Softening hard water with sodium carbonate (Na2CO3)

Sam Fromson 4c

Softening hard water with sodium carbonate (Na2CO3)

Prediction

In a preliminary experiment, we discovered that adding 1g. Na2CO3 to hard water softened it slightly. We tested both tap water and distilled water with the sodium stearate and discovered that tap water was far harder than distilled water, we then tested tap water that 1g of Na2CO3 had been applied to. This solution was softer than the original tap water. The exact results were;

This tells us that Na2CO3 softens hard water. However 1g Na2CO3 did not render it as soft as distilled water. Now our aim is to determine whether increasing the quantity of Na2CO3 added to the hard water softens it more effectively. We are measuring the hardness of the water by the volume of soap solution required to produce a lather. Thus, the less soap solution required to form a lather, the softer the water, and the more successful the Na2CO3.

We know that 1g Na2CO3 did not fully soften the water. My prediction is that as the quantity of Na2CO3 is increased, the water will become softer. Thus requiring less soap to form a lather. Additionally I predict that after a certain point the Na2CO3 will become less effective at softening the water until finally it will not effect the softness of the water if more Na2CO3 were to be added.

Water is made hard by dissolved calcium ions, Ca2+, when placed into a solution Na2CO3 splits up into Na+ ions and CO32- ions, the CO32- ions combine with the dissolved calcium ions to create calcium carbonate, this is a solid precipitate that doesn’t alter the softness of the water. The sodium ions stay dissolved in the water, but they don’t cause water to become hard.

This justifies my prediction because a greater quantity of Na2CO3 added to the water, means an increased number of CO32- ions. The more CO32- ions there are, the greater the amount of calcium that can be removed from the water, thus making the water softer. I can also justify my prediction that after a certain point the Na2CO3 will begin to become less effective, until finally it becomes ineffective, i.e. when all the calcium ions have been removed from the water. Some of the calcium ions form bonds with the stearate ions, this forms scum. This occurs before a lather forms. A lather will only form when all the calcium ions have been removed from the water. Therefore when there won’t enough CO32- to remove all the Ca2+ from the water, scum will have to form in order to remove the last remnants of the Ca2+ from the water in these cases all the stearate ions will be used up in forming the scum and there won’t be enough left to form a lather. When there are fewer Ca2+ ions remaining in the water, due to them being used up to form Ca CO3, then less stearate ions are used up in the formation of scum, and a lather will form more quickly due to the greater abundance of spare stearate ions.

The equations for these reactions are;

Ca2+ + CO32- → CaCO3

In tap in Na2CO3 dissolved in

Water the water

Ca2+ + 2St- → CaSt2

In tap stearate ions scum

Water in soap

Method

Obtain 5 boiling tubes and place 25ml of water into each tube

It must be kept at the same volume of water for each sample of Na2CO3 otherwise it will ...

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The equations for these reactions are;

Ca2+ + CO32- → CaCO3

In tap in Na2CO3 dissolved in

Water the water

Ca2+ + 2St- → CaSt2

In tap stearate ions scum

Water in soap

Method

Obtain 5 boiling tubes and place 25ml of water into each tube

It must be kept at the same volume of water for each sample of Na2CO3 otherwise it will not be a fair test

Place 0.75g Na2CO3 into the first tube, place 1.5g into the 2nd , 2.25g into the 3rd , 3.0g into the 4th and 3.75g into the 5th
Place a bung on each tube and shake each tube 20 times

It is important to ensure that all the tubes are shaken an equal number of times so that in each case the Na2CO3 has an equal chance to dissolve, thus splitting the Na+ ions and CO32- ions.

Fill the burette with Sodium stearate. Add 0.5 cm3 of soap solution (calcium state) from the burette into each boiling tube. Shake each tube 20 times and watch for a lather.

The soap solution must always be made to the same concentration, other wise it will be easier or harder to form a lather with an equal volume of it.
Use a burette because they are more accurate a t dispensing liquids than a beaker or a measuring cylinder.
Do not allow any flames to come near the soap solution because it contains ethanol which is highly flammable, additionally wear safety specs to prevent any splashes going into your eyes.
It is important to ensure that all the tubes are shaken an equal number of times so that in each case the softened water has an equal chance to form a lather with the soap.
The definition of a lather must remain constant throughout the experiment, a lather is of a height of 2 cm. or more and lasts for at least 10 seconds.

Increase the amount of soap solution in each boiling tube by 0.1 cm3 and shake each tube twenty times. Repeat until a lather is formed in each tube and note down how much soap solution was required.
Repeat the experiment a second time

This is in order to get a more accurate final result, this will be done by finding the mean of these two results.

