Year 9 Chemical Reactions Worksheet
Early theory on combustion: Phlogiston Theory
Combustion, fire, and flame have been observed and speculated about from earliest times. In
ancient times, there were sometimes considered to be four elements: earth, air, fire, and
water. This theory lacked the predictive power to make alchemy a science. Eventually, early chemists
decided that combustion was the most important chemical reaction, that understanding combustion
would actually revolutionize chemistry.
The phlogiston theory was put forward in 1667 by Johann Joachim Becher as an attempt
to explain the combustion process and the rusting of metals. Becher suggested that there was
the existence of a fire-like element called phlogiston which was contained within flammable or
combustible substances that were released during combustion.
The Phlogiston theory said that all flammable substances had phlogiston within them. It
was something that did not have any colour, smell, taste or mass that was released upon
burning. Only after everything was burned would the ‘dephlogisticated’ substance would take
its true form. For instance substances that could be burned in air were determined to be rich in
phlogiston. When air was taken away, burning would cease leading to a conclusion that air had
a capacity to absorb a finite amount of phlogiston.
This theory didn’t survive for very long. Chemists were beginning to conduct
quantitative experiments and taking measurements. It was observed that some metals gained
weight after being burnt, magnesium being one of them. If phlogiston had been released during
the burning process, why was there a gain in weight? If anything the weight should have
decreased on the release of phlogiston, or perhaps remained the same given that phlogiston
was supposed to not have any mass.
Question
How did society back in the 1700s come to believe in the Phlogiston Theory?
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Task 1: Burning Magnesium and paper – Testing Phlogiston Theory
Aim: To test the Phlogiston Theory by demonstrating if the mass of a substance would increase
or decrease after burning it.
Equipment:
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Magnesium Strips
A piece of paper
Bunsen Burner
Goggles

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Analytical Balance
Tongs
Evaporating Basin
Method:
1)
2)
3)
4)
Put on goggles provided
Weigh the evaporating basin (to 2 decimal places) and record the result below.
Use the tongs and place one strip of magnesium into the evaporating basin.
Weigh the evaporating basin with the magnesium strip in it on the analytical balance.
Record results onto the table below.
5) Calculate the mass of the magnesium and record your result in the table.
6) Set up the Bunsen burner then turn the collar to open the air holes and create a blue
flame.
7) Use the tongs to hold the magnesium strip to the Bunsen flame and quickly place it back
on the evaporating basin. DO NOT LOOK DIRECTLY AT THE MAGNESIUM STRIP WHEN
BURNING.
8) Observe the physical change after the reaction
9) Weigh the evaporating basin again (after the magnesium has stopped burning),
calculate the mass of the magnesium after burning and record your results.
10) Repeat steps 3-9 with a folded piece of paper
Results:
Mass of Evaporating Basin: ___________________
Weight (to 2 decimal places)
Before burning
After Burning
Magnesium Strip + evaporating basin
Magnesium strip
Paper + evaporating basin
Paper
Questions:
1. What was the appearance of the magnesium before it was burned?
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2. What was the appearance of the magnesium after it was burned?
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3. Identify 2 things that were released from the magnesium and paper when placed in the
flame.
1. __________________
2. __________________
4. If ‘Phlogiston’ (from phlogiston theory) was a substance that was thought to be released
from matter when burning, would you have expected the weight of the magnesium strip
and paper to have increased (weigh more) or decreased (weigh less)?
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5. Do your results support the Phlogiston theory? If not, what do you think may have
happened? (Clue: The magnesium has reacted with another element to form a
compound)
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Year 9 Chemical Reactions Worksheet
Joseph Priestley and Antoine-Laurent Lavoisier: Proving the
Phlogiston Theory was incorrect
Phlogiston Theory survived as the dominant theory until Antoine-Laurent Lavoisier
demonstrated that combustion requires a gas with a measurable weight to occur. This gas was
oxygen. He used closed vessels to make these measurements and his findings started the
caloric theory of combustion.
Antoine-Laurent Lavoisier was born in Paris, France, on August 26, 1743. His mother
came from a wealthy family and his father was a respected lawyer. When his mother past away
when he was fire years old, he lived with his aunt, Constance Punctis. Lavoisier grew up to
follow his father’s footsteps and become a lawyer but realised his true passion was in science.
He was a man who could not dismiss what he could not explain, he especially favoured
chemistry and is now often known as the Father of Chemistry through his He explored was in
contact with many scientists all over the world.
