KS3 BURNING
Lavoisier:
The first chemist?
Treetops
Studio
Elliot
Williams
THE UNDERSTANDING OF BURNING AND THE
LAW OF CONSERVATION OF MASS (1789)
The story of Antoine Lavoisier
It is the 1780s and Lavoisier is investigating what happens
during burning
In this topic you will first cover two examples of burning
1. Burning Candle
2. Burning magnesium
<<< teacher demo
or class practical
•During the 18th century the most obvious example of
burning was wood which was the main fuel at the time
•Class Practical: Simply burn a wooden splint and leave it
burn out on the heat proof mat.
•
What happens when something burns? For two hundred
years scientists had believed that burning was explained
by the “Phlogiston” theory
THE PHLOGISTON THEORY OF 17th and 18th century
When something burns, then a substance called
“phlogiston” leaves the substance.
CLASS/GROUP DISCUSSION/ FEEDBACK : Do you
think the Phlogiston theory works for these three
examples? (burning candle,magnesium and wood)
Hint: Think about the mass of each substance
before and after heating.
Scientists at the time would say that they can all
be explained by the phlogiston theory because
they all changed to a smaller mass when they
were burnt
The candle and the wood both lose mass. So
this checks out with the phlogiston theory
Unfortunately, accurate measurements show that
the magnesium doesn’t lose mass. The white ash has
a greater mass than the starting piece of magnesium.
To put it simply. When magnesium burns it gets
heavier!
CLASS/GROUP DISCUSSION/ FEEDBACK : Why
exactly does the magnesium example show that the
phlogiston theory is wrong!
CLASS/GROUP DISCUSSION/ FEEDBACK
QUESTION: If it is so easy to show that the
phlogiston theory is wrong then why hadn’t anyone
spotted it before?
HINT: Remember how long ago this time period was
ANSWER: One important factor must have been that
balances at the time were not very accurate. Lavoisier
invested in the finest balances available to him
LAVOISIER (1743- 1794)
Was not convinced about the
Phlogiston theory.
He did some very carefully measurements of
masses of substances before and after burning
Here is some of the actual apparatus he used…
In the days before bunsens, and other similar gas
burners, he had problems getting certain things to
burn, especially metals. So he invented a burner that
used “amplified Sunlight”
CLASS/GROUP DISCUSSION/ FEEDBACK
It’s on the next slide. How do you think it worked?
Experiment on burning using “amplified sunlight!”
Notice the
“shades”….
Cool or
what!
Antoine Lavoisier was born to a wealthy family and
inherited a fortune at the age of 5 through the death of
his mother. He studied chemistry, astronomy, biology,
mathematics, and law.
His main passion was for chemistry.
At the age of 28 he married Marie-Anne Paulze. She was
only 13 years old! She proved to be a great scientific
colleague to her husband. She was fluent in English, and
excelled in illustration and engravings.
CLASS/GROUP DISCUSSION/ FEEDBACK
QUESTION: Why do you think his wife’s skills in
language and drawing would be so useful? HINT
remember the time
ANSWER:
Most of the leading scientist of the day were British and
the main means of spreading their findings was in scientific
papers written in English.
Marie-Anne would translate these English papers into
French for her husband to be able to understand them
She also translated her
husband’s work from
French into English so he
could share his ideas.
Her sketching skills were used to produce engravings
of the apparatus and methods he used
Here is one of her sketches turned into an engraving
showing Lavoisier at work. She has drawn herself
taking notes at the right hand table.
So LAVOISIER did very careful
weighings of substances before and
after burning.
He found that sometimes the mass
INCREASES when you burn
substances e.g. metals get heavier
when they burn.
He said this showed that the
phlogiston theory must be wrong.
Do you remember why?
Lavoisier thought that there was a
gas in the air which joined to
substances when they burned.
He called this gas “oxygen”.
He thought that when things burn
they don’t lose phlogiston, but they
gain oxygen.
