The Combination with water is also pretty nice:
HCl (Hydrochloric Acid - Very dangerous) NaOH (Caustic Soda - Also very dangerous)
Combine to literal Saltwater (H2O and NaCl).
Interestingly, because the reaction goes both ways, in any amount of salt water, there will be a miniscule amount of Hydrochloric Acid and Caustic Soda. The concentration is so low because they immediately react again, going back to water and salt.
I’m currently in the process of trying to 100% Cat Quest, since I already have all achievements on Cat Quest 2, before Part 3 comes out this august. Very much enjoyed the first two games, so I recommend as well :)