Ionisation

[Snorlax]

I'm having difficulty in understanding Ionisation.
Is water always reactant? (does it mean in ionisation, there will always be a hydronium ion? - hydrolysis?)

Some help with explaining this would be great, thanks.

[lzxnl]

What sort of ionisation? If you mean ionising in water, then strictly speaking water is a necessary part of the reaction. The ionising of hydrogen chloride, for instance, requires water and if you have a HCl solution and the water is boiled off, the HCl will escape as a gas. We just don't tend to write water as part of the equation.

[Snorlax]

Polar covalent molecular compounds.
So in these, a hydronium ion is always produced?
Also the ions are always hydrated when this ionisation occurs?
I guess i'm asking this because i was confused on the differences between ionisation and dissociation, more so on what ionisation is.

[lzxnl]

Ok, not all polar covalent molecular compounds will ionise to form H+. As an example, CHF3 is polar covalent, but that thing has an acidity constant on the order of 10^-25. It really won't dissociate.

Also, polar molecular compounds that DO ionise can ionise differently. For starters, ammonia ionises to form OH-.
If you mean the ionisation of acids to form H+ (yes you'll have to spell that out for the examiners) then the ions are hydrated when this ionisation occurs, yes. Bonding with water is one of the major factors that stops the ions from rejoining each other.
You'll have to look at the compound in question. Generally, acidic properties of the compound will be mentioned by the name "something acid" or a Ka value.

Ionisation has to form ions. Dissociation does not. Dissociation just means when one molecule breaks up into smaller parts. Think carbonic acid breaking up to form H2O and CO2. That's dissociation, but not ionisation. Dissociation CAN involve ionisation.