How Hard Is This Water?

How Hard Is This Water?

How Hard is This Water?

Have you ever had difficulty lathering soap or find that the scum in your shower constantly needs to be removed? These are signs of “hard water”. Soap doesn’t lather as well in hard water because metal ions, such as Ca2+, form precipitates, creating soap scum. Because this happens, soap is a less effective cleanser in hard water. Even laundry can appear dingy or feel rough when washed in hard water.

Hard water can contain various metal ions, including Ca2+, Mg2+, and Fe3+. In order to fully determine the hardness of the water, each ion must be isolated. When a particular ion species needs to be isolated it is possible to use the tendency of the ion to form an insoluble compound by a precipitation reaction. The quantity of ion in the mixture can be determined by a process called Gravimetric Analysis whereby the precipitate is filtered, rinsed, dried and massed. From the mass and the known composition of the precipitate, the amount of the ion in the original solution can be determined stoichiometrically. When done carefully, gravimetric analysis can yield very precise results.

An important consideration in Gravimetric Analysis is that the ion must be completely precipitated. This can be accomplished by ensuring that the ion is the limiting reactant in the precipitation reaction.

In this activity, you will analyze 6 samples of water from different sources for their degree of water hardness through the principles of metal ion precipitation and separation. The samples will then be ranked in order of increasing water hardness. The samples have been boiled to concentrate metal ions. Since water hardness measures calcium carbonate in mg/L, saying that a sample of water is 60mg/L calcium carbonate means that there are 60g in 1x106 g of solution(this is also sometimes called ppm). The volume of the water has been reduced 200 times. That means that 0.24g calcium carbonate can be collected from a 20mL sample.

You must design your own procedure for quantifying the calcium content, and thus, the hardness of the water. You may, but do not have to, use the equipment listed below. If you want to use something not on the list, you must check with your instructor to see if it is available and safe.

Beakers, graduated cylinders, drying oven, sodium carbonate,

vacuumfiltration apparatus, distilled water, deionized water,

metal scoop, watch glass, analytical balance, calcium chloride, filter paper

After devising a procedure, collect your data and rank the samples in order of water hardness. Write a partial lab report, focusing on the materials and methods, calculations, and conclusions sections.