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Time until the cubes go clear Essay

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An investigation into the effect of diffusion on the size of a cell Introduction What I am going to do is place some agar phenolphthalein in a HCl solution and time until the cubes go clear. Theory Diffusion is the passive transport of how substances cross membranes. It is the movement of particles from of particles of a higher chemical potential to a lower one. In all diffusion the net flux of the particles transported is equal to physical property multiplied by the gradient.

Diffusion happens because of the second law of thermodynamics; the entropy of closed system will always increase in time. Also substances being diffused go from a state of higher order to a state of lower order, so diffusion is a completely natural process for it requires no energy to happen. The rate of diffusion is dependant on a number of factors-  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.

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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. Surface area time to go clear (secs) rate of diffusion 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. 2

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