I need some SCIENCE!

[Lvl 2 Expert]

Pre-emptive notice removed, due to having lost its relevance.




For this weekend I need some pop science living room experiments. Think cola and menthos or "try to throw the tennis racquet without it starting to spin" or "does a pepper taste hot on different areas of your tongue?". So I'm looking for cool suggestions. I know there are tons of these out there, but I haven't done that many yet.

[Peelee]

Theres always the easiest, cheapest one you can possibly do; instant ice. Put distilled water in the freezer for a few hours. If it's still liquid, you can either give it a good thunk and watch it freeze in your hands, or slowly pour it out into a dish and watch an ice stalagmite form.

Water is cool.

[georgie_leech]

If it's related to D&D (and thus magic), then an undeniably awesome but DANGEROUS trick is lighting your hand(s) on fire. Live out your wizard fantasies.

[Bucky]

You can demonstrate how ferromagnetism works at macro-scale using a grid of compasses.

Wave a magnet over them and it disorders the grid, but reapply it slowly and they'll all line up to point at the magnet and stay that way even after it's removed.

[Khedrac]

How much of air is O2?
Take a tea-light candle, float it in a bowl of water then light it.
Now put a clear glass upside-down over the candle (it needs to not seal with the bottom of the bowl)
Watch as the water level in the bowl goes up as the O2 is used up before the candle goes out.

Air pressure:
fill a small glass with water (to the brim).
Place a piece of card on top of the glass.
Hold the card to the glass while you turn it upside down and hold it in mid-air.
Remove your hand - the card and water remain in place due to air pressure.

Maths is silly:
Take a strip of paper (about 12 inch long and 1 inch wide works well) and join it to itself giving the paper a single half-turn so it does not form a smooth cylinder - it should now be a Moebius strip.
Moebius strips 1:
Ask a child to colour one side one colour and the other side a different colour - they will find it only has one side.
Moebius strips 2:
Cut the paper in half down the middle of the strip (with no twists this would make two 1/2" cylinders) - ask the children to guess how many twists the resultant cylinder will have.
(Off hand I cannot remember the answer, but I think it is 2.)
Moebius strips 2:
Cut the paper down the strip about 1/3 of the way across - ask the children to what they think will happen.
(This is my favourite as you get 2 interlocking loops of different sizes.)

[Excession]

Heat a small amount of water in an old soda or beer can. Once it is boiling, carefully pick up the can with tongs and put it upside down into cold water. The condensing water vapour will very quickly crush the can. Video: https://www.youtube.com/watch?v=TrluA_tcjVE

[Lord Torath]

How about the boiled-egg-sucked-into-the-bottle trick?

Or blow up a balloon by placing it over the top of a glass bottle, then putting that bottle in boiling water.

[Kato]

I think about two years ago I started a similar thread and got some decent answers.. Not sure if it's still in the archive. Sadly I can't remember much of what was suggested.
Two cute things I like to do once a while with students are..

1) match rockets. A little fidgety but with a match and a bit of tin foil you can make a decent rocket.
2) easier and usually more impressive : film canister rockets : if you can get your hands on a film canister, or something similar, fill it with water and a tablet of Alka seltzer or something and watch it soar.


Other things not yet suggested.. Burn an empty tea bag. Try to pop a balloon on a needle cushion. Pick a small hole in a plastic bottle filled with water. Or a can. If you want to be a bit more fancy, get some pure alcohol, douse paper with it and burn it.

Depending on what exactly you aim to do, you might also want to look for magic tricks, instead of experiments..

[Bucky]

How much of air is O2?
Take a tea-light candle, float it in a bowl of water then light it.
Now put a clear glass upside-down over the candle (it needs to not seal with the bottom of the bowl)
Watch as the water level in the bowl goes up as the O2 is used up before the candle goes out.

I would expect the opposite effect for various reasons.

[Peelee]

How much of air is O2?
Take a tea-light candle, float it in a bowl of water then light it.
Now put a clear glass upside-down over the candle (it needs to not seal with the bottom of the bowl)
Watch as the water level in the bowl goes up as the O2 is used up before the candle goes out.

I would expect the opposite effect for various reasons.

I would expect zero effect, since water is incompressible, so the water level certainly shouldn't go down, and heating the rest of the gas expands it. Even if the gas doesn't expand the gas enough, the lower the pressure, the faster the water will evaporate. So yeah, I'd expect either no change in water level or (very minimal) lower water level.

Of course, what I'd expect and what actually would happen are not necessarily correlated.