NB. Ensure that temperature does not vary too much while the experiment is being conducted

Results

Analysis

From the preliminary work (see above) we knew that 1g Na2CO3 did not fully soften the water. My prediction was that as the quantity of Na2CO3 is increased, the water would become softer. Thus requiring less soap to form a lather. This was accurate as you can see in the table above detailing the results, the quantity of soap required to form a lather decreased by approximately 0.4cm3 for each additional 0.75g of Na2CO3 that was added to the water. Additionally I predicted that after a certain point the Na2CO3 would become less effective at softening the water until finally it will not effect the softness of the water if more Na2CO3 were to be added. That effect is just starting to be visible towards the end of the results. This can be seen more clearly in my graph of results. I had hoped for a clearer demonstration of this effect however my results were reasonable within the margin of experimental error.

This justified my prediction because a greater quantity of Na2CO3 added to the water, meant an increased number of CO32- ions. The more CO32- ions there were, the greater the amount of calcium that could be removed from the water, thus making the water softer. I can also justify my prediction that after a certain point the Na2CO3 will begin to become less effective, until finally it becomes ineffective, i.e. when all the calcium ions have been removed from the water. Some of the calcium ions formed bonds with the stearate ions, this forms scum. The scum was visible floating in the water after the lather had subsided. They were formed before the lather formed, but were only clearly visible after. A lather will only form when all the calcium ions have been removed from the water, and there are some stearate ions left. Therefore when there wasn’t enough CO32- to remove all the Ca2+ from the water scum had to form in order to remove the last remnants of the Ca2+ from the water in these cases all the stearate ions were used up in forming the scum and there wasn’t enough left to form a lather. That is why in the cases where a lather did not fully form, the scum was far more visible.

The equations for these reactions are;

Ca2+ + CO32- → CaCO3

In tap in Na2CO3 dissolved in

Water the water

Ca2+ + 2St- → CaSt2

In tap stearate ions scum

Water in soap

Evaluation

In my experiment I attempted to ensure that the test was a fair one. I think that for the most part I succeeded. The results that I obtained supported my theory well, and there were no extremely anomalous results. However there were several minor ones. These were the results for 0.75g of Na2CO3 and 3.75g. The result for 0.75 was too low, this could be explained by either me accidentally weighing out too large a quantity of Na2CO3 , this would mean that less soap solution would be needed, or it could be explained by me not reading the burette accurately and putting in more soap solution than I was supposed to. The result for 3.75g Na2CO3 was too high, this could either be because I weighed out too little Na2CO3 or because I misread the burette and I thought that I had added more soap solution than I had really added, or possibly it could be because I drew the line of best fit incorrectly and the graph was levelling out more than I thought that it was. This would mean that the water was becoming as soft as it could get. This would be clarified by further experiments.

There were several flaws in the way that I carried out this experiment and if I were to repeat the experiment then there are several points that I would change.

I would measure out the tap water more accurately

In this experiment we only used a measuring cylinder to define the quantity of tap water needed for the experiment.
If I were to repeat the experiment then I would use a burette to measure out the quantity of tap water that was needed.

I would ensure that all the Na2CO3 that I weighed out on the scales, ended up in the test tube.

In this experiment a small quantity of Na2CO3 stuck to the ceramic bowl that I was using to measure out the correct quantity.
If I were to repeat this experiment then I would ensure that no Na2CO3 was left in the bowl.

I would read the burette more accurately.

In this experiment the burette was placed on the desk and the water level was above my head, my perspective of the soap solution level may not have been entirely accurate.
If I were to repeat the experiment then I would place the burette on the floor and kneel down so that the reading would be taken with me looking straight at it.

Overall I think that the results are reliable and that I would trust them to draw the conclusion of my initial prediction that as the quantity of Na2CO3 is increased, the water will become softer. Thus requiring less soap to form a lather. Additionally that after a certain point the Na2CO3 will become less effective at softening the water.

I would however like to do a further experiment in order to completely certify my prediction that after a certain point the Na2CO3 will become less effective at softening the water. For this further experiment I would like to essentially repeat the previous experiment however I would start the quantity of Na2CO3 added to the water at 3g. and test tap water with 3g, 3.5g, 4g, 4.5g, 5g & 5.5g.

The results from this experiment would certify or prove to be false, my initial prediction that after a certain point the Na2CO3 will become less effective at softening the water. At this point I am basing my certification of this prediction on the result for 3.75g Na2CO3 . This result however could be anomalous, and my further experiment will tell us for certain.