Answer this first: Are covalent compounds soluble in water?
Then answer these:
Why then, is HCl gas able to dissolve in water to form
hydrochloric acid?
SO3 able to form sulphuric acid when dissolved in water?
Ammonia able to dissolve in water to form the ever-common
aqueous ammonia?
Chlorine gas and ammonia gas not being able to be collected by
downward displacement of water?
Are these just exceptions in chemistry concepts?
Next, titration.
Which do you put, acid or alkali, into the burette and
pipette(which ends up in the conical flask)
What colour does indictor phenolphthalein turn in neutral
solutions? I know it turns pink in alkalis, and colourless in
acids.
Third, period III compounds (or is it aluminium chloride
only?).
Aluminium chloride is an ionic compound when solid. But then,
why does it turn into a covalent compound when it's a gas? Or have
I ran into an unreliable source which gives false info?
Does it apply to period III chlorides only?
Fourth, sublimation.
In the 'O' Level syllabus, which substances that sublime are we
required to know?
Ammonium XXX (chloride?)
Carbon dioxide
Iodine
Anhydrous aluminium chloride
Anhydrous iron (III) chloride
Right, this doesn't stop here. I'll need a lot of help on these
things
Thanks for answering my doubts. I appreciate it :D
And hey, I'll add more when I think of somemore.
>>> Are covalent compounds soluble in water?
<<<
Some are, some are not. Case by case basis. Even those that are
soluble, may be soluble for different reasons (eg. hydrogen
bonding, ion-dipole interactions, etc).
>>> Are these just exceptions in chemistry concepts?
<<<
No, these are not "exceptions" in the sense that they are
anomalies of nature or the laws of Chemistry; rather, the
problem (why they seem to be so) lies only in that 'O' level
Chemistry (like 'O' level any subject) is much oversimplified to
general principles that are really for the sake
of convenience and simplification. It's not that these
principles or concepts are wrong or flawed, just that they do not
(by deliberate oversimplification design) fully encompass the
proper mechanisms at a level of any significant depth, required to
fully understand various phenomena in Chemistry, such as the ones
brought up by Garrick.
An illustration of this would be the H-Cl example (see below).
This is already a simplified explanation, but the fact
remains that 'O' level students are simply not equipped (ie. based
on what their schools taught them) to understand the explanation to
any satisfactory depth.
>>> Why then, is HCl gas able to dissolve in water to
form hydrochloric acid? <<<
Because HCl ionizes (ie. electron transfer, bond pair on H-Cl
becomes lone pair on Cl, forming H+ and Cl- ions) to form H+ and
Cl- (and this occurs only in water because of reasons indicated at
the end of this paragraph), and it is these ions, not HCl itself,
that is soluble in water, due to favourable ion-cipole interactions
and hence theromdynamic stability in the various states or
phases (considering both enthalpy and entropy in the solution
process, which is the sum of lattice dissociation process and
solvation process).
So Garrick, the bottomline is, endure just a bit more, you'll
understand all of these questions you've asked today, when you
study 'A' level Chem in just a couple more months time. For next
week's exam paper, simply work with what you're taught at the
simplified 'O' level stage, it will suffice for the paper
itself.
>>> Aluminium chloride is an ionic compound when solid.
But then, why does it turn into a covalent compound when it's a
gas? Or have I ran into an unreliable source which gives false
info? <<<
Even at lower temperatures, aluminium chloride is largely a
(polar) covalent compound, with some ionic character (you'd be
penalized if you say its "ionic with covalent character", because
it's the other way around in the case of aluminium
chloride). At lower temperature, its structure is similar to
an ionic lattice solid (even though it it still not considered an
ionic compound).
At higher temperatures approximately 180 - 190°C (depending
on the pressure), it exists as a dimer, Al2Cl6. At such
temperature, it exists as a simple covalent molecular compound,
with dative bonds donated by chlorine to the electron deficient Al
(to achieve a stable octet).
At even higher temperatures, it breaks up into individual AlCl3
molecules.
Solid aluminium chloride doesn't conduct electricity at room
temperature because the ions aren't free to move. Molten aluminium
chloride (only possible at increased pressures) doesn't conduct
electricity because there aren't any ions any more.
All of these are not expected to be understood or learnt by the
'O' level candidate. I've given this info only because Garrick
asked about this compound.
Bottomline : just endure a couple more months, you'll soon get
to enjoy understanding all these at a deeper, more accurate, and
more truthful level, when you study these in the JCs or in the
Polytechnics.
>>> In the 'O' Level syllabus, which substances that
sublime are we required to know? <<<
Pretty much those which you listed. Read the data given in your
exam question carefully. The data (eg. melting point, boiling
point, solubility, conductivity, etc) will hint to you as to the
nature of the substance, even if it's something seemingly 'new'
that you've not come across in your course of study. This is called
"examining the candidates' ability to apply what he/she has learn
to new situations/scenarios, new compounds, new problems". Every
exam, including the 'O' levels and 'A' levels, will have such
questions.
Enjoy! 
