The Free-Ride offspring are magnets for questions not easily answered in the framework of the grade school science curriculum. This means, I think, that the Free-Ride parents are going to have to work out some age-appropriate ways to offer explanations. And I have a feeling my molecular model kit is only going to take us so far.
For example:
What’s the deal with dry ice?
As Hallowe’en approaches, the Free-Ride offspring make more frequent encounters with plastic cauldrons spilling over with spooky vapors. They know that what produces those vapors is called “dry ice”, but what exactly is that stuff? It’s not “regular” ice (which is made out of water), so calling it dry ice is OK. But how does carbon dioxide (the stuff we exhale) get turned into solid blocks? And why doesn’t it do the regular thing of melting to a liquid before it goes to gas phase?
They’re not quite ready for the intermolecular forces explanation, so we’re reaching for some other way to describe how carbon dioxide’s molecules make it play with others quite differently from substances like water.
Where does the salt go in salt water?
Of course you can dissolve things like salt and sugar in water … but what does that mean exactly? Where does it go? You can’t see it any more (although you can sure taste it, so it hasn’t gone away).
And do you mean to tell me there’s carbon dioxide in that soda?
How do you dissolve a gas like carbon dioxide in water (along with all that high fructose corn syrup) to make soda? Can you really dissolve it? (The bubbles seem pretty happy to escape when you open the bottle or the can.)
Hmmm. To talk about solutions, we probably can’t avoid talking about intermolecular forces on some level. (We might also have to show the offspring what happens when you sprinkle some sugar or salt into a carbonated beverage.)
If heating a liquid usually turns it into a gas, what’s up with egg whites?
Oh egg whites, you and your albumin getting opaque and solid when heated! Now I need to see if I can explain the structure of proteins (and what it means for a protein to denature) to grade schoolers. (I know my molecular model kit is not the way to go here.)
Actually, the cartoony drawings in The Epicurean Laboratory seemed to satisfy the sprogs’ curiosity as to what kind of change the egg white protein experiences on heating. But what they really liked was the idea that an explanation of the properties of a particular ingredient, or of the transformation behind a certain technique, should be accompanied by a recipe.
There are certain kinds of lab work that are inherently rewarding — especially if you’re too young to be stuck washing the glassware afterwards.
I just wanted to tell you that I really admire how you engage your children in these discussions. With no children of my own (and an uncertainty of whether I’ll want any someday), I still look forward to these Friday posts.
Well, can you tell them that there are intermolecular forces, without explaining exactly what causes them? It should be easy to believe at least that there must be something which holds molecules together, or else all matter would fall apart. From there, you can say that (for reasons of chemistry that are hard to go into), carbon dioxide molecules don’t stick together as well, so they can break free and go into the air more easily (direct sublimation at standard pressure). Water holds together better, so their molecules stay in a jumble (liquid) for a while before finally escaping (as steam).
I could be wrong, but would a useful way of explaining the salt be that it is broken up into REALLY tiny little things, so tiny that you cant see them?
To add to what Donalbain is saying, I think a window screen might be a good analogy for what happens with the molecules in sugar/salt when it’s dissolved.
If you take a bundle of wire and hold it all together, you can’t see anything through it – it’s solid. If I spread those wires out far enough (as in a window screen), light passes through the gaps and it becomes transparent. The wires are still there, but the properties of the system have changed. A similar concept applies to sugar molecules.
Umm, it might be a bit early to be talking about intermolecular forces.
For instance, the salt/salt water question can be handled pretty simply just by mixing two different granular solids, for instance marbles of two different colors. Blue marbles in a bowl can be “water” and white ones can be “salt”; at first when the marble clusters are brought together they’ll be discrete, but a little stirring will soon get the white marbles quite well in suspension.
Bonus points for using a massager to simulate Brownian motion in the water marbles, displaying the way salt dissolves even without stirring; and if you really wanted to be clever you could use steel bearings for the “salt”. Add enough to the lighter “water” and you’ll have a nice analogue of saturation as the steelies all sink to the bottom.
When I was about the same age as the Elder Offspring the explanation I was given of dry ice was that it was a frozen gas. How did it get frozen? In much the same way as water, but with (in essence) a much colder freezer. It remained “dry” because of sublimation; that is, it didn’t pass through a liquid phase on its way to the gaseous.
An explanation more technical than that would have been well over my head anyway; and I came away from the discussion with a tolerably decent understanding of the properties of frozen carbon dioxide, sufficient to suit me until I was in chemistry classes in HS. I mean, there was more to know about it, but as a young child I simply did not need that much information.
As for fizz: Isn’t that basically injected into the soda by compression? Or is that not a sufficient explanation for the sprogs?
I think sprogs will like Howtoons.
http://www.howtoons.com/