Chemical Change – Observing and Measuring Chemical Changes
Lessons 9 and 10
Observing and Measuring Chemical Changes
Lesson Objectives
Choose from:
 Know that some materials dissolve in a liquid to form a solution, and that others may not.
 Explain what happens to a material during a chemical reaction.
 Make careful observations using scientific equipment and use to explain your ideas.
Essential Knowledge/Science Explained – for teachers
1.
The Blue Bottle Reaction: An alkaline solution of glucose acts as a reducing agent and changes methylene blue
from a blue to a colourless form. Shaking the solution raises the concentration of oxygen in the mixture and
this changes the methylene blue back to its blue form. When the dissolved oxygen has reacted, the methylene
blue is slowly reduced back to its colourless form by the remaining glucose; the cycle can be repeated many
times by further shaking. For this reason the methylene blue is known as a Redox Indicator. For more
information on Redox Indicators follow the link: http://en.wikipedia.org/wiki/Redox_indicator
2. Chemical worms: Sodium alginate is a natural polymer which can be extracted from giant kelp (brown
seaweed). It is one of the polymers that help to build the cell walls of these plants. It has some unusual
properties and a wide variety of uses.
When sodium alginate is put into a solution of calcium ions, the calcium ions replace the sodium ions in the
polymer. Each calcium ion can attach to two of the polymer strands. This is called crosslinking and turns the
solution into a gel.
Alginate is used in many applications e.g. Gaviscon and new ones are being found all the time. It is used in the
food industry, medicines and dental impression materials. Calcium alginate (the cross-linked polymer) is used in
wound dressings. Part of the blood clotting mechanism involves calcium ions. On contact with blood, the
calcium alginate releases calcium ions in exchange for sodium ions as observed in the experiment above. These
extra calcium ions help the blood to clot and encourage healing.
http://www.scitech.qut.edu.au/documents/community/events/pd/Polymers/Teacher_guides/AlginateWorms.pdf
Common Misconceptions
1.
Once a material has dissolved, it has disappeared. No, the molecules have simply become incorporated into a
liquid.
2.
Steel or Iron only requires the presence of oxygen to rust. Water is also necessary for the process.
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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Chemical Change – Observing and Measuring Chemical Changes
Lessons 9 and 10
Cross curricular links
Literacy
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Numeracy
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Other subjects
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Chemical worms may inspire some poetry (personification) or work on adjectives
and adverbs.
Comic strip style pieces on the use of concrete.
Take accurate measurements and control time intervals.
List other occasions when colour changes occur: in nature – chameleon, leaves, or
materials when wet – sand. You could even find out about Rachel Riley’s famous
dress.
Painting with Sound. Using music to inspire colour changing artwork. A similar
example is available at https://www.tes.co.uk/teaching-resource/ks2-music-unit13-painting-with-sound-cornwall-6304652
Preparation
For the demonstration (less than 20 minutes beforehand, preferably) HEALTH & SAFETY: Eye protection should be
worn by the teacher.
 Take one of the pots of methylene blue, unscrew the lid and add 50 ml of the alcohol.
 Next take one of the pots of potassium hydroxide and empty the contents into the Schott bottle, add 300 ml of
water and 10 g of glucose and swirl until the solids are dissolved. Add 5 ml of the methylene blue solution. The exact
quantity used is not critical.
 Use the mass balance to weigh the glucose.
 The resulting blue solution will turn colourless after about one minute. Screw the top onto the bottle.
For the investigation:
Prepare the two solutions needed.
Add the stated amount of water to the calcium chloride powder. Shake well.
Repeat with the sodium chloride powder.
Decant into smaller quantities for the children to use.
Prior Discussion
Have you ever been told to bring your bike in out of the rain? Have you ever wondered why? After all, rain is just water,
and the rain could clean your bike, right? The problem is that some parts of your bike are made of steel. Steel sounds like
a strong metal, and it is, but the main element in steel is iron, and in the presence of water, iron combines easily with
another element, called oxygen, to form iron oxide, also known as that reddish-brown substance called rust. (Corrosion)
Rust changes steel into a different material, one that is weaker than the original steel. Rusting is a big problem because
so many things people use every day are made out of steel, like cars, trucks, bridges, roofs, machinery, nuts, and bolts.
When these objects are unprotected and exposed to water, they rust, and this damage costs a lot of money. Today you
are going to mimic the different conditions that cause iron to rust.
