A blue Cu (II) cation salt solution was mixed with a green Ni (II) cation salt solution and an aqua colored solution resulted. Is this a chemical change or a physical change?
This is a physical change or a chemical change depending on the concrete example. For example, we can take Copper(II) sulfate - CuSO4 (gives a bright blue solution) and Nickel(II) nitrate – Ni(NO3)2 (gives a green solution). The mix of these salts in solution gives another color – not bright blue and another shade of green. But no chemical reactions occur. Ni(NO3)2 + CuSO4 -> Ni(NO3)2 + CuSO4 CuSO4 is still CuSO4, Ni(NO3)2 is still Ni(NO3)2. There is no reaction between soluble salts possible if there is no gas or solid as a product. But when it comes to another case for instance Copper(II) sulfate - CuSO4 (gives a bright blue solution) and Nickel(II) carbonate – NiCO3 (gives a green solution). The mix of solutions gives another color and this is the result of chemical change, because insoluble CuCO3 is formed. CuSO4 + NiCO3 -> NiSO4 + CuCO3↓