This came up in a game the other day and I need a ruling. The PCs were fighting two living statues (animated objects) each with hardness 5. The wizard cast acid arrow (or acid splash, I don't remember). Since the total damage was less than 5, the ruling was that the hardness negated any damage dealt. A couple people argued that energy attacks and/or spells aren't subject to the rules for hardness, and are only reduced/negated by energy resistance. I could see that argument, but after extensive searching, I still can't find any reference to this rule in the core rulebook. Can someone help with this ruling? And please, give me the page number that states that energy attacks ignore hardness.

I believe I read somewhere that hardness is about similar as damage resistance, so I would treat it that way.

Edit: Found this on SRD:

Hmmm. I wonder what happens if an animated object wears adamantine armor.

Hardness applies to all damage, energy damage included. Damage reduction only applies to physical damage.

Hmmm. I wonder what happens if an animated object wears adamantine armor.

The damage reduction from the adamantine armor reduces damage, then the hardness reduces it more. Hardness and damage reduction are two separate things, and so each would apply.

According to Page 173 of the core rulebook: "Energy Attacks: Energy attacks deal half damage to most objects. Divide the damage by 2 before applying the object’s hardness. Some energy types might be particularly effective against certain objects, subject to GM discretion."
I know the old rules had Acid and sonic only had to get through the Hardness(in your example the damage would have been reduced by 5), Fire and Lightning had to get through twice hardness(The 5 Hardness would have effectively been 10) while Cold had to get through four times hardness(effectively turning 5 Hardness into 20 resistance).

Some energy types might be particularly effective against certain objects, subject to GM discretion.

Because of this sentence, I would rule that acid does full damage against stone objects, but as per RAW it's up to GM discretion.

I know the old rules had Acid and sonic only had to get through the Hardness(in your example the damage would have been reduced by 5), Fire and Lightning had to get through twice hardness(The 5 Hardness would have effectively been 10) while Cold had to get through four times hardness(effectively turning 5 Hardness into 20 resistance).

In 3.5, hardness was not multiplied. Damage was divided by 2 (fire and electricity) or 4 (cold). Acid and sonic weren't divided, but hardness still applied.

40 points of cold damage vs hardness 5
Your method: hardness effectively becomes 20, 40 damage-20=20 cold damage getting through
Actual 3.5 method: 40 cold damage/4 = 10 cold damage -5 hardness = 5 damage getting through.

Your method only works if you both multiply the hardness, and then divide the total. Though I don't know why anyone would bother to add an extra step to get the same total.

So, not only were your players wrong in this case, but they'll find out that it's more difficult to damage objects with energy attacks (per GM discretion).

Ouch.