Investigate the effect of sodium carbonate on hard water

I am going to use titration in this experiment. To do this, I will use soap solution in a burette, I will use a pipette and a filler to get 25.0cm3 of hard water (the laboratory tap water). Then I will weigh out and add sodium carbonate (starting at a control of 0.000g and not going any higher than 6.000g) to the water and dissolve it. After doing this I will add soap solution (0.5cm3) to the solution, and shake it for 10 seconds. We will time this using a stopwatch/stop clock. We will then see if a permanent lather forms (a permanent lather is a lather that lasts for at least 10 seconds), which is also timed using a stopwatch/stop clock. I will continue to add soap solution until a lather forms. This will tell me if the sodium carbonate has made the water softer of harder. If a lather forms after only using 1cm3 – 3cm3, then it has been softened by the sodium carbonate, because all the calcium ions have been used up quickly; however if it takes over 6cm3 of soap solution to form a lather, then the sodium carbonate hasn’t really softened the water and more sodium carbonate is needed. During the experiment I am going to record my results. I am going to record the measurements of the soap solution each time and the mass of the sodium carbonate each time.

After doing that, I will take a mean of the masses of sodium carbonate and a mean of the amount of soap solution used each time. Then after finishing the experiment I am going to write a table of results of all of this detail, to collect evidence and show what I have found out. After doing this, using the table of results I am going to plot a graph to show these results in another way and to see what effect the sodium carbonate had on the water. The apparatus I am going to use is a pipette and a filler, to get an accurate amount of water, to make sure a good level of hygiene is kept and to produce reproducible results; a burette, to measure accurate amounts of soap solution going into the solution each time, as it is a long thin glass tube with graduations and can deliver accurate amounts of liquid to 0.1cm3; a weighing machine that is accurate to 3d.p, to weigh out small amounts of sodium carbonate; a stopwatch/stop clock to time the shaking and intervals. I also need the soap solution, hard water, sodium carbonate and a conical flask with a rubber cork.

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In this experiment, the sodium carbonate should soften the water as the calcium/magnesium ions react with the carbonate ions.

calcium sulphate + sodium carbonate =; calcium carbonate + sodium sulphate

CaSO4(aq) + Na2C03(aq) =; CaC03(s)+ Na2S04(aq)

The actual experiment is very simple however there are many key factors that we need to look at. When doing the experiment, I will have consider the amount of sodium carbonate used each time , the amount of water, the strength of the soap solution, the type of water, the amount of soap added and how many times we are going to do the experiment for each measurement of sodium carbonate. I am also going to change the amount of sodium carbonate to see if the water gets softer or harder when more or less is added. The variables which I can control are the hardness of the water as it all comes from the same source, the volume of soap solution, the height of lather, the volume of water and the type of water. The vigour of our individual shaking can be controlled (to an extent, as the shaking can’t always be exactly the same vigour but can be the quite close), but the similarity in the strength of my shaking and the strength of other people’s shaking can’t really be controlled. I am unable to control the similarity in the way that I do the experiment each time and the similarity in the way that I do the experiment compared to the way that people that I’m comparing results with. I am also unable to control the temperature each time, the humidity of the air and the wind each time.

The temperature, wind and humidity don’t really affect the experiment as I am not looking at any of these variables as results, so it isn’t a problem that I can’t control them. However the similarity of the way I do the experiment each time and the similarity of the shaking compared to other people could be a problem. So I need to wash the apparatus with tap water and distilled water every time I do the experiment and I need to use the same apparatus each time. For the shaking we really can’t do anything apart from just trying to make people with similar strength do the shaking, so we can keep it as similar as possible in each experiment. In this experiment, I predict that the sodium carbonate will soften the water and the more sodium carbonate added, the softer the solution will get. I predict that with no sodium carbonate to soften the water, the water will need at least 6cm3 of soap solution to use up the calcium ions and show that the water has been softened. However as more sodium carbonate is added, less soap solution is needed to use up the calcium ions and soften the water, until eventually at one point there will be enough sodium carbonate to use up all the calcium ions and soften the water that no extra soap solution will actually be needed.

I think that the graph is going to look like this because, at first when the sodium carbonate is added, there will be a large decrease in the volume of soap needed, however eventually, after a certain amount of sodium carbonate added, there will hardly be any soap solution needed. From then on, the volume of soap needed will be the same or there will only be a small decrease, so therefore the curve will start to slow down and instead of the volume of soap needed, decreasing rapidly, the volume of soap needed, will decrease but not so quickly. In the experiment, we are going to range the mass of sodium carbonate. However we are not going to use more than 6g of sodium chloride. I am going to use 8 different masses (0.0g, 0.5g, 1.0g, 2.0g, 3.0g, 4.0g, 5.0g, 6.0g).However, for each mass, 1 result is definitely not enough as that 1 result could be a rogue result. So I am going to get 3 results for each mass, so I can find out if one of the results was wrong. With the three results I will take an average of the exact amount of sodium carbonate (to 3d.p) used and the exact amount of soap solution used.

