- Particles will diffuse quicker when there is a high concentration gradient.
- A short distance fro particles to diffuse across.
- If the substance is small molecules they will diffuse quicker.
- Larger surface area increases rate of diffusion. For if there is larger surface area then it allows particles to diffuse faster as there is less resistance.
The last one is the key one, as this one plus knowing the concentrations of the solutions allows you to work out the rate of diffusion by Ficks law.
Surface area * difference in concentrations is proportional to rate of diffusion
Thickness in membrane
Hypothesis
My hypothesis is that as you increase the surface area of the “cells” the rate of diffusion will increase.
Preliminary Work
For my preliminary I gathered these results, I used 30ml of HCl for each cube, the results show that as you increase the surface area that the rate of diffusion deceases, this will be down to the fact that there is more indicator in the larger cubes. It also shows that my hypothesis is wrong, but I have already explained why.
Apparatus
5 cubes of agar-phenolphthalein; 0.5cm^3, 1cm^3, 2cm^3, 4cm^3, 6cm^3
250ml beaker
10cm^3 measuring cylinder
mm rule
Scalpel
100 ml of 2M HCl
Distilled water
It is necessary to have a rule that measures in mm to ensure the cubes are measured accurately, it is the same for using the measuring cylinder as this is needed for the precision it posses in making the solution of acid. Distilled water is needed to ensure the results are reliable as tap water contains number of things that will offer resistance to diffusion.
Variables
- The independent variable is the size of the agar cells, these will be made by cutting them up with a scalpel and measuring them to confirm, and I will have five different sizes and repeat the tests three times.
- The dependant variable is the time that it takes for the acid to make the phenolphthalein in the agar go clear, I will hold a piece of white paper behind the cube to ensure I am clear whether the test is over.
- Control variables are the time which will be ten minutes, this will be time on a stopwatch, temperature will be kept at room temperature, the concentration of the acid will be kept the same, and it will be all made up at the same time this will ensure precision. The amount of phenolphthalein in the agar cells should be in equal proportion in all the cells as the agar jelly will come as one block.
Safety
This experiment is particularly dangerous but a number of precautions should be taken, goggles and gloves should be worn when handling acid as it comes at quite a high molarity. A cutting plate should also be used when cutting up the agar cubes. Disposal of the agar should be in the bin.
Method
- Prepare agar cells using cutting plate and scalpel.
- Measure cubes using mm rule and trim if essential.
- Measure out 30ml into a test tube
- Place the cubes in the beaker and then fully submerge them in the HCl solution. Start stopwatch.
- Turn the blocks frequently over the time.
- When blocks are clear against the piece of paper stop the stopwatch.
Results
Bibliography
Sources are CGP revision guide and Cambridge advanced sciences biology 1.
