T carry out an investigation to test the strength of vinegar supplied in three various fish and chip shops, so we can find out if it has been watered down, and if so by how much.
What Vinegar is:
The kind of vinegar you get at the grocery store is about 95% water, but the vinegary part is acetic acid (also called ethanoic acid or methane carboxylic acid). The formula is written: CH3COOH
Vinegar samples A, B and C
1. Make sure you are wearing safety goggles.
2. Measure out 20mis of each sample of vinegar.
3. Add five drops of phenolphthalein to the 20mls of vinegar sample.
4. Add alkali, sodium hydroxide, to the vinegar solution, 1ml at a time using a pipette.
5. Continue to do this until the mixture of vinegar, phenolphthalein, and sodium hydroxide had created a slightly pink liquid.
6. Do this two more times, making sure that you are not cross contaminating the samples or chemicals. This should be done by washing the affected apparatus after each use.
7. Wash your hands
8. Calculate an average of the three results for each sample.
I predict that the samples A, B and C will be less concentrated, than the manufactures sample because the fish and chip shops will have added water to them so less vinegar is needed.
This table shows the amount of sodium hydroxide needed to neutralise each vinegar sample.
These are the averages for each of the four samples, from the above table.
The results show that only two of the four samples of vinegar were watered down.
I have worked out that Sample A is 2/3 water and sample b is only 1/3 water. The vinegar had been watered down this much so that for every litre of sample A sold, they must only use 333mls of vinegar. Assuming they sell 50 litres of vinegar a year, that means they are saving over 33 litres.
Although one could assume that they weren’t watering down the vinegar to save money, just because they think it was too strong and simply want to make it more to their own personal taste- although I find this unlikely.
This table shows the amount of vinegar in proportion to water.
Mls of water in a 20 ml sample
Mls of Vinegar
These pie charts illustrate this data more clearly.
The graphs show clearly, how extensively the sample A had been watered down. There is very little vinegar in the solution, and an overwhelming amount of water, this will therefore compromise taste and strength, and chances are people will just use more.
When the two chemicals, vinegar and sodium hydroxide, are mixed together, they effectively cancel each other out. This may not happen quickly if one is a stronger chemical than the other, you will need more of the weaker chemical to make a neutral solution.
The chemical word equation for a neutralisation reaction is-
Acid + Base = Salt + Water
Neutralisation happens because acids contain hydrogen atoms with a positive (+) electrical charge, these are called ions. The chemical symbol for this atom is H+
While alkali solutions contain hydroxide ions, which have a negative (-) electrical charge. The chemical symbol for this atom is OHï¿½
When an acidic solution and an alkali solution are mixed, each hydrogen ion joins up with a hydroxide ion to form a water molecule. This is therefore why in the word equation the acid and base form water, and the fact that water is formed is why the final solution is neutral.
Therefore the chemical formula for the vinegar and sodium hydroxide would look like this-
CH3COOH(aq) + NaOH(aq) –> CH3COONa(aq) + H2O(l)
This show how the hydrogen and oxygen atoms have joined together, leaving a salt and water, like in my explanation.
The results where not as I had expected, this is only because I thought that all of the chip shops that we where going to test would water down their vinegar, in reality only two of the three did do so.
Although two of the samples where watered down, so I suppose this is as I expected, as you rarely get you prediction spot on to the final results. Also I didn’t mention any figures in my prediction, so I cannot compare anything to the actual answers.
This prediction wasn’t based on any high brow scientific knowledge, it was simply a guess, that is therefore why I can say little more than this about why the results where different to my prediction.
Figures where not included in my prediction because these would have simply been guesses, as I don’t have a clue how much a fish and chip shop would water down their vinegar by. Although I could have guesses a figure like 1/4 of the sample would be water.
On the whole I feel that I carried out quite a hard experiment to the best of my ability, bearing in mind the equipment we had to work with. I think that it was done well because all three of my tests for each of the four samples where relatively close together, within 1 or 2mls of each other. This on the other hand doesn’t mean were very exact although I feel it is a good indication.
Though looking back I now fell that maybe I should have used clean instruments for every individual test, as this would have eliminated cross contamination. But this really wasn’t possible, as had everyone in the class wished to do this then we would have needed over 80 sets of apparatus.
What I did do was to wash each of the instruments after each use; this obviously stopped cross contaminations of samples to some extent, but may have watered down my vinegar and sodium hydroxide samples. Other than I don’t think 1 or 2mls of water in a 20ml sample of vinegar would have had a huge effect, especially seeing as we weren’t being completely precise.
A problem I found with the instruments was that it is very hard to be precise when measuring liquids in a pipette or measuring cylinder. This is because the water doesn’t sit level in such a thin tube. Instead it creates a convex shape, which makes measuring 1ml of sodium hydroxide quite difficult indeed. This may not affect the results when measuring out 1 or 2mls, but when you’re measuring out 20 or thirty the extra 1/4 of a ml soon adds up.
Although, the test was still fair as all four samples had this bias’- therefore all the results where proportionate.
A problem we found when using phenolphthalein, as an indicator, is that the colour suddenly changed from clear to red, but we wanted the colour pink, as this shows a neutral solution. Pink is in between clear and red, and the colour abruptly changes, often causing the solution to become alkali.
We could have solved this problem by starting with the alkali, sodium hydroxide, and adding vinegar to that. This would have meant that it will be a slightly slower change from red to pink, as opposed to a quick change from clear to pink.
But even though I have found these problems I still feel my data is both accurate and validated.