frothy stuff

Catalysis helps a lot of "oooh" demonstrations, cos you don't have to wait around for things to happen. We can buy 27% hydrogen peroxide here and it is good to use for demos as it breaks down readily releasing oxygen gas. I like to mix it with some detergent ( dishwashing liquid) and then to catalyse the decomposition with some solid KI. I carry the reaction out in a tall thin glass vessel- sometimes throw in a bit of Universal Indicator so you get more than the faint brown colour of iodine ( some Iodide ions get oxidized) The way you get a thin column of froth oozing out of the container amuses the kids. It looks harmless but be careful as peroxide can damage skin eyes etc. Kapai!

If a goat is tethered to the side of a circular field of radius 50.0 metres, how long should the rope be so that he can graze on exactly half the field?

Let R = the radius of the rope and the point O = the centre of the field
let A & B be the points where the circle extended by the rope meets the edge of the field
let the angle OCA = x radians

The area required is sector of radius R and angle 2x + the two segments from the chords AC & BC of equal area.

The area of the sector is given by 1/2Rsquaredtimes 2x = Rsquared times x

the area of the two segments is 2[ 1/2X2500(pi-2x) - 1/22500sin9pi-2x)]

this transforms to 2500( pi-2x-sin2x )

From geo R = 2X50Xcosx
so R squaredX x is 4 X 2500 X xcos2x

Adding together we get 10,000X x cos2x + (pi-2x - sin 2x ) = 1/2 pi

transforms to cos2 x + 1/4x(1/2pi-sin2x) -1/2 = 0

solve for x in terms of some form of approximation (Newton etal)

Are Rabbits clever?

One day a fox bumps into a rabbit and says " Hello little rabbit, what are you up to?"
" I'm studying about how rabbits eat foxes"
" Come on, you know that is impossible"
" Well, follow me and I'll show you" said the rabbit.
They both go into the rabbits dwelling and after a while the rabbit comes out with a satisfied smile.
Along comes a wolf. " Hello little rabbit, what are you up to?"
" I'm studying how rabbits eat wolves"
" Are you crazy?"
" Come with me and I'll show you."
As before , the rabbit comes out with a satisfied grin.

I'm sure you've guessed- inside the rabbit's dwelling, there is a huge big lion with the carcass of a wolf and a fox beside it.

The moral of this story is- its not the quality of what you learn, its the teacher that counts!!

Golden Rain

I guess most chemists know that the lead halides are sparingly soluble salts which are reasonably soluble in hot water. Lead iodide is a startlingly yellow precipitate when lead nitrate is mixed with potassium iodide. Usually I give the students ( 13 y.o. level) sample bottles of 0.1 mol/L of the two salts, a 10mL and a 100mL measuring cylinder, two beakers, one with distilled water and the other with ordinary tap water and a 250 mL conical flask.
I then tell them the instructions will be delivered verbally, so they must listen carefully
" Look at me when I'm talking to you".
Then - " put 5 mL lead nitrate into the 10 mL measuring cylinder and transfer to the conical flask- wash out the 10 mL measuring cylinder first with tap water (fill 3 times and empty) then with the distilled water( fill 4 times and empty) - then add 40 mL of distilled water to the conical flask- then put 5mL of the potassium iodide into the 10 mL measuring cylinder and carefully add to the conical flask".
By now the kids imagine this complicated set of steps is the only way to achieve the bright yellow colour, so they are proud of their achievement.
-then I get them to heat the contents of the flask until it just reaches boiling point- the yellow ppt. will now have disappeared. I let them allow the contents to cool while they write up what they did. Soon the oohs and aahs are heard around the room as the Lead iodide reappears as sparkly little crystals - with the blinds down and the side lights on the contents glitter like golden rain.
They love it


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