Final answer:
According to solubility rules, RbNO3 or rubidium nitrate is soluble in water, as all nitrates are soluble in water. Other compounds listed such as Cu(OH)2, PbSO4, and Ca3(PO4)2 are generally not soluble in water based on these rules.
Step-by-step explanation:
To determine which of the substances listed is soluble in water, we need to refer to solubility rules. According to these rules:
- All nitrates are soluble, meaning compounds containing the nitrate ion (NO3) will dissolve in water.
- Most sulfate salts are soluble, but there are important exceptions such as PbSO4, which is not soluble.
- Rubidium nitrate (RbNO3), as a nitrate, is expected to be soluble in water.
- Most hydroxides are slightly soluble or insoluble, with exceptions for Group I elements, making Cu(OH)2 generally insoluble in water.
- Lead sulfate (PbSO4) is one of the sulfates that is not soluble in water.
- Calcium phosphate (Ca3(PO4)2) is generally insoluble in water as most phosphates are.
From these solubility rules, we can conclude that RbNO3 is the compound that is soluble in water.