One day he met Joseph Priestley, a chemist, philosopher and lecturer, who shared him
his discovery of a gas that gave fire a more intense flame on a candle. Priestley called it
‘dephlogisticated air’ as he theorized that this air lacked phlogiston, therefore readily able to
absorb the phlogiston in the candle. From here, Lavoisier did extensive research on this gas and
later showed that the Phlogiston Theory could not have been correct, and that the
‘dephlogisticated air’ was what he later named ‘oxygen’. He did this by performing hundreds of
experiments which demonstrated that some material gained weight after being burned, and
this did not make sense because if phlogiston was found in flammable substances, and
phlogiston was lost into the air when the substance is burning, the material should have gained
eight instead.
Other scientists also had similar results that could not have explained by the Phlogiston
theory, and so Lavoisier came up with a different theory that it must be a reaction where
something in the air is combining with the burning substance to make it heavier during
combustion, oxygen. During the eighteenth century, as it became clear that metals gained weight
when they were oxidized, so phlogiston was increasingly regarded as a principle rather than a material
substance.
Law of Conservation of Mass: Lavoisier changed chemistry from a qualitative to a
quantitative science. He freed society from the disillusionment of the phlogiston theory
by showing that the mass of the products in a reaction are equal to the mass of the
reactant
Question
1.
Would we know about oxygen today if Priestley did not share his ideas?
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2.
How did Lavoisier begin to think that the Phlogiston Theory might have been incorrect?
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3.
How did Lavoisier demonstrate phlogiston theory may be wrong?
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Year 9 Chemical Reactions Worksheet
Testing Lavoisier’s Theory of Combustion
Task 2: Observing the need for oxygen in a combustion reaction
Aim: To demonstrate oxygen is needed for the process of combustion.
Equipment:
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Test tube stoppers
Goggles
Hydrogen Peroxide
Manganese dioxide
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A Test-tube
A test tube rack
2-3 Paddle pop sticks (or splints)
Bunsen burner or lighter
Method:
1) Put on a pair of goggles
2) Pour about 30mL(3-4 centimeters) of hydrogen peroxide into the test tube
3) Use the tea spoon to scoop a small amount of manganese dioxide into the test tube
then quickly place a test tube stopper on the test tube
4) Light the tip of the paddle pop stick (or splint) with the Bunsen burner and blow it out
5) Move the tip of the paddle pop stick into the test tube and above the solution
6) Observe what happens
Questions:
1. What did the gas that was given off by the hydrogen peroxide and manganese oxide do
to the burnt splint?
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2. If Lavoisier’s theory is correct, name this gas
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Lesson Summary Questions
6. What were reactants and products in the magnesium and paper burning experiment?
Reactants: ________________________
Products: _________________________
7. Complete the word equation:
Magnesium
+
________________
=
Magnesium ______________
8. Complete the equation using the above answer and the periodic table:
Mg
+
___
=
Mg __
9. Complete the diagram to show how you think the particles change in the reaction:
10. Why is the result from burning magnesium different to those from burning paper?
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Year 9 Chemical Reactions - Homework
CHEMICAL REACTIONS: COMBUSTION
What is a combustion reaction?
Chemical reactions are when bonds between atoms in the original chemicals, the
reactants, are broken. The atoms are neither created nor destroyed, but are rearranged and
new bonds are formed. In this way, new chemicals - the products – are produced.
Combustion reactions or burning is when substances in a compound combine with
oxygen, which then produces carbon dioxide and water. Combustion is commonly called
burning. It is an exothermic reaction, which means heat is produced and is easily distinguished.
Examples of combustion reactions include the burning of petrol in a motorcycle engine,
wax vapour in a candle flame, and natural gas in a kitchen stove. In all these cases,
hydrocarbons (compounds containing only the elements carbon and hydrogen) combine with
oxygen in the air to form carbon dioxide gas and water vapour.
The fire triangle or combustion triangle is a simple
model for understanding the ingredients necessary for
most fires.
The triangle illustrates a fire requires three
elements: heat, fuel, and an oxidizing agent (usually
oxygen). The fire is prevented or extinguished by
removing any one of them. A fire naturally occurs when
the elements are combined in the right mixture.
Questions:
4. What is a combustion reaction?
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5. What are some examples f combustion?
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6. What are hydrocarbons, where are they found and what role do they play in combustion?
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7. If Lavoisier’s theory is correct, name this gas
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