In this experiment, we aim to investigate the effect of sodium carbonate on hard water.

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Jakub Figurski, 4A

GCSE CHEMISTRY COURSEWORK: INVESTIGATING HARD WATER

1. PLANNING

Aim

        In this experiment, we aim to investigate the effect of sodium carbonate on hard water.

Method

        Firstly, 25.00cm3 of a sample of water was pipetted into a clean conical flask.  The pipette was used because this measures and delivers a very accurate amount of liquid (to the nearest hundredth of a cm3, or 0.01cm3), but care had to taken to make sure it was not held by the bulge, as this could have made the amount inaccurate (as body temperature heats the apparatus up and makes it expand).  Also, the pipette is used to avoid spillage, as the thin tube can put the water straight into the appropriate container.  Furthermore, a pipette filler was used for safety purposes, because blowing or sucking could get unwanted and potentially dangerous substances into the mouth. 0.500g of sodium carbonate (a white powder) was added to the water, having been carefully weighed on an electronic balance — this was very exact and had a cover to maximise accuracy (it protected the balance from being affected by wind or other particles).  The balance measured the mass of the powder to the nearest thousandth of a gram (i.e. 0.001g).  Then a burette on a stand was obtained, a considerable amount of soap solution funnelled into it through the top (so the level reached near the top) and 1cm3 of the solution added to the water.  The burette (glass tube with measurements marked on the side and a stopcock at the bottom) was used because it very accurately lets out 1cm3 every time.  The conical flask was then corked (so no substances were able to escape through the top) and shaken for ten seconds (this timed by a stop-watch).  It was observed whether a permanent lather (one that lasted for more than ten seconds — again a stop-watch was used) was created.  If so, the result of 1cm3 being added was recorded.  If not, then the process of adding a cm3 of soap solution and shaking the flask was repeated, as many times as necessary to produce a permanent lather, and the exact amount of solution needed was noted down.  The flask was washed out carefully, once with tap water and duly with distilled water (so that the apparatus was thoroughly cleaned, avoiding any contamination).  The experiment was promptly repeated all over again, but this time with other amounts of sodium carbonate (1.000g, 1.500g, 2.000g, 2.500g), and the results recorded also in a table (using a tally chart).  Afterwards, the different results of the tests were compared to make out any trends.

Diagrams

Prediction

        I predict that the more sodium carbonate is added to the water, the less soap is needed to create a lather. Eventually, though, the powder will lose its effectiveness, due to the fact that all the calcium ions will have already been removed.  So these are the results I would expect to obtain, in graph form:

(N.B. The amount of sodium carbonate is on the x-axis because it was the quantity which we controlled, but the amount of soap solution needed was the quantity which we were investigating, and was therefore beyond our control.)

Theory

        Now, the explanation of the theory behind the experiment.  Soaps are compounds of sodium or potassium.  A common soap, and the one used here, is sodium stearate (Na+St- where St- = C17H35COO-).  Sodium stearate is soluble in water, but calcium stearate is not.  Hard water contains dissolved calcium ions — Ca2+(aq)  — which cause the water to form ‘scum’, a solid white precipitate.  Hard water also makes soap very difficult to lather, again because of the calcium ions.  

So when soap is mixed with hard water, this is what happens:

sodium stearate + calcium hydrogencarbonate    

                                               (soap)                                (in hard water)

 calcium stearate + sodium hydrogencarbonate

                                                      (scum)

2NaSt(aq) + Ca(HCO3)2(aq)  CaSt2(s) + 2NaHCO3(aq)

       The ionic equation for this reaction is:

Ca2+(aq) + 2St-(aq)  CaSt2(s)

(N.B. Only ions which make the scum are shown, not the ‘spectator’ ions, such as the ones from 2NaHCO3(aq).  Two stearate ions are needed to remove one calcium ion.)

       Instead of calcium hydrogencarbonate, calcium sulphate (CaSO4) could be written in the equation:

2NaSt + CaSO4  CaSt2 + Na2SO4

                                                                   (from soap)(from hard water)(scum)(left in solution)

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       Therefore, in the experiment, this is what happened once the sodium carbonate powder (Na2CO3(s)) was added:

CaSO4 + Na2CO3  CaCO3 + Na2SO4  

We can see that this is a double-decomposition reaction, with the sodium ‘winning’ the sulphate away from the calcium because it is higher in the reactivity series of elements.  So the sodium carbonate effectively lessens the effect of calcium ions in the water, thus softening it and making it easier for a lather to be formed.  Therefore, all this must mean that the more sodium carbonate is used, the more calcium ions are removed, the softer the ...

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