I saw a video about slant bending styles that tried to do chemical science to bending, and while I think this is an understandable reaction to what we know of bending in the Avatar universe... I think it's the wrong approach entirely to understand questions like, "why can't earthbenders bend ice?"
Ice, after all, IS a rock, from a chemical perspective. Water, from a chemical perspective, is a form of lava, just with a much lower melting point and more abrupt transition point than we're used to. But that's not what matters in bending. Remember the words of the gurus Hu and Patik: The separation of the elements is an illusion.
Now, it's an illusion so powerful that it is functionally a physical law, one that only powerful spirits can alter, but what matters is not that the avatar can break the rules, or that most benders can't, or even the implication that maybe, just maybe, you could get a two-element bender if their chi was really weird.
What matters here is that figuring out if a kind of bending is possible isn't about chemistry, it's about spirituality. And that's a weird thing to say about the physical world, at least if you're from the cultural context that gave us Descartes.
To the people on the Earth of Avatar though? The definition of dirt is spiritual.
And clearly chemistry exists in the avatar universe, they have explosives after all, but bending isn't a physical science, exactly. It's more like an extension of the spirituality of Avatar: the body itself has a spiritual dimension, and so do the objects in it.
Aside: benders losing their bending if they project into the spirit world is a really interesting question and the fact that Zaheer unlocked permanent levitation is probably related to why he was so powerful in the spirit world as to be able to just teleport around are probably related, but that's an entire other can of worms.
So why can't earthbenders bend ice? Because it's ice. Water ISN'T Earth, not in a chemical sense (that chemistry can detect this illusion is WILD if you think about it) but in the sense that the illusion of separation is very, very sturdy.
How sturdy? So sturdy it's functionally a physical law. And I think that's the hard problem with thinking about bending in the Avatar universe: there are illusions at play, and those illusions are strong enough that nobody, not the avatar, not the great gurus, not even the great spirits of light and dark can truly, completely step outside of them. (The avatar can SOMETIMES come close though.)
If we start from the view that the separations are illusions, just so powerful no one can break them, everything about slant bending immediately makes perfect sense. Only benders who are able to connect powerfully with their element can do any slant bending at all, and the slant bending they achieve is constrained by their connection to that element. The powerful illusion of separation of the elements can, with great spiritual connection, be bent, just not broken.
So you get earth benders who are able to bend earth into lava, fire benders who can channel lightning, water benders who can work on water within plant life, and air benders who can connect to spirit energy with a sensitivity that comes from the relationship air has with spirit. (By the way, Guru Patik literally spelled this out with jump cuts to Toph inventing metalbending.)
Aside 2: All forms of "violation bending" IE bloodbending, ripping air out of someone's lungs, bonebending, and presumably some horrific form of firebending-based life-draining are difficult because you're overcoming someone else's intrinsic spiritual will. That's why. That's also why they're horrible.
In every case, it's not at all about what physical science tells us about the elements of the periodic table, but what spiritual discipline tells us about the elements of spiritual physicality. And the elements of spiritual physicality are, on some level, an illusion, but in a world where the spiritual energy of a person can reach out and throw rocks, illusions aren't that different from physical laws.
When we consider the design of powers in Avatar, along with its lore and the changes that will happen, we need to think along these lines, not along lines of relational logic. Spirits defy relational logic, and often, so does bending itself.
