0live has it pretty much covered.
The other advantage to scratching the rock is that not only are you creating more surface area for a reaction, but you also may be able to penetrate a weathered 'rind' on the rock, which could be resistant to the reaction.
And adding an acid (acetic from vinegar, but stronger like HCl/muriatic is better) reacts and releases CO2, which is what causes the fizzing.
Also, not an issue for most people, but if you happen to be collecting your rocks in the winter, sometimes they can be cold enough to not get a noticeable reaction in the field, warming the rocks and the acid can also help get a better reading.
And for soaking in a bucket, make sure you get a baseline reading after letting the water set for a day or two before you put the rock in (it will probably change from what it is straight out of the tap).