This way, at least I can be safe and stop myself using the wrong data. This will also mean that at the end there will be a better answer to the question, “What effect does sodium carbonate have on hard water?” and I will be able to draw a more detailed graph illustrating these points. In this experiment being precise is extremely important and that is why we use the apparatus that we are using. A burette and pipette are used instead of measuring cylinders as they are more accurate and precise. In this experiment I need to be very careful; I must make sure that I don’t spill anything, I must make sure that I don’t add too much or too little soap solution to the solution and I need to be safe. I am also going to follow the school lab safety rules which include, “Always wear eye protection when needed; spit out anything you might accidentally get in your mouth, wash your mouth out; wash off any chemicals that you might get on your skin with plenty of water; report all accidents, breakages, or faulty apparatus and equipment; only use bins for rubbish, not broken glass; wash your hand after practical work; know the safety labels for toxic, corrosive, highly flammable and irritant/harmful,” (as shown below).

Corrosive Highly Flammable Irritant/Harmful Toxic

In this experiment, the calcium ions react with the carbonate ions when the sodium carbonate and the hard water are mixed and most of the calcium ions are used up.

calcium ions + carbonate ions =; calcium carbonate

Ca� + CO3� =; CaCO3

Therefore the more carbonate added, the less calcium there is left in the water after the sodium carbonate has been added to the water. After this, if there are still calcium ions left, stearate ions react with the rest of the calcium ions and all the calcium ions are used up and a scum is formed.

calcium ions + stearate ions =; calcium stearate(scum)

Ca� + 2St =; CaSt2

Calcium Ions make the water harder, so that means once the calcium ions are removed the water is softer. So, if you put more (sodium) carbonate in the water, the water gets softer and softer until there are no more calcium ions left. The stearate (soap solution) just tests if the water has become hard, by seeing if a scum is formed. If there is no scum, then we know the water is soft. .Previously, I have tested the hardness of different types of water. I have used tap water from different places, distilled water, mineral water, tap water with calcium hydrogen carbonate in it (temporarily hard water) and tap water with two different masses of sodium carbonate in. In these experiments I used the same method that I’m going to use for this experiment, however I only took 1 result for each type of water, which I shouldn’t have done in case it was a rogue result. In this experiment, I will do 3 results for each time I do the experiment with a different mass of sodium carbonate.

In the last experiment, I added 1cm3 of soap solution (stearate) at a time, this time I am going to add 0.5cm3 of soap solution for more precise, reliable results. In the previous experiment, I used 5 types of tap water (laboratory(central London), Sussex Coast, Scarborough, Surbiton and Greenwich), distilled water, mineral water (Scottish natural), laboratory tap water with calcium hydrogen carbonate in it and lab tap water with sodium carbonate. We used the distilled water as our control, because we knew the distilled water when titrated would be very soft, so knowing this we could check that all the apparatus was working by seeing if the result for the distilled water was that it was very soft. The results all seemed to be correct, except for the Scottish Natural Mineral Water, which was hard water but we expected it to be a lot harder as it contained a lot of minerals.

Source of Sample

Volume of soap

for a lather/cm3



Distilled Water


very soft

Lab Tap Water


very hard

Sussex Tap Water



Scarborough Tap Water


very hard

Surbiton Tap Water


very hard

Greenwich Tap Water


very hard

Scottish Natural Mineral




Lab Tap Water + Calcium

Hydrogen Carbonate


extremely hard

Lab Tap Water + 0.5g of

Sodium Carbonate



Lab Tap Water + 2.0g of

Sodium Carbonate


very soft

From these results, we can tell that the more calcium ions there are in the water the harder the water is and therefore more soap solution (which contains stearate) is needed to get rid of all the calcium ions. In the experiment we are about to do, we know that we need more soap solution to get rid of calcium ions, but we also have sodium carbonate (which contains carbonate ions) which also helps to use up calcium ions. As shown in the results, we can see that using sodium carbonate definitely softens the water. In the experiment that we are about to do, instead of only using two masses of sodium carbonate with water, we are going to use more masses (8 to be precise), so that we get more accurate and reliable results. So, we know from previous experiments that the more sodium carbonate added, the less stearate/soap solution need to soften the water, and the more sodium carbonate added, the softer the solution.


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