Final answer:
The pairing of cations with nitrate and anions with sodium is based on solubility guidelines, as nitrates and sodium salts are generally soluble, allowing for the prediction of precipitation reactions in chemistry.
Step-by-step explanation:
The reason why all the cations are paired with nitrate and all the anions are paired with sodium has to do with the solubility guidelines in chemistry. All nitrate salts are soluble because the nitrate ion is a large, singly-charged anion that forms highly soluble salts with various cations. Similarly, sodium ions, which are singly charged cations, typically form soluble salts with various anions. This general rule helps predict when a precipitation reaction will occur. Other ions like chlorides, bromides, and iodides are also generally soluble, with a few exceptions for ions like Ag+, Hg2+, and Pb2+. Therefore, when conducting a double-replacement reaction, pairing nitrates with cations and sodium with anions often results in soluble compounds, unless one of the products is an exception to the solubility rules.