The second thing I needed was a soluble substance to alter the concentration of the water. My two initial thoughts were salt and sugar, but as potato has little sugar because it is converted into starch, I decided to go with salt.
Preliminary Test
I wasn't sure how much potato to use so I set up a preliminary test which would be designed to test which size of potato would give the largest reaction. The two main variables in this test where the thickness of the potato and the length of the potato.
To keep the test fair I kept the concentration of salt to water the same and I kept the amount I used the same. I used 25m of the highest concentration I had available (44g/200ml) to get the biggest change in weight.
These were my results
To make this table clearer, I produced a graph from its data.
As you can see from the graph, the 5mm thick and 3cm long was the best opinion as that gave the largest change in weight. I want the change to be large so I have a larger margin for error, and it will also make my results easier to analyse
Plan
Apparatus
For the experiment I will need
- A beaker (100ml) – I will use a beaker of this size because it gives a wide enough area so that the potato can lay on its side and also be submerged.
- A 5mm thick hollow tube – This will be to cut my bits of potato into equal rods
- A knife – This will be to cut the potato to the right length
- A ruler – To measure the potato's length
- Different salt water concentrations (5 different concentrations starting from 4g/200ml and going up in 4grams each time. I will need 75ml of each solution, 25ml for the 3 tests per solution) – These will be used to see at what rate osmosis occurs. I have chosen these concentrations, as they were readily available and will give me a good spread of results.
- A stop clock – I will use this to time the how long each potato rob has been in the solution.
- A scale correct to 2DP of a gram – This will be used to weigh the potato before and after each test.
Method
I will set up the apparatus like this.
The arrows show the process the potato goes through, start and ending at.
I will first record the potatos weight and put it in a solution a 4g salt to 200ml of water. I will leave it then for 15 minutes. After 15 minutes I will record the weight again.
I will repeat the experiment 3 times for each solution, the solutions being 4/200ml, 12/200ml, 20/200ml, 28/200ml and 36/200ml.
To make this a fair test I will
- Make sure the size of the potato stays the same
- Make sure there is no skin on the potato
- Make sure I have no exess water on the potato, but I will not dry it with a paper towel as different concentrastions of salt water could absorb at different rates and/or leave salt behind.
- Make sure that there is the same ammount and solution in each beaker.
- Make sure the beakers are clean.
- Make sure the potato cuttings come some the same type, age etc of potato.
Results
This is a table of my results.
As this table is a little unclear I used the data from the table to produce a graph
As you can see from the results, the graph's line of best shows a steady decrease of weight as the salt solution gets more concentrated. This is because as the beginning of the test, the salt concentration was low, which means that the water content in the potato was higher than the salt water solution.
This is a diagram of how osmosis occurs
I expected the blue line to be the same as the line of best fit. I think that the weight loss is only slight at the beginning because the difference in concentration was small, end therefore the transfer will not have happened as quickly. When the concentration was at a larger difference, osmosis occurred more quickly.
I don’t think that these result are anomalous because of the pattern they make 7 6 1 6 7
These results support my prediction to an extent. If it weren’t for the two irregular results, my prediction would be correct.
Evaluation
This experiment was successful because it supported my prediction that the weight will steadily decrease as the salt water concentration increases.
I expected the blue line to be the same as the line of best fit. I think that the weight loss is only slight at the beginning because the difference in concentration was small, end therefore the transfer will not have happened as quickly. However, this theory doesn’t work for the end result. I suspect this was because the potato had reached a concentration where the transfer could not happen more quickly.
To prove this theory I would have to repeat the experiment with higher concentrations, and see whether the results changed at all, or just more slowly.
I don’t think that these result are anomalous because of the pattern they make 7 6 1 6 7
I could’ve made the experiment more accurate by
- Bringing the scale closer to the experiment, as I had to walk across to the other side of the room to take the reading.
- Repeat the experiment more times
- Find some method of drying the potato without taking some of the salt away. I did not dry the potato as I thought it would be more unfair to dry them with cloth because it would take salt the too. The amount of salt on the outside of the potato would change for each experiment, but the amount of water wouldn’t change greatly.
If I were to do this experiment again I would take the potato out and weight it again at regular intervals to see the rate at which osmosis happens.
Daniel Hill 10S 25309.doc Page of