Help us settle this chemistry dispute...

[prairiewindmomma]

Today, after finishing cleaning the bathrooms (note the irony in the question that follows), ds took a chemistry exam.  In the exam, the question is written as a true/false.  "The presence of calcium or magnesium ions in water makes it 'hard'."  Ds posits that the answer is false, because it requires both calcium and magnesium to make it hard.  The test answer key and I believe the answer is true as it isn't the interaction of the ions together that make it "hard". 

That said, in doing research, I came across this today: https://water.usgs.gov/edu/hardness.html  The USGS notes that the simple definition is calcium and magnesium.  

I'd ask on Facebook, but I have good friends who work at USGS and I'm going to take a ribbing if I am wrong. 

[Dicentra]

Based on the wording of the question, the answer would have to be "True". 🙂  You don't need both calcium and magnesium ions to make water hard - in fact, you can have ions other than calcium or magnesium present that would also make water hard.  You're correct - it isn't any kind of interaction between calcium and magnesium ions that defines "hardness".  Quite often, both calcium and magnesium ions will be present together in a hard water sample because of the geology of the aquifer that the water is coming from (the parent geological formations that would produce hard water would often contain both calcium containing minerals and magnesium containing minerals).  You wouldn't need both to be present for the water to be considered hard, though.

Hope that helps!

[prairiewindmomma]

Thank you, Dicentra.