[Lord Torath]

I would expect zero effect, since water is incompressible, so the water level certainly shouldn't go down, and heating the rest of the gas expands it. Even if the gas doesn't expand the gas enough, the lower the pressure, the faster the water will evaporate. So yeah, I'd expect either no change in water level or (very minimal) lower water level.

Of course, what I'd expect and what actually would happen are not necessarily correlated.

I'd expect the upside-down bowl to drop a bit relative to the water level, as there is now a smaller volume of gas inside it to hold it up. Similar to how the lower volume of air on the bottle after burning sucks the egg into the bottle.

[Peelee]

Also, I missed that the bowls weren't sealed, and assumed it was a closed system. What I said probably makes a lot more sense with that in mind. My bad.

[Bucky]

I'd expect the upside-down bowl to drop a bit relative to the water level, as there is now a smaller volume of gas inside it to hold it up. Similar to how the lower volume of air on the bottle after burning sucks the egg into the bottle.

The burning isn't just removing oxygen from the air. It's adding the same volume of CO2. Since the fuel also has a little bit of oxygen in it, there will be slightly more CO2 added, than oxygen removed

[nineGardens]

So... perhaps not quiet right for your age group... but if you string a helium balloon up in a car, and then accelerate forward, which way does the balloon go?

[Peelee]

So... perhaps not quiet right for your age group... but if you string a helium balloon up in a car, and then accelerate forward, which way does the balloon go?

That's right for any age group, because it's intrinsically weird. Even knowing how it works, it's just weird to see. It's great.

[Khedrac]

The burning isn't just removing oxygen from the air. It's adding the same volume of CO2. Since the fuel also has a little bit of oxygen in it, there will be slightly more CO2 added, than oxygen removed

I didn't invent this one so, oddly enough no - this one is supposed to work - it doesn't go up much, but it does go up.

I am also surprised that no one commented that you are heating the air inside so expansion...

Now, I have not see this tried, but since there is so much discussion - why doesn't someone give it a go and let us know what really happens?

[Lvl 2 Expert]

Thanks everyone for the tips. There's certainly a lot in here I can use. Some things are not quite as practical with my means, like finding a source of helium for that cool balloon in car experiment, but I'm still happy things like that are in this thread, because they're fun. I'll let you know how it went.

I didn't invent this one so, oddly enough no - this one is supposed to work - it doesn't go up much, but it does go up.

I am also surprised that no one commented that you are heating the air inside so expansion...

Now, I have not see this tried, but since there is so much discussion - why doesn't someone give it a go and let us know what really happens?

Assuming candle fuel has the formula CyH2y, assuming no volume change from heating, and assuming identical density ~ identical volume per proton/neutron for O2 and CO2 and no volume at all for H2O or candle fuel:

2CyH2y+3O2=2CO2+2H2O.
So you exchange 3O2 for 2CO2. Molecular weight O=16, C=2, so 3 O2 = 96, 2CO2=88.

Theoretically it goes down a little bit, the 20% of the air that is oxygen is going to shrink to 88/96th of its former self. But the heat is going to negate this a bit, so it's fair to say the total comes out around neutral. Would be a fun quiz question, since it's hard to figure out what will actually happen, perhaps not the greatest demonstration of how much of the air is oxygen.

[Peelee]

I am also surprised that no one commented that you are heating the air inside so expansion...

BOOYAKASHA!

heating the rest of the gas expands it.

g

[Kato]

Assuming candle fuel has the formula CyH2y, assuming no volume change from heating, and assuming identical density ~ identical volume per proton/neutron for O2 and CO2 and no volume at all for H2O or candle fuel:

2CyH2y+3O2=2CO2+2H2O.
So you exchange 3O2 for 2CO2. Molecular weight O=16, C=2, so 3 O2 = 96, 2CO2=88.

Theoretically it goes down a little bit, the 20% of the air that is oxygen is going to shrink to 88/96th of its former self. But the heat is going to negate this a bit, so it's fair to say the total comes out around neutral. Would be a fun quiz question, since it's hard to figure out what will actually happen, perhaps not the greatest demonstration of how much of the air is oxygen.

It took me a moment to follow your line of thought there... In case I understand you properly, you're mislead.
The molecular mass is (almost) meaningless here. The effect is the 3 Mol of oxygen turning into the 2 Mol of CO2 and water, with the water probably quickly turning liquid, so you exchange 3 Mol of air for 2 Mol, which assuming about 21 percent oxygen in the air and assuming all being converted means about 7 percent of gas lost. That's not a lot, considering heating and some water vapor but even 5 percent loss with this experiment is a pretty obvious rise of water level.


