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Author Topic: How many school demonstrations are inaccurate?  (Read 2893 times)

Offline thedoc

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How many school demonstrations are inaccurate?
« on: 21/03/2012 10:09:01 »
annes  asked the Naked Scientists:
   
Re: The candle in the jam jar experiment

I remember this experiment from my childhood - in those days teachers didn't do many experiments in class so that's probably why i remember it - and I have even done it for kids myself - but we were told, and I have repeated, that a fifth of the glass is filled with water when the flame goes out because the flame has used up the oxygen in the air which is a fifth.

This was shown to prove that oxygen is a fifth of the air and it is quite convincing - shows the truth but not because of what is actually happening. Rather a paradox. I wonder how many teachers are still teaching this.

And, more important, how many science experiments can be done to show a fact when what is actually going on is not the real cause.

annie 123

What do you think?
« Last Edit: 21/03/2012 10:09:01 by _system »


 

Offline CliffordK

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Offline RD

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Re: How many school demonstrations are inaccurate?
« Reply #2 on: 21/03/2012 14:55:44 »
Do schools still have a tongue map on the wall ?  ... https://en.wikipedia.org/wiki/Tongue_map

and tell pupils they only have five senses ... http://human-body-physiology.knoji.com/how-many-senses-does-a-human-have/
« Last Edit: 21/03/2012 15:03:01 by RD »
 

Offline chris

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Re: How many school demonstrations are inaccurate?
« Reply #3 on: 21/03/2012 18:48:45 »
The tongue example just goes to show how pervasive the impact of disinformation can be; the wikipedia ref supplied above by RD suggests that the disproof of the regional sensitivity claim occurred in 1971; yet the concept of localised tongue taste sensitivity is still widely held...
 

Offline Airthumbs

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Re: How many school demonstrations are inaccurate?
« Reply #4 on: 22/03/2012 19:56:29 »
Are we getting stuck in the stone age or what? 

Because of my education at school I believed that there is a line of gravity around the Earth and that if you pass through it you are in Zero G.

I also thought that light was just the colours of the rainbow, and still see this stated by American documentaries.  Why don't they just say the light spectrum and what we see is the visible spectrum!

Try telling most people that radio waves are a form of light and they look kinda confused.

I also remember the jar and candle test and again we were misinformed.

 

Offline damocles

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Re: How many school demonstrations are inaccurate?
« Reply #5 on: 22/03/2012 20:56:51 »
Before getting too disparaging about the candle in jar experiment, let us take careful stock of what it involves.

Candle wax is an organic fuel. Its exact formula or purity does not matter -- the important point is that it contains carbon, hydrogen, and possibly a little oxygen, but no other elements. When it burns, it reacts with oxygen from the air, and produces carbon dioxide gas and liquid water.

In the burning candle experiment, the candle wax is clearly in excess, so that 100% of the oxygen in the available air will be consumed. There are three reasons why the volume of gas will not (initially) decrease by the expected 20%-ish:
(1) the oxygen will be replaced with another gas -- carbon dioxide -- in a quantity that will amount to roughly 2/3 of the amount of oxygen consumed.
(2) the residual air will be significantly heated by the experiment, and expand, probably by about 20%.
(3) in the final stages of the reaction when the oxygen supply is nearly exhausted, it is likely that carbon monoxide will form instead of carbon dioxide.

If the candle experiment is performed, there will not be an immediately observable reduction of air volume by 20%,
but
if the experiment is left set up and revisited after 24 hours a reduction of this magnitude will be observed, and the removal of 20% of the air through the consumption of oxygen will be a valid and correct explanation.
(1) The solubility of carbon dioxide gas in water at ambient temperatures is roughly 1 volume. The volume of the residual air will almost certainly be much smaller than the volume of the water in the trough, and so nearly all of the carbon dioxide produced will slowly dissolve. Making the water alkaline (lime or caustic soda, not sodium carbonate/bicarbonate) will assist in this process.
(2) Any expansion of the residual air through heating will be reversed as it slowly returns to thermal equilibrium with its surroundings.
(3) Carbon monoxide is fairly insoluble, and will probably remain in the residual air, but the amount produced in this experiment should be fairly small -- a very minor side reaction.

If a candle is burnt in a small pocket of isolated air over a large volume of water until it extinguishes, and then left for a long time, a 20% reduction in the air volume will indeed be observed. Consumption of the oxygen content of the air will indeed be a valid explanation.
 

Offline syhprum

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Re: How many school demonstrations are inaccurate?
« Reply #6 on: 31/03/2012 20:09:18 »
When I was at school we were told that the nucleus of atoms contained Protons and electrons, Neutrons had been around for ten years but like the workings of radar they were a military secret in case the Germans found out about them. 
 

Offline damocles

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Re: How many school demonstrations are inaccurate?
« Reply #7 on: 01/04/2012 21:55:41 »
When I was at school we were told that the nucleus of atoms contained Protons and electrons, Neutrons had been around for ten years but like the workings of radar they were a military secret in case the Germans found out about them. 

Wal, that can't be right! The 1935 Nobel Prize in Physics was awarded for the discovery of the neutron, and the prize award and the nature of the discovery would have been published long before the outbreak of the second world war.

I think that the absence of neutrons from your school instruction in chemistry was almost certainly because (i) they are fairly irrelevant to chemistry, and (ii) your teacher may not have been particularly well-informed about them.
 

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Re: How many school demonstrations are inaccurate?
« Reply #7 on: 01/04/2012 21:55:41 »

 

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