I am planning to investigate plasmolysis in beetroot cells and need two substances to compare in order to perform a t-test. I was thinking of NaCl and CaCl₂ but this is too easy as the tri-ion CaCl₂ will have a greater attraction for the water and thus increase plasmolysis.

Yes, that sounds "easy", but do both of those compounds ionise to the same extent? Are any other factors coming into play such as carrier molecules in the membranes responding to Ca and Na differently? Biological systems are rarely as simple as we might think.

You can certainly make a prediction, based on sound chemical principles. The real question is whether the material behaves exactly as you have predicted - ie a) do you get a significant difference between Na and Ca chlorides and b) is it in the predicted pattern?

You could include a non-ionic substance like sucrose and compare the effect of similar molarities of NaCl and sucrose to find which molarity causes "incipient plasmolysis" (50% plasmolysis).

Often, the way round a situation like this is to conduct a "trial experiment" to have a quick look and see whether both substances cause plasmolysis and whether the pattern is approximately as you predict. (Don't forget to allow for water of crystalisation in your calculations of molarity.)

John Hewitson