Subject: why does Calcium Carbonate CaCO3 LOWER the boiling point of filtered water?

I did the experiment with increasing salt concentrations {NaCl} and boiling 
point increased until 110 gms in 250ml of filtered water.  Then temp was same 
as 100 gm salt.  I got the same results (incr temp boiling point) with NH4Cl.
(another salt electrolyte)
Then I tried CaCO3 (10gm and 20gm in 250ml filtered water) which didn't 
dissolve too well AND the Boiling point was lower than my filtered water only 
(4 control points ) WHY DOES CaCO3 LOWER the boiling point over filtered plain 
water?
My sugar solution was the same boiling point as the filtered water boiling 
point. I expected CaCO3 to act this same way?
 I am using an chemistry thermometer bought at a science store. .1 degree 
accuracy in my range of temps
I live in Denver, so elevation is 5280ft and it's May. Thank you for taking the 
time to answer if you can.
Christine Stewart

Re: why does Calcium Carbonate CaCO3 LOWER the boiling point of filtered water?