Final answer:
The pair of compounds that is soluble in water is NH₄I and Ni(NO₃)₂. This is because nitrates and ammonium compounds are generally soluble in water according to solubility rules. The correct answer is O NH₄I and Ni(NO₃)₂.
Step-by-step explanation:
The question asks which pair of compounds is soluble in water. To determine this, we can refer to solubility rules, which give us a guideline on the general solubility of various ionic compounds in water. According to these rules:
- All nitrates (Ni(NO₃)₂) are soluble in water.
- All compounds of alkali metals (Li+, Na+, K+, Rb+, Cs+) as well as ammonium (NH₄+) are soluble.
- Most chlorides, bromides, and iodides are soluble, with exceptions including those of Ag+, Hg₂²+, and Pb²+.
Given these rules:
- Hg₂Cl₂ and Hg₂Br₂ are not soluble due to the presence of Hg₂²+.
- NiS and Li₂S are sulfides, many of which are not soluble in water unless they are combined with alkali metals or ammonium.
- CuCO₃ includes a carbonate, which is generally only soluble when paired with an alkali metal or NH₄+.
Therefore, the pair that is soluble in water as per the solubility rules is NH₄I (since it contains ammonium) and Ni(NO₃)₂ (since it is a nitrate).