they made last lesson to see how much weight they can bear before they
snap. This is a measure of their tensile strength.
Health & Safety: Safety glasses should be worn by all, as pieces of concrete could fly out when the composites
break.
Can the pupils predict which composite they think will be the strongest and which the weakest of their samples?
Record this at the side of their results tables.
Set up and demonstrate as shown below and explain how they will carry out the tests, emphasise the safety
aspects, such as making sure the bubble wrap is correctly in place under the hanger.
They must keep their feet away from the area to avoid broken toes.
1.
2.
3.
4.
Each pair of pupils should take: Safety glasses, their thoroughly set hard composite samples, a G clamp,
several 100 g masses, one mass hanger, some string, and a piece of bubble wrap about 60 cm x 30 cm
Pupils should fold the bubble wrap double and place on the floor under the mass hanger to absorb the
impact of the weight when it falls.
Get children to set up their sample to test as in the diagram above and your demonstration.
They should add one 100 g mass to the hanger at a time and continue to do this until the sample
eventually breaks. Make a note of this in the table.
Bear in mind that the hanger itself is a 100 g so when one mass has been added there will be 200 g in total and so
on. They may be surprised as to how much weight it will take to break it. Nine masses plus the hanger is 1 Kg and it
could take that and more for some of the composites to eventually break.
Strong loop
of string
G-clamp
Concrete
Bench
Weights
on hooks
Bubble wrap
Possible Questions/ Suggestions for discussion
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What other additives could be tried in the mix?
How could the concrete be made even stronger?
How might this experiment be improved on in the future?
What other materials could this kind of test be used for?
How else could you record your data and what else could you test?
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Chemical Change – Irreversible Chemical Changes
Lessons 7 and 8
Learning Outcomes
All children should
 Practise the skill of making a prediction.
 Control variables.
 Collect data using appropriate ranges, numbers, values, for
measurements and observations.
 Learn that chemical reactions are either reversible or
irreversible.
Some children could
 Explain their observations using justified arguments.
 Recall that exothermic reactions increase temperature and
endothermic reactions decrease temperature.
A few children could
 Independently undertake a practical technique.
 Know that a composite material is made by combining two or
more materials – often ones that have very different
properties.
Plenary/Review including Skills Progression focus: Recording and analysis of scientific data
and observations
1.
2.
3.
4.
Did the results match the predictions? Were there any surprises? Discuss what the children discovered from
the results.
Were the results reliable and can any conclusions be drawn from them as to what composition makes the best
and strongest concrete?
Is this a fair test? Is there a better method of testing the concrete? How could it be improved?
From the results what particular additive in the strongest composite can be identified as the ingredient that
made the difference between it and the other composites?
Useful websites
History of concrete: http://www.historyforkids.org/learn/architecture/concrete.htm
Or: http://www.teachprimary.com/learning_resources/view/ks2-science-the-romans
Soda Snake video: www.youtube.com/watch?v=8jNG8w0lJ4U
Template mould for making concrete
Shaded tabs, place glue on the outside to stick to the inside of side
pieces
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