Investigating how the concentration of a the metal ion affects the hardness of water.

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Investigating how the concentration of a the metal ion affects the hardness of water

Introduction

Causes of hard water

        Almost 60% of people live in hard water areas. As the rain falls, it reacts with the carbon dioxide in the air to form carbonic acid (1). The rain then falls into streams and as it passes through rocks and soil, the carbonic reacts with minerals. One example of this is limestone; the carbonic acid reacts with calcium carbonate to form calcium hydrogen carbonate (2). This ionic compound in water, separates into calcium ions and hydrogen carbonate ions(3). These calcium ions are what make water hard. Another rock, which causes hard water, is dolerite. It is a fine-grained igneous rock.

1) H2O   +  CO2                 H2CO3

    Water +Carbon                 Carbonic acid

                 Dioxide

2) H2CO3(aq)    +  CaCO3(s)                      Ca(HCO3)2 (aq)

Carbonic acid + limestone                Calcium hydrogen carbonate

3)  Ca(HCO3)2 (aq)                 Ca2+(aq)           + 2HCO3-(aq)

Calcium hydrogen                 calcium ions + hydrogen carbonate ions

Carbonate  

Soap

        One way of testing for hard water is to see its reaction to soap. Soft water will form a lather, but hard water will form a scum, with solid pieces floating around. Soap is sodium stearate, and all sodium compounds dissolve into water. Soap has two parts, a head and a tail. The tail is said to be hydrophobic (water hating). The head on the other hand is hydrophilic (water loving). The tail of the soap, attaches itself to the dirt, as it is trying to avoid the water, while on the other hand the head is staying in the water. The dirt is then pulled off the object as it is rinsed again. But, when there are calcium or magnesium ions, then the calcium or magnesium reacts with the soap to form calcium stearate or magnesium stearate. This solid is scum.

Ca2+(aq)   + 2St-(aq)                  CaSt2(s)

Calcium + Stearate        Calcium Stearate (scum)

Mg2+(aq)          +     2St-(aq)            MgSt2(s)

Magnesium + Stearate         Magnesium Stearate (scum)

The chemical formula for soap is:

Prediction

        For my preliminary experiment, I needed to find out which metal ions form a lather with soap solution and which metal ions form scum. Approximately 2cm3 of the metal ion was put into a test tube and approximately 2cm3 of soap solution was added. A bung was put into the top of the test tube and was shaken. The results were as follows:

        This experiment proves that magnesium and calcium ions cause hard water. Also, on the next page there are two maps, 1 showing where limestone is present in Britain and the other showing where hard water is present in Britain. There is a definite relationship between the areas with limestone (CaCO3) and hard water. Therefore form this I can conclude that calcium causes hard water. I have chosen Calcium as the metal ion in my experiment as I know it causes hard water.

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As this experiment is testing the concentration of the metal ions, I think that as the concentration of the calcium ions increases so will the amount of soap solution needed to form a lather. I think this because when the number of calcium molecules increase, the amount of stearate in the soap needed to react with it will also increase.

        

Also, I predict that as concentration is doubled, so is the amount of soap solution needed to form a lather. The equation for the reaction between stearate and calcium is as follows:

Calcium + Stearate     ...

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