hey... why is no one here like a* material...
what the guys said before me is wrong.
i mean the ionic equations, and what to use for measurements (i havent seen the question u posted in the attachment yet, but when i do, another rant)
to write ionic equations, you first have to understand what they are. basicly all they do is split up every single compound that is reacting into what it really is(eg NaOH(aq)=Na
+ and OH
-) (this is because when something dissolves in water, it splits up into ions)
when you have done that, you must cancel out all "spectator" ions. this means that any ions which appear on both sides of the equation must be crossed out. (remember covalent compounds arent made of ions, and only aqueous compounds can be split up into ions)
eg
NaOH(aq) + H
2SO
4(aq) ----> NaSO
4 (aq) + H
2O (l)
change into ions...
Na
+ + OH
- + 2H
+ + SO
42- ---> Na
+ + SO
42- + H
2O
Now simply cancel out all spectator ions (any that are not taking part in the reaction) to give:
Na+ + OH
- + 2H
+ +
SO42- --->
Na+ +
SO42- + H
2O
and so
2H
+ (aq) + OH
- (aq) ---> H
2O (l)
the state symbols are simply the state symbols of the original reaction (remember, putting in state symbols is one of the best ways to impress the examiner, even if they aren't required!)
ok, now the measuring part.
i assume 25cm
3 is meant for a titration. the best way to accurately measure a fixed volume of liquid is
always to use a volumetric pipette (as in if you need 15cm
3 to add to something, not to measure the volume of for example the reactant produced in a reaction). in igcse at least. a beaker and a measuring cylinder are used only to
estimate the volume of a liquid.
to measure constantly changing volumes of liquid, the best is of course a burette. this for us is mainly in titrations, i doubt you would use it anywhere else. a pipette is not used to ever measure anything-volumetric pipettes are though. you cannot call a volumetric pipette a pipette in your exam, or else you'd probably get it wrong.
some stuff about pipettes:
http://www.chem.yorku.ca/courses/chem1000/equipment/pipette.htmlregular pipettes (aka dropper, dropping pipette (best name), teat pipette) are used for transferring small amounts of liquid, eg indicator.
until now no question in igcse chemistry extended has asked for measurements of a substance, eg volume of liquid obtained from experiment. and they probably wont. if they do, the best way to measure is a measuring cylinder. (remember to try to avoid this as much as possible, and mention how inaccurate it is when you do write up an answer)
hmm... what else... its so sad you guys dont have a good chemistry teacher. you wont be able to do the whole syllabus properly in even a month of constant study without a good teacher. what does a good teacher do? make sure you dont do too much outside the syllabus, and make sure you know everything in the syllabus.
ok whatever bye.
