Burgia, you should be checking that conversion is complete before you start taking gravity readings. A lot of people will do this by pulling out a tsp. of the mash (liquid only) then using a drop or two of iodine in it. If it changes color (blue, black,purple) then it is not fully converted. There shouldn't be any change of color in the iodine if it has converted. If conversion isn't complete then you should mash longer. As for the specific question you asked it sounds logical that starch is heavier than water but I couldn't really answer one way another.