I agree about what ur saying.. funny also lolya cause naruto is soo scientific lmfao cause ya know human anatomy totally allows people to summon black fire with their eyes, go into sage mode, use jutsu and remove poison with only chakra, ya i must have missed those classes in schools. my theory comes from the idea that u heat up water enough like say if lava flows into a lake the water becomes acidic so basically by mixing water and fire u get acid. and chakra u said it might be steam...last time i checked steam isnt a liquid but a gas and her jutsu looked like it was liquid eating away at the wall while steam would have just left a whole in the wall rather than eating away at it
Looking at the pic of her Yoton no Jutsu, it seams to look like something extremely hot or acidly.. It also looks like something NOT-water and NOT-gas but something more creamy like said before "lava" the body of the fluid is either way able to eat up a wall.. perhaps it's something liquidish water combined with Mizukage's chakra or something new invented material by Kishi..
We're yet to tell..