Teacher Demonstration
The Blue Bottle Reaction
HEALTH & SAFETY: The teacher should be wearing eye protection.
Apparatus & Chemicals
 1000 ml Schott bottle with screw top lid
 400 ml glass beaker
 Potassium hydroxide pellets (Corrosive), 8 g
 Glucose (dextrose) (Low hazard), 10 g
 Methylene blue (Harmful), 0.05 g
 Ethanol/methylated Spirits (IDA – Industrial Denatured Alcohol) (Highly flammable, harmful)
 Mass balance
 Safety glasses
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Chemical Change – Observing and Measuring Chemical Changes
Lessons 9 and 10
Procedure
You may wish to present this experiment as a magic trick. Alternatively you could use this as a discussion on dissolving,
for example where are the particles? Which materials dissolve etc.
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Take the bottle containing the solution you prepared earlier. Screw the lid securely in place; shake it vigorously so
that air dissolves in the solution.
The colour will change to blue and will fade back to colourless over about 30 seconds.
The more shaking, the longer the blue colour will take to fade.
Alternatively for a more dramatic effect you can pour the solution into the large beaker and then back into the
bottle and vice versa.
After some hours, the solution will turn yellow and the colour changes will fail to occur.
A white background helps to make the colour changes more vivid.
On a cold day it may be necessary to warm the solution to at least 20 °C otherwise the changes are very slow. This
can be done by running the base of the flask under the hot water tap for a few minutes.
Possible Questions/ Suggestions for discussion
Expected responses from children are:
 Something is on the stopper causing the solution to turn blue when the flask is shaken.
 There are two colourless liquids in there and they mix to cause the colour change.
 Shaking the solution adds heat and causes the colour change. The contents then cool and the blue colour fades.
Children’s Investigation
Class activity – Experiment 1: Rusting of Iron Wool (Corrosion)
Apparatus and chemicals
Health & Safety – Safety glasses and gloves to be worn by all.
Each pair will need:
 2 Thermometers
 2 polystyrene cups with lids
 6 cm iron wool
 1 pair scissors
 1 Ruler
 Lemon juice
 Tap water
 pH paper (3 pieces)
 Disposable gloves
 Safety glasses
Procedure
In this experiment the pupils are going to see the effects of acid compared to tap water on iron wool. For this activity,
the children work in pairs.
Each pair and the teacher need to put on some disposable gloves before handling the iron wool (it is quite easy to get
metal splinters from it, so gloves should be worn).
The teacher must cut the iron wool into 6 cm pieces and get the children to measure each piece before they put them in
the cups as they are going to re-measure them at the end of the experiment. After the experiment the solutions can be
washed down the sink and the cups and iron wool thrown in the bin.
1.
2.
3.
Get each pair to collect their apparatus: A ruler, 2 cups and lids, 2 thermometers, some pH paper, a piece of iron
wool about 6cm long (they are going to cut it into half measure both pieces accurately using the ruler and place
one piece into each cup).
In one cup they will pour some tap water, just enough to cover the iron wool. Make sure they press it down
well, so it is firmly fixed in the bottom of the cup.
In the second cup they will pour some lemon juice, again just enough to cover the iron wool.
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Chemical Change – Observing and Measuring Chemical Changes
Lessons 9 and 10
4.
5.
6.
They need to take a piece of the pH paper rip it in half dip one piece in the lemon juice and one piece in the tap
water to test the pH of each solution and measure the initial temperature with the thermometer, which should
be room temp ( approx. 20°C).
Place the lids on the cups and insert a thermometer into each one through the centre hole in the lid. The
reaction will already have started and it is important to note the temperature initially and at regular intervals
(e.g., every minute for the next 10 mins.).
At the end of the experiment, get the pupils to check the pH of the two solutions as the oxidation occurs; you
would expect the pH to change and become less acidic. Also get the pupils to inspect the iron wool to see if
there is any notable difference in the two pieces. How do they compare to the initial measurements?
Class activity – Experiment 2: Chemical Worms
Apparatus and chemicals
Health & Safety: Wear eye protection and gloves. Although the chemicals are all low hazard they might sting if you got
them in your eyes.
