[enahs]

Does anybody know of any really unique and fun chemical demonstrations appropriate for ~8th grade students?

[mike]

Slime
Luminol
liquid nitrogen
nylon
thermite
ferro fluid

[Dan]

Freeze a banana in liquid nitrogen and hammer a nail into a block of wood with it. That's fun.

Check out sulfur hexafluoride fun.

[Yggdrasil]

I remember a really insightful gas law/boiling point demo that one of my chem professors performed.  She heated water in a round bottom flask until it boiled then removed it from the heat, let the boiling subside, and capped the flask.  Next, she cooled the glass above the level of the water (with an ice cube) and caused the water to start boiling again.

This is a great demo because the result is counterintuitive: by cooling the flask you can make the water boil.  Of course, this experiment is to show that boiling point depends on pressure, so you can get a solution to boil by either raising the temperature or lowering the pressure.

[constant thinker]

I remember a really insightful gas law/boiling point demo that one of my chem professors performed.  She heated water in a round bottom flask until it boiled then removed it from the heat, let the boiling subside, and capped the flask.  Next, she cooled the glass above the level of the water (with an ice cube) and caused the water to start boiling again.

This is a great demo because the result is counterintuitive: by cooling the flask you can make the water boil.  Of course, this experiment is to show that boiling point depends on pressure, so you can get a solution to boil by either raising the temperature or lowering the pressure.

I've seen that demo performed before too. Instead of using an ice cube my teacher just poured cold water over the flask. I also thought it was pretty cool, and a really good demo. The fact the it isn't what a regular person unfamiliar with that concept would expect makes you really think about it.

[Ψ×Ψ]

Along the ice-cube-boiling lines (sorta)...
Take 100 mL EtOH (95%) and add 100 mL H₂O.  Combine.  Confuse people when total volume is less than 200 mL.  (Best to do this in graduated cylinders, and I might have seen it with 50/50/<100mL instead of what I have listed.)

[movies]

I've done luminol for that age group a couple of times and it's a big hit.

Other easy ones are sodium in water and various solubility demos (I like the triphasic dichloromethane/water/hexane with indigo and iodine!).

I want to try the sugar/sulfuric acid one too.

[mike]

Last Thursday I did some demos for a group of year 10 students 13-14 year olds. I shoed them the oscillating clock (traffic light reaction) which they absolutely loved, all the pretty colours. Then we did the catalytic decomposition of hydrogen peroxide in a volumetric flask with detergent, cool! This was followed by the sulfuric acid and sucrose, carbon tower, also a hit. Finally we finished it off with a spectacular thermite reaction that was eagerly anticipated and did not disapoint.

[movies]

I did the very simple penny in hydrochloric acid trick.  I never thought it was all that exciting, but the kids loved it!  Make sure you get a penny that is zinc filled (older pennies are solid copper) and file off a little bit of the edge to expose the zinc.

[Alexander]

try to make volcano using (NH₄)₂Cr₂O₇

[pizza1512]

Last Thursday I did some demos for a group of year 10 students 13-14 year olds. I shoed them the oscillating clock (traffic light reaction) which they absolutely loved, all the pretty colours.

Does anyone know what was used for that experiment?

[constant thinker]

How about copper wire in a silver nitrate solution. Kind of boring, but demonstrates the concept reactivity series.

You could also put some sodium polyacrylate in a styrofoam cup secretly. Then you add some water and tip the cup upside down. It would confuse people as to why the water isn't coming out.

Something that even the students could do would be making slime. I think It's polyvinyl alcohol and sodium borate that have to be mixed. You get a cool slime that's fun to play with. I did this a long time ago.

[Custos]

Fun with CO₂. Generate a beaker of carbon dioxide with sodium carbonate and acid. Carbon dioxide is denser than air so it will stay in the beaker so long as you don't move it around too much. Blow a soap bubble and allow it to fall into the beaker. When the bubble reachers the carbon dioxide layer it will literally bounce -- seemingly in mid-air -- and then come to rest floating on the invisible layer of  CO₂.

Another good experiment while you're at it, is to put a lit candle in the bottom of another beaker. You can then carefully "pour" the  CO₂ from the first beaker into the second. When the second beaker fills up with  CO₂ the candle will be extinguished.

Both good demonstrations of the fluid form of gasses and the concept of differing densities of gasses.

[hmx9123]

It should be mentioned that thermite and sodium in water can be dangerous if not handled correctly.  The former has a great fire and moderate explosion risk, and the latter has an explosion risk.

Thermite should never be dropped into water--it can cause an explosion.  It should not be put out with a CO2 extinguisher, either--use a lot of sand to smother it and let it burn itself out if it gets out of control.

The sodium in water can not only explode, but also throw sodium hydroide from the water into kids' eyes, which is hazardous.

The sulfuric acid/sugar stinks a lot if you don't have a fume hood.

Don't mean to be a party-pooper, but no one has mentioned the safety aspects of these experiments, and 8th grade rooms generally don't have fume hoods.

[Jacob]

I've Demonstrated flame colours in a whole school assembly using a solid fuel and solutions of metal-chloride salts.

For the solid fuel I simply produced it before their eyes using a saturated solution of calcium acetate, to which I 'squirted' ethanol using a squezzy bottle. There should be about 50cm³ of calcium acetate in a 200cm³ beaker, the ethanol is just added until a solid precipitate is formed, its around 75cm³ to 100cm³. I produced 3 sets of fuel which I laid out on heatproof mats beforehand then produced a fourth in front of the students.

Then I used solutions of metal chloride salts, strontium chloride, potassium chloride & Barium chloride are all eyecatching. The solutions are simply one or two spatulas of the salt in 100cm³ of water in a small spray bottle.

Its very effective and works well with large numbers as i said i did it twice in assembly once with years 10 & 11 then again with years 7, 8 & 9 the higher years seemed to appreciate it more!