The inital speaker for the talk "Misconceptions in Physics Teachers" didn't turn up. Alas, I don't know his name, but there was an excellent stand in who gave a talk. He mentioned two common misconceptions.
Firstly:
The model that we teach for particles in science syllabi is like this (from BBC Bitesize):
Now, imagine a 50p piece. What happens when it gets cold? It shrinks.
Now, if the particles have no space to move into, how does the coin shrink? The atoms themselves would have to get smaller.
This is a problem with the model used above, the so called "ball model". It's easier when thinking of the "cloud model" where the edge of an atom isn't defined, and is based on probability. In this model, electron clouds can overlap. This could be what's happening as the coin shrinks. Or it could be that the atoms themselves do change size. However, as the speaker understood it, what's actually happening is up for debate (though he came down on the side of the atoms changing sides).
The misconception here is thinking of atoms as having fixed boundries, and not recognising the limiatations of the model used.
The second misconception was dissolving salt:
The speaker had witnessed a girl adding 60 cc of salt to 200 cc of water (1cc of water = 1ml). The final volume after the salt had dissolved was 240 cc, not 260.
The explanation given was that the salt crystals had moved in between the gaps in the water moelcules. This is a commonly given answer, and one that often gets taught. However, it is wrong.
The speaker, having his doubts, carried out the experiment himself, with salt. He noticed, when the water gets added to the salt, air bubbles escaped. He tried this again, but this time with sand, which doesn't dissolve. Sure enough, he got the same result. The decrease in expected volume is actually down to air escaping from inbetween the salt/sand, and not because particles are fitting in between gaps in water particles.
What he found most interesting though, with both of these, is that many teachers often get angry when presented with their misconceptions. "But it's in the text book" they'll say.
The take home message for me is to be more aware of the limitations of the models used in science teaching, and to try and root out any misconceptions I may hold.
Certainly air will escape when you mix water and grain salt, because there is air in the granular solid. Unfortunately this disguises any true volume change. But I expect you will find a volume change even when you take a solid block of salt with no included air, and dissolve it in water. Also, most pairs of miscible liquids show a volume change on mixing, water and alcohol being the best known case.
ReplyDeleteIn the specific case of dissolving an ionic solid like salt in water, there is generally a net volume reduction. This is because the water molecules are polar - they have some areas with a net positive charge and some areas with a net negative charge. So the negatively charged part of the water molecules cluster around the positive ion, and the positive parts cluster around the negative ions. This tends to result in a volume reduction.