This would be the perfect graph of results, I believe, because the temperature where the cell membrane breaks all the pigment should leak out, leading to the sharp, possibly vertical, line on the graph. I would also expect there to be 0% pigment leakage at temperatures below the temperature at which the cell membrane breaks down; this is because I believe the membrane will still be fully intact and operational at the temperatures below the temperature where the cell membrane breaks, ergo no pigment leakage. A more realistic graph would be an S-shape, similar to that found in cumulative frequency graphs (see below).
My hypothesis is that as you increase the temperature the red cabbage is cooked at, there will be more pigment leaked that leaks out, which means there will be more absorbance of light at greater temperatures; there should also be a graph that looks something like the above diagram (the S-shaped curve).
There are several variables I must control and keep constant, whilst the variable I will change (the independent variable) will be the temperature I ‘cook’ the red cabbage at, whilst the dependant variable (the variable that changes and that I take readings of) will be percentage absorbance of light of the cooked red cabbage samples. I will record percentage absorbance of light because that gives me quantitative data (figures and numbers), rather than qualitative data (opinions) which I would get by recording the colour of the cooked red cabbage samples. My control variables will be:
- Time the cabbage is left to be cooked and also time the cabbage is left to let the pigment leak out.
- Part of cabbage used; is the sample from the same part of the cabbage?
- Volumes of water; both for the water the cabbage is cooked in and the water it is left in to let the pigment leak out.
- Size of cabbage sample, are they all the same size?
Procedure
Equipment:
- Raw red cabbage
- Knife
- White tile
- Ruler; to measure cabbage sample
- 8 boiling tubes, one for each cabbage sample and each temperature reading
- Test tube rack
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Plastic beaker, 250cm3
- Kettle, to boil water
- Thermometer
- Colorimeter and cuvettes
- Stopwatch
- Measuring cylinders
- Tweezers
Method:
- Choose a leaf of the red cabbage and cut out 8 pieces roughly 1cm by 0.5 cm. Take care when using the knife to cut out cabbage pieces
- Collect 8 boiling tubes and a test tube rack and place one piece into each boiling tube. Label each test tube from 30°C to 80°C in 10°C intervals.
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Now, boil a kettle full of water. When it is boiled, pour a 250cm3 roughly half full with the boiling water. Next, with your thermometer, take the temperature of the boiled water: if it is colder than 80°C then boil another batch of water until it is warmer than 80°C; if it is hotter than 80°C then slowly add cold tap water until it is closer to 80°C. NB: take care with the boiling water as it could possibly scold or burn.
- You are now ready to cook your first red cabbage sample. Take the piece from the boiling tube marked ‘80°C’ and using a pair of tweezers, pick up the cabbage sample from that boiling tube and place it in the beaker full of 80°C water. Leave it to cook for a minute then put it back into its boiling tube, remembering to measure out 10ml of cold tap water to put in the boiling tube with the cabbage sample. Record the time for when the cabbage sample was placed in the boiling tube and leave for 30 minutes to let the pigment leak out, if any does.
- Now repeat the previous step for the remaining temperatures: 70°C to 30°C. If the temperature in the ‘cooking’ beaker isn’t right either add more boiling water to make it warmer or add more cold water to cool it down. Also, if the beaker starts getting full, you can pour some out into a nearby sink, but remember to keep the desired temperature in the beaker.
- Once all the cabbage samples are prepared and they have all been left for 30 minutes, prepare cuvette samples of each of the cabbage samples. Do this by collecting a cuvette, remembering to pick it up by the rough sides, leaving the clear sides unmarked, and then pour it full with your red cabbage sample. Repeat for all the cabbage samples remembering to note down the temperature the sample was cooked at.
- As you have used tap water to leave the cabbage sample to settle in, the water would have gone blue; this is because the tap water in my area is hard water and is quite alkaline. So the colorimeter has to be set to a yellow filter. But if your samples are more red/purple, use a blue filter.
- The technique to use when using the colorimeter is a ‘blank-test-blank’ technique. This is where we take a cuvette sample with only tap water in to ‘zero’ the colorimeter, then we put the sample in, then the tap water cuvette is placed back in to ensure the colorimeter has read the cabbage sample correctly and that the colorimeter goes back to zero.
- Collect your readings in a table like this:
- Repeat experiment at least twice, but preferably three times, and on repeating the experiment use the same section of cabbage leaf.
My Results
Results Table Showing the Light Absorbance of Cooked Red Cabbage Samples At Varying Temperatures
NB: the averages gave values with more than two decimal places, so I left the averages at two decimal places because the colorimeter reads only to two decimal places, so I can’t have results with several decimal places.
Analysis & Evaluation
My graph shows that there is an S-shaped curve, which fits my hypothesis. Also it shows that the temperature the cell membrane breaks down would be somewhere between 60-70°C, where the curve is at its greatest gradient. I could have found the temperature that the cell membrane breaks easier if I had more made But, the graph is only as valid as the results.
I have eliminated any systematic errors that could have occurred when using the colorimeter by the technique I used, which ensured the colorimeter went back to zero, ready for the next red cabbage sample. This not only eliminated systematic errors with the colorimeter readings, but also gave me precise results.
But there are some unavoidable random errors in this experiment. Firstly, the red cabbage samples didn’t all have the same amount of pigment and weren’t the same volume. This would have introduced errors that would have created a wide range of results, which would have been above and below the line of best fit.
My experiment wasn’t perfect; there are several areas where improvements could be made. Firstly, instead of using a beaker full of heated water, I could have used several water baths, set to the required temperatures. This would be better than using a beaker full of heated water because the water bath’s temperature remains almost constant; whilst the beaker full of heated water’s temperature changed whilst the cabbage was being cooked, the biggest change being at greater temperatures. Although, the beaker was plastic and prevented heat loss through the sides of the beaker, but the most heat loss was where the water was in contact with the air. Next, my method of cutting out the pieces of cabbage could be altered by using a cork borer. Although in my method I have made efforts to keep the pieces roughly to the same dimensions, the cork borer would have made all pieces to the same dimensions. Also, by using pieces all from the same leaf does it make a fair test? I found that not all the pieces from the same leaf were the same volume or had the same amount of pigment in it/colour to it. To eliminate any problems from this, I could have cut many pieces from several sections of the cabbage and chose the ones I would use by a method of random sampling.
In conclusion, my results back up my hypothesis that as temperature increases the percentage of light absorbed increases (or the amount of pigment to leak out will increase), and the graph produced would be S-shaped. The temperature that the cell membrane breaks down is between 60°C and 70°C.