What Is the White Layer on Water?

[mathsTKK]

I met a customer who face a problem in the water quality in the house. The water supply is clean and clear but when it is boiled or cooked, a white oily layer floats on top of the water. When the water is left cooled, the white layer hardens and sticks on the wall of the pot and container. The customer claims that a sand filter has been used to filter the water but the white layer still appear. Currently, I predict that's actually a layer of oil and I can use a paper layer filter to remove the oil. However, I wonder whether that's really oil layer or something else. Can someone tell me what's actually the white layer in the water? and what's the solution to the problem?

Thank you very much^^

 

[russ_watters]

Probably just calcium or "hardness". http://en.wikipedia.org/wiki/Hard_water

 

[mathsTKK]

Thank you russ_watters^^ But can I ask why the white layer only appear when the water is heated? I still don't understand why this situation occurs...

Anyway, thank you once again and I hope for more replies soon^^

 

[NobodySpecial]

In solution it's Calcium bicarbonate, when you heat it the reaction forms calcium carbonate (limescale)
Ca(HCO3)2(aq) → CO2(g) + H2O(l) + CaCO3(s)

It means your water tastes nice and will have lots of useful minerals in it - at the cost of using a bit more soap and needing a new kettle every few years.