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25 ml Gaviscon (which contains sodium alginate)
100 ml 1% w/v Calcium chloride solution (irritant)
75 ml saturated Sodium chloride solution
2 sealable glass jars (approx. 150ml each)
Labels for jars
10 ml plastic syringe (without needle)
Safety glasses
Gloves
Procedure
Forming calcium alginate:
 Pour the calcium chloride solution into one of the sealable jars (Label it Ca2+)
 Take up some of the Gaviscon into the syringe and gently squeeze out a thin stream into the calcium chloride
solution by gently and slowly pushing in the syringe plunger.
 Flexible, translucent “worms” will form instantly as the sodium ions are exchanged with calcium ions and the
polymer becomes cross linked. The longer the calcium alginate is in contact with the calcium chloride solution, the
more rigid the worms will become.
 You can lift the worms out of the solution to feel their texture.
Removing the cross-linking the calcium alginate polymer:
 Pour some of the saturated sodium chloride solution into the other jar (Label it Na +).
 Take a few worms from the Ca2+ jar (as soon as they are formed) and drop them into the Na+ jar. Shake both jars,
after a few shakes the cross linked polymer in the jar of brine (saturated sodium chloride) will dissolve and form a
cloudy solution, while the alginate worms in the calcium chloride solution will remain intact.
Pupils should make a table of their results of time intervals and temperature measurements and include observations of
the visual effect that hopefully has occurred and/or the pH values, depending on age and ability.
More able pupils could produce a graph of Time v Rise in temperature.
Ask pupils how the iron wool could be measured more accurately, bearing in mind that measurements aren’t all about
size. (i.e. weight) This could be done by weighing the mass of the wool both before and after the reaction.
Possible Questions/ Suggestions for discussion
Experiment 1
 What happens to iron wool in the presence of acid? ( It rusts quickly)
 What happens to iron wool in the presence of water? ( It rusts slowly)
 What is rust and what is its chemical name? (Iron oxide)
 Where have you seen this reaction before? (Burning iron wool)
 What type of reaction is taking place? (Exothermic)
The reaction is an exothermic one which means that heat is released by the reaction. This heat increases the rusting
effect on the iron wool: the combination of the acid and heat speeds up the reaction. Hence the rusting effect should be
greatest on the iron wool in the lemon juice.
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Chemical Change – Observing and Measuring Chemical Changes
Lessons 9 and 10
For more technical detail and excellent pictures of the effects of rusting, follow the link:
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p079.shtml
Experiment 2
 What is alginate an example of? (polymer) Where can it be found? (seaweed)
 Where have they come across other materials which react in a similar way?
Learning Outcomes
All children should
Some children could
A few children could
 Use scientific language when explaining simple scientific ideas or
phenomena.
 Make observations or measurements, identifying the ranges and intervals
used.
 Know that chemical reactions occur at a range of different speeds.
 Give examples of some of the factors that affect the rates of a reaction e.g.
temperature, movement, catalysts.
 Make observations and analyse the results.
 Propose scientific explanations for unexpected observations or
measurements making allowance for anomalies.
 Give examples of slow, medium and fast reactions e.g. – rusting, growth of
organisms, fermentation, and explosions.
 Explain the terms corrosion and polymerisation.
 Know that the speed of chemical reactions is affected by ‘Surface area’ –
the greater the surface area the faster the reaction.
 Develop an understanding that the chemical structure of solids differs
thereby causing different chemical reactions.
Plenary/Review including Skills Progression focus: Recording and analysis of scientific data and
observations
Get the pupils to enter a discussion and kick off with these little mind bogglers.
 Why were the ‘worms’ able to be so firm and stable in the Ca2+ solution?
 Why did the ‘worms’ become unstable and in fact dissolve in the Na+ solution?
 What do the results obtained tell us about the reaction of the alginate firstly with the calcium chloride and then with
the sodium chloride?
 What have we learned from this experiment about polymers and how they are formed?
Answers will clearly vary from the realisation that a different reaction took place, to an understanding that the
chemical structure of the solids used must be different.
Useful websites
Endothermic and exothermic reactions explained: http://www.sciencemadesimple.co.uk/chemistryblogs/exothermic-and-endothermic-reactions
https://www.youtube.com/watch?v=yvyHVA1Ww_M
Sodium ethanoate in a different form – reminder:
http://www.bbc.co.uk/bitesize/standard/chemistry/revision_videos/endothermic_exothermic/video/
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Chemical Change – Observing and Measuring Chemical Changes
Lessons 9 and 10
NOTES:
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