Particle Theory – Introduction to Simple Chemical reactions (4)
Lessons 11 and 12
Introduction to Simple Chemical Reactions (4)
Essential Knowledge
Disappearing plastic
The expanded polystyrene does not actually dissolve in the propanone; it merely collapses as air escapes when
the propanone is absorbed. An interesting example of a gas formed not by a chemical process, but by a physical
process. The resulting colloidal gel consists of propanone molecules dispersed in a network formed by a tangle of
large polystyrene molecules – a similar structure to ordinary jelly in which water molecules are dispersed in a
network of protein molecules.
You could use the analogy of spaghetti Bolognese with the long polystyrene molecules being the spaghetti and
the propanone being the mince.
Burning Paper
Paper is mostly made from a chemical called cellulose that comes from plants and consists of C (carbon) H
(hydrogen) and O (oxygen).
When it is burnt, the chemical reaction is effectively most like respiration i.e
cellulose + oxygen → carbon dioxide + water (+ energy)
The ash that is left is largely composed of unreacted cellulose, kaolin, clay, alum, talc and carbon.
Burning Iron Wool
Iron in steel wool reacts with oxygen from the air to form iron oxides. The oxygen from the air has weight and
adds to the weight of the iron. The result is material that is heavier than the iron alone.
The reaction is:
4Fe + 3O2  2Fe203
Common Misconceptions
1. Materials do not disappear when they dissolve.
2. Mass is associated with the term ‘massive’ and thus related to the size or volume of an object. The
term mass relates to the amount of matter in an object.
3. Gases do not have mass or weight. Fact: Gases are matter, and thus they have mass and weight.
4. There is a loss in mass when matter burns. Fact: Mass is conserved. Gases produced by combustion
have masses.
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.
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Particle Theory – Introduction to Simple Chemical reactions (4)
Lessons 11 and 12
Cross curricular links
Literacy
Numeracy
Other subjects
 Creative writing linked to the beautiful image of the burning iron wool, eg,
fireflies.
 Personification/poetry writing – using the iron wool as a stimulus.
 Accurately recording data, comparing data and commenting on its accuracy
and precision.
 Calculating the change in mass in the iron wool investigation using inverse
operations.
 Rounding/converting measurements using Newtons.
 Research Lavoisier and his work on combustion. Regardless of his
extraordinary services to the nation and to mankind, Antoine Lavoisier’s
connections to the fax agency proved to be fatal to him, for he died in May
1794 during the reign of terror. The Revolutionaries guillotined some 28 tax
farmers, including Lavoisier and his father-in-law.
 Research the life and times of Joseph Priestley.
Prior discussion
Ensure children are aware of the difference between weight and mass:
Weight is different from mass. Weight is the measure of the force of gravity on an object. The mass of an object
will never change, but the weight of an item can change based on its location. For example, you may weigh 100
pounds on Earth, but in outer space you would be weightless. However, you will always have the same mass on
Earth as you have in outer space.
Weight is a force caused by gravity. The weight of an object is the gravitational force between the object and the
Earth. The more mass, the object has the greater its weight will be. Weight is a force, so it's measured in
Newton’s.
Children could try converting the weight of some objects into Newton’s.
(1N = 9.8 kgs)
Teacher Demonstration
Disappearing plastic:
You will need: mug, acetone, polystyrene blocks/chips
Engaging children through an investigation where a solid seems to ‘disappear’. When expanded polystyrene is
placed in acetone, the polystyrene apparently disappears, and the gas bubbles within the material create a fizzing
effect as they are released. A small volume of propanone can absorb an impressive volume of expanded
polystyrene, making this an attention-grabbing demonstration.
1. Place about 5 ml of acetone into the bottom of the mug without the class seeing you do so and ‘casually’ place
small handfuls of the foam into the mug from a significant volume on your desk during an unrelated discussion
with the class.
2. The expanded polystyrene will quickly ‘dissolve’ in the propanone and a quite staggering quantity will
‘disappear’ into the mug.
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Particle Theory – Introduction to Simple Chemical reactions (4)
Lessons 11 and 12
Combustion of Paper
You will need: tin lid, heatproof mat, balance scales, sheet of paper match
1. Place the tin lid on the mass balance and ‘Tare’ or ‘Zero’ the display.
2. Lightly crumple up 1 sheet of A4 paper on the tin lid on the balance to check and record the mass.
3. Place the tin lid and paper on a heat proof mat and ask the class whether they think the mass will go up or
down when it is burnt – get the class to suggest this change in terms of grams and percentages.
4. Ignite the paper and burn it completely and then reweigh.
5. The mass should have decreased by approximately 90% which pupils should find quite surprising.
6. Ask the class to explain why there was a change in mass and where this mass (mostly molecules of CO2 and
water) has gone.
Possible Questions/ Suggestions for discussion
This investigation raises discussions over dissolving and disappearing, and allow you the opportunity to correct
common misconceptions about mass and volume changes in chemical reaction.