*not a chemist, everyone is welcome to correct me

[Lvl 2 Expert]

It took me a moment to follow your line of thought there... In case I understand you properly, you're mislead.
The molecular mass is (almost) meaningless here. The effect is the 3 Mol of oxygen turning into the 2 Mol of CO2 and water, with the water probably quickly turning liquid, so you exchange 3 Mol of air for 2 Mol, which assuming about 21 percent oxygen in the air and assuming all being converted means about 7 percent of gas lost. That's not a lot, considering heating and some water vapor but even 5 percent loss with this experiment is a pretty obvious rise of water level.


*not a chemist, everyone is welcome to correct me

A gas' density is determined solely by the amount of mol? So CO2 does have a much higher density than O2? I should have paid more attention in physics class, this ideal gass stuff and such.

Hey, I do biochemistry, and when a human goes into the gaseous phase they have a bigger problem than knowing their density.


Anyway, we had trouble keeping the cup upright, but it did seem as if we lost a little volume of air under it. The heating effect was negligible.

The balloon on a bottle failed spectacularly. I tried putting a burning piece of paper inside the bottle, but it extinguished as soon as I pulled the balloon over it (well dug, in hindsight) and the cooling air pulled it vacuum. Thinking back I think it can be done with some water in the bottle, keep it over a flame to heat the water to steam and blow up the balloon that way (or melt it, that would be fun as well), but I'm not heating a beer bottle like that until I have enough safety goggles for everyone. (I know beer ir probably sterilized on the bottle, and I know laboratories are full of glass for heating, but still...

This failure was made even cooler when a mixture of baking soda and vinegar one of my co-organizers brought did manage to inflate a balloon spanned over a bottle.

The question "where do I need to hold the lighter to ignite this lamp oil" went very well. The trick is lamp oil needs a fuse, something that can soak up lamp oil while burning, heating it to create enough vapor and then that can ignite. You can't ignite it by holding a lighter near it. (Or maybe if you put in some serious effort, preferably with an extra big flame.) Ones I threw a little bit of burning paper in the cup of lamp oil it was actually quite hard to extinguish.

The crumbling soda can experiment went really well, it crumbled more than in that video even. (I'm more comfortable heating metal objects I don't know the properties off to cooking temperature than glass objects, because metal doesn't shatter.)

Some of the match head rockets in the rocketry competition burned pretty well, but only one took off for a short nose dive to the ground. This is very suitable for a longer session focusing on several elements of their design. As a pro-tip: it probably works best with the kind of matches where both the propellant and the oxidizer are in the head, rather than the oxidizer being on the side of the match book.

Taste zones turned out to sort of exist.

Menthos and soda pretty much failed to go off. We blame the budget menthos and budget regular cola.

Edit: Oh, and punching Oobleck is fun.

Everyone thanks for the experiments, the ones we didn't use as well!

[Lord Torath]

A gas's volume is dependent only on its moles (assuming temperature and pressure don't change). A mole of any gas at STP (standard temperature and pressure) takes up the same volume as a mole of any other gas at STP. Density depends on the molecular mass/weight. In this case, we only care about the volumes the gasses take up, and going from 3 units of O₂ to 2 units of CO₂ results in a 33% decrease in volume. The heat added from the chemical reaction will increase the volume (pressure remains the same), but it might not be enough to offset the decrease from 3 to 2 units of gas.

[Bucky]

There is a tiny effect where the extra mass of the carbon in CO2 increases the pressure-difference between the top and bottom of the container. But that's negligible.

And the mass didn't come from nowhere. The burnt-out candle is now displacing less water because it has less mass, but this shouldn't affect the relative water level because it displaced most of that water completely outside the container.

[Knaight]

The heat added from the chemical reaction will increase the volume (pressure remains the same), but it might not be enough to offset the decrease from 3 to 2 units of gas.

Room temperature is about 300 K (STP is 298.15, room usually runs a bit higher). Offsetting that decrease would require the temperature to raise to 450 K. That's a pretty common temperature for baking, and not a particularly cold one. We're talking about around 350 F here.

There's absolutely no way that temperature increase is happening, and so the volume decrease is going to be barely mitigated.

[Lord Torath]

Room temperature is about 300 K (STP is 298.15, room usually runs a bit higher). Offsetting that decrease would require the temperature to raise to 450 K. That's a pretty common temperature for baking, and not a particularly cold one. We're talking about around 350 F here.

There's absolutely no way that temperature increase is happening, and so the volume decrease is going to be barely mitigated.

See? I knew someone would do the math! Thanks, Knaight! (I love this place!)