For a closed system, you need two things:
1) a conservation of mass within the boundaries of the system
2) the ability to freely exchange energy to & from the "closed" system with a surrounding external system
So, the answer is <u><em>never</em></u>, since your defining the "system" as the water within the bathtub, and an open bathtub is exposed to evaporation, which is not conserving mass within the defined "system".
You can use dimensional analysis:
Colligative properties are properties of solutions that result from adding solute to a solvent and that depend on the concentration of solute particles, but independent of the identity of the solute molecules or ions. The four colligative properties are boiling point elevation, freezing point depression, osmotic pressure, and vapor pressure lowering.
Explanation:
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