A splash weapon is a ranged weapon that breaks on impact, splashing or scattering its contents over its target and nearby creatures or objects. These include such classic favorites as hornet's nests and bottles of alchemical fire, which is basically magic napalm and the best friend of people fighting trolls.
To attack a creature with a splash weapon, make a ranged touch attack against the target creature. Thrown splash weapons require no weapon proficiency, so you don’t take the –4 non-proficiency penalty, which makes alchemical splash weapons a great choice for starting spell-casters to use in a pinch. If you hit the creature's touch AC, the splash weapon deals direct hit damage to the target as described in the item. It also bursts messily and inflicts a lower amount of splash damage (half) to all creatures within 5 feet of the space of the targeted creature. Splash damage does not inflict additional damage to the target of the direct hit damage, even if the creature targeted is large enough to be in both the directly-hit square and one or more splashed squares.
Creatures hit with the direct damage of a splash weapon do not get a saving throw to attempt to reduce its damage. However, any creatures affected by the splash damage of a splash weapon automatically take only half damage from the weapon, just as though they had succeeded on a Reflex save for half. No actual saving throw is rolled; they just succeed. However, any creature that has an ability that further reduces damage on a successful reflex save, such as the Evasion class feature, may apply that ability, since they are treated as having succeeded on the saving throw.
If the target creature is sized-large or larger, you choose one of its squares and the splash damage affects creatures within 5 feet of that square. In no cases (except by GM fiat) can splash weapons deal precision-based damage (such as the damage from the rogue’s sneak attack class feature).
Now, if you do not wish to attack a creature, splash weapons allow you to attack an area, instead. You can choose your target to be a specific square. Treat this as a ranged attack against AC 5. When you target an area in this way, you must have some sort of a solid object to break your splash weapon against. In most cases, the ground is just about as solid as you can get, and works fine, but what if you are throwing a flask of alchemical fire up into the branches of a tree to burn up that nasty group of stirges? The branches of a tree constitute difficult terrain that is not on the ground, so does the flask break, or not?
The GM adjudicates all thrown splash weapons that are not targeted at a creature's touch AC or blindingly obvious area targets such as floors or walls. Such attacks are always against difficult, impeded, or blocked squares. Blocked squares are as solid as the ground, and break flasks reliably, unless they are made of things like flesh or rubber. As a rough rule of thumb, impeded squares are usually sufficient to break a thrown splash weapon, and squares of difficult terrain are often good as well. The exact character of the obstacle is left up to the GM. A flask of acid thrown into dense spider webs is unlikely to break, but that same flask thrown into a tangle of melted sword-blades is almost certain to break, while a tangle of thorny brush falls somewhere in between.
If you target a non-creature square (such as the ground) with a splash weapon and it breaks on impact, creatures in the target square or any adjacent squares are dealt only the splash damage (half damage, no save). The direct hit damage is not dealt to ANY creature.
If you miss the target (whether aiming at a creature or a non-creature square), roll 1d8, to determine how the missed splash weapon scatters from its intended target. The d8 roll, called a 'scatter die', moves the target square of the splash weapon by 1 square in a random direction away from its intended target. A roll of a 1 falls short by one square in a straight line toward the thrower, while results of 2 through 8 are counted clockwise, circling around the originally-intended target square, from the 'falls short' point of a 1 result. After you determine where the weapon landed, it deals splash damage to all creatures in that square and in all adjacent squares. Missed splash weapons never deal direct-hit damage, even if there is a creature in the square that the splash weapon actually hit.
In the example to the right, a splash weapon was thrown (by the thrower), and missed its intended target. A d8 scatter die was rolled, with a result of a 2. The second square, counting clockwise from the "falls short" result of 1, is the new target of the splash weapon. All creatures in square 2's square and its adjacent squares take the splash damage from the splash weapon. None of the creatures take the direct-hit damage, since the splash weapon failed to directly hit anything.