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I have this booked as a prac for 2 classes thurs and fri, being tuesday is it soo soon to make it?
I thought if i mess it up then i will have a chance at making it again..

(first time doing this prac - done agar plates 100's of times)

I don't think it is too soon to make up the agar. I believe it says in "The Laboratory" that the longer it has to sit sit the colour gets stronger or something to that effect.

I wish the colour would get stronger.. my experience is the opposite and I have been reduced to cheating - as in pouring weak sodium hydroxide solution over the agar until the colour reappears...
Fudging pracs is what we become good at isn't it..
as long as it shows the students what they need to see!

ok, i will make it this morning and see how i go...there is that awkward moment in agar making between pouring it in and it setting....

When you make it, forget the receipe just make sure it is bright red
Then cover with Gladwrap and store in fridge!

The teachers said 'its not a prac exam so dont' worry' put my mind at rest a bit!!!

Do you understand what the prac is all about?
Surface area to Volume ratio
You start with 3 cubes of bright red jelly 3cm, 2 cm & 1cm
drop them in 1 or 2 M HCl depending on time then cut the cubes in half to see how far the acid has penetrated into cube by measuring the bit that's still red

yes i do now, the new grad teacher explained it to me - i am open to learn.

My last lot came out a funny grey/pink, I thought I may of airated in too much by using a whisk so made a second lot using a stirring rod and it came out the same colour but they both still worked fine so dont worry too much MariaC

We tried a different version this year that avoids the problems with getting a consistent colour..........just make up plain agar & add a bit of starch powder. Pour into unitrays with dividers & set (thanks to Ian for that genius idea )
Drop your cubes into dilute Iodine-pot iodide solution, & you get the same basic result only in reverse. The cubes don't decolourize, they turn black instead as the iodine diffuses into the cube & stains the starch.
The reaction doesn't happen quite as fast, so you need to leave the cubes in the iodine for 15 minutes, & when the kids cut the cubes open, they need to blot the cut surfaces so any iodine dragged across the cut by the knife doesn't continue to cause staining, because it can make the zones harder to see.
A recipe & prac method are attached. It's a breeze because you can make up the agar a lot faster. You just boil, dissolve as usual & add the starch straight away, then pour. No waiting for it to cool before adding the NaOH or phenolphthalein & checking all the time in case the colour fades.
No mess no fuss & also less hazardous

very hot pink colour for the first lot.....there was only enough to do the 3cm tray, so making more now.
When i get a chance i will post a photo!

here is one i made earlier...

Do you have access to a microwave?
Being Scottish & Careful all the agar you trim off chop up real fine then shove in microwave oven for about 20 sec stir then back till it's melted you don't want it to boil if you can help it then recast into more blocks
How miserable is that?

We have always cooked up agar and phenolphthalien, then the cubes are put in a dilute sodium hydroxide solution to turn pink.
Very easy, never had any complaints.

Thanks to the wonderful labbies on here!
I made the agar, and the class has done it this afternoon.

I pulled out the 1cm one, and it was dull, so i 'painted' it over the sodium hydroxide again (a trick i learned here!)

and the results were excellent!!!! so i have attached a photo!

i will also not be sad to see the end of this prac!!!

Made some yesterday, should have checked this thread first. It was originally quite pink but has faded considerably. I will try the layer of phenolphthalein on the top trick, if that doesn't work its back to the drawing board.

paint it with Na Oh - that should turn it pink quick!

I love this prac - one of the few Biology pracs that actually works (probably because it relies on Chemistry - which always works!)...spoken by a true chemist........

Oh David, I say that just means Chemistry's not as much fun as Biology coz it's too predictable

I reckon the more organic & less predictable biology pracs are cool....even if they don't always work as expected .....& do Chem classes get to play with anything living? Nup.....but we biologists get to have lessons with stick insects, bearded dragons, bunnies, all sorts of plant life, gorgeous microscopic critters....& we get to make capsicum soup & blow lungs up with a bicycle pump! ....spoken by a true biologist