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Did the plastic disappear?
Ask the class about changing mass in chemical reactions and if they think there has been one here.
Ask the class how they would go about proving there has not been a change in mass but simply a change
in volume. See if they can suggest any other examples of where this happens such as compressing gas in
a football pump etc.
Children’s Investigation
Combustion of iron wool
Pairs will need: ball of iron wool, balance scales, tin lid, heatproof mat, 2x leads, cell
In small groups learners can enjoy the sizzle of burning iron wool in the investigation, whilst also drawing on their
existing knowledge to explain what is happening. Accurate repeated measuring allows for the identification of
anomalies and an ideal opportunity for comparing and graphing results.
In many ways this is one of the most important chemistry investigations for pupils to carry out as it follows in the
footsteps of Lavoisier. It was this experiment (although in the case of Lavoisier who used mercury instead) that
allowed him to single handedly revolutionise (excuse the sad pun! .. as he had is head chopped off and it was said
‘it took but a second to cut off his head; a hundred years will not suffice to produce one like it’) the study of
chemistry by overturning the so called Phlogiston theory and proving how respiration works.
He discovered, as the pupils will see, that there is a direct law governing the ‘ratio’ of reactants in all chemical
reactions.
Show the pupils how to make their iron wool balls. Explain the procedure to them and emphasise the importance
of accurate mass measurement and recording. As part of this, pupils should try really hard not to lose iron oxide
before they make their final mass measurements.
HEALTH & SAFETY: Pupils must wear eye protection
1. Place the tin lid on the mass balance and ‘Tare’ or ‘Zero’ the display.
2. Put the ball of iron wool on the balance to check and record the mass.
3. Take the ball of iron wool and physically expand it by pulling the bundle of fibres apart to create a large ‘fluffy
ball’ (about 10cm across). Do this over the tin lid taking care not to lose any ‘bits’.
4. Remove the tin lid from the balance and place on a table on a heat proof mat. Using two wires connected to
either side of a 9V cell with crocodile clips on each end – lightly touch either side of the wire wool ball at the same
time for about two seconds.
5. The wire wool should quite spectacularly catch fire. Once it has burnt completely, and allowing a few minutes
for the tin lid to cool. Reweigh the wool and record the result.
6. Calculate how much oxygen was used from knowing the mass of the iron wool and iron oxide.
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Particle Theory – Introduction to Simple Chemical reactions (4)
Lessons 11 and 12
Possible Questions/ Suggestions for discussion
1.
2.
3.
4.
5.
6.
How do we know a chemical change took place?
Did the materials change their state?
When chemicals undergo combustion does their mass change?
What happens to the mass of the iron after it is burnt? Why?
What happens to the mass of the paper after it is burnt? Why?
Where did the energy come from?
Learning Outcomes
All children should
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Some children could
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A few children could
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Learn the term mass and use when explaining observations.
Understand that chemical reactions can involve an increase and a decrease
in mass.
Decide upon the best way to present their data.
Explain whether or not their data is valid.
Learn the term ‘combustion’, and that it involves a reaction with oxygen.
Understand that chemical reactions can involve an increase and a decrease
in mass, but that the mass is not destroyed – simply changed in form and
that some mass may enter a reacting system and others leave.
Calculate and compare mass changes in reactions.
Accurately explain mass changes in chemical reactions in terms of
particles.
An interesting and useful reaction pupils could do is removing the dark
tarnish from any silver objects they have – visit the site below and ask
pupils to write up a short experimental report on what they did and what
they observed with extra points for clear demonstration of understanding.
Pupils should explain why the tarnished spoon’s mass decreases and the
aluminium foil increases in mass.
http://scifun.chem.wisc.edu/homeexpts/tarnish.html
Plenary/Review including Skills Progression focus: Recording and analysis of scientific data
and observations
 How could this experiment be improved to make the data more precise accurate and valid?
 In what ways might data from this type of experiment be useful in the real world?
 There is an excellent opportunity here to place all of their data in a class table during the experiment to
discuss and demonstrate averages, anomalies, reliability and validity.
 Initiate a class discussion on what the data really shows and introduce them to the fact that sometimes
experiments do not apparently yield any meaningful numbers.
 A discussion about the best type of graph in which to present this data could be done and then the class
instructed to try and complete one of their own or work together on a single type.
Useful websites
http://scifun.chem.wisc.edu/homeexpts/tarnish.html
http://www.bbc.co.uk/learningzone/clips/conservation-of-mass-in-chemical-reactions/10619.html
https://www.youtube.com/watch?v=x9iZq3ZxbO8
https://www.youtube.com/watch?v=_wUVhO_rVXo
http://www.bbc.co.uk/schools/teachers/bang/videos/lesson9_conservation_of_mass.shtmlhttps://www.youtube
.com/watch?v=AE0kuHKoitE
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