Elliot Williams

Treetops Studio

KS3 BURNING

Lavoisier:The first chemist?

THE UNDERSTANDING OF BURNING AND THE LAW OF CONSERVATION OF MASS (1789)

The story of Antoine LavoisierIt 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 burntThe candle and the wood both lose mass. So this checks out with the phlogiston theoryUnfortunately, 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

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

(Reactants) (Products)

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.

LAW OF CONSERVATION OF MASS

Magnesium + oxygen = magnesium oxide

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

Airmercury

mercury oxide

Heat the flask strongly for 10

minutes

reweigh

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.

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

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.