These days we would explain the increase in mass of
magnesium when it burns by writing the following
WORD EQUATION
(A word equation just shows all the chemicals involved
before and after a reactions)
magnesium + oxygen = magnesium oxide
The magnesium gains mass ( gets heavier) because
oxygen adds its mass to the magnesium
BUT wait a minute – How can
adding oxygen make something gain
mass. Does oxygen have any mass?
Oxygen is in the air – can you
weigh air?
Demo weighing the air in a balloon
Lavoisier went one stage further and stated that
the Total mass of substances at the beginning and
the end of burning must be the same
Magnesium + oxygen = magnesium oxide
(Reactants)
(Products)
Total mass
is the same
From this came his LAW OF CONSERVATION OF MASS
During a chemical reaction the total
masses before and after stay the same
LAW OF CONSERVATION OF MASS
Magnesium + oxygen = magnesium oxide
These days this law might seem pretty
obvious.Take the example of magnesium.
Since the magnesium joins to the oxygen in the
air, then if you add together the mass of the
piece of magnesium and the mass of oxygen it
joins to, then it must be equal to the mass of
the magnesium oxide formed.
In your exercise book……….
BURNING AND THE LAW OF CONSERVATION
OF MASS
This law states that the total mass of reactants is
always equal to the total mass of products
1. Explain the old theory of burning (Phlogiston)
2. What made Lavoisier think that the phlogiston
theory was wrong? ( Hint: he did some careful
experiments burning metals)
3. How did Lavoisier explain what happens during
burning?
4. Why do you think Lavoisier found it difficult to get
other scientists to believe him?
5. Rust is iron oxide. Why is rusting a bit like burning?
6. Does rusting make the nail get heavier or
lighter? Explain your answer?
7. Rusting also obeys the law of conservation of
mass? Explain why.
EXTENSION QUESTION
8. The equation for a candle burning is:
Wax + oxygen = carbon dioxide + water
A candle burning might seem to disobey the law
since it gets lighter. Explain how a candle burning
does obey the law of conservation of mass.
CLASS/GROUP DISCUSSION/ FEEDBACK
QUESTION: Here is one of Lavoisier’s famous
experiments
A. Try to predict the result of this experiment
B. Try to explain why this experiment ON IT’S
OWN does not provide sufficient evidence to
disprove the phlogiston theory.
One of Lavoisier’s famous experiments
was to heat mercury in a sealed flask full
of air. He weighed it before and after
mercury
mercury
oxide
Air
reweigh
Heat the flask strongly
for 10 minutes
A.Will the total mass get heavier, lighter, or stay
the same? Explain………………………
B.Does this prove the phlogiston theory was wrong?
A. The total mass will stay the same
because the mercury gains mass by joining
to some of the oxygen in the air. This gain
in mass is the same as the loss in mass of
the air, so the overall total mass remains
the same.
B. This experiment on its own does not
prove that the phlogiston theory is wrong.
You could say that when the mercury burnt,
the phlogiston left the mercury and the
total weight of the burnt mercury plus
phlogiston is therefore the same as the
weight of the starting mercury.
TEACHING POST SCRIPT
The phlogiston theory is often wrongly
attributed to the ancient Greeks (even in the
brand new Hodder Science book!) It was in fact
first postulated by Becher a German alchemist
in 1667.He made up the name phlogiston from
ancient Greek ( meaning “burning up”). Becher
was an interesting guy but not one of life’s
winners. He was an alchemist, a science which
of course is based on nonsense. He made up an
international language of 10,000 words which
everyone ignored, and was totally convinced
that given the right materials he could make
himself invisible!. He is only remembered now
for his phlogiston theory, which of course was
also rubbish.
TEACHING POST SCRIPT
Lavoisier made a huge contribution to chemistry and
science in general . For example he named hydrogen,
introduced the metric system to France, devised
systematic chemical nomenclature and was the first to
prove that diamond was a form of carbon. He was
executed by guillotine by French Revolutionists aged 50,
because he had intervened to try to stop non-French
scientists losing all their possessions by a law passed by
the new Revolutionary Government. 18 months later they
admitted it had been “a mistake”! They made a statue of
him.
Lavoisier is now thought of as the father of modern
chemistry – perhaps the first true chemist.