The relationship between size and the rate of diffusion

My prediction is that the smaller the cube of gelatine, the less time it will take to diffuse, and the larger the cube of gelatine will take a longer time to diffuse. My reason for this prediction is because the smaller the blocks, the larger surface area to its volume, and the larger the blocks the smaller the surface area in comparison to the volume.

The apparatus for this experiment is as follows.

* Gelatine block

* Scalpel

* Boiling tube and bung

* 250mls Hydrochloric Acid (Dilute)

* Tile

* Ruler

* Stop clock

To make this a fair test we measured the cubes accurately, while cutting the gelatine with the scalpel, using the ruler. The time was also timed accurately, each gelatine block was to be at the same place of diffusion when we took note of the time. The amount of dilute hydrochloric acid remained the same at 250mls. As the temperature can affect the rate of diffusion, we did not touch the boiling tube for long periods of time.

The experiment took 20 minutes to complete, we placed the gelatine block on the tile, we used the ruler to measure the block at 10mm(1cm3), then we used a scalpel to cut it. We repeated this until we had 5 cubes of gelatine blocks at 5 different sizes, the sizes were, 10x10x10, 10x10x5, 10x5x5, 5x5x5 and finally 5x5x21/2.

We then placed 250mls of dilute hydrochloric acid into the boiling tube. Starting with the largest block, we placed each gelatine block into the tube, next we tilted the tube, so we could spread out the gelatine blocks along the side of the boiling tube. We observed the blocks and waited to watch for diffusion to take place, each block turned from red to orange in different times.

Result table

CUBE

2

3

4

5

Cube dimensions (mm)

0x10x10

0x10x5

0x5x5

5x5x5

5x5x21/2

Surface area (mm)

600

400

300

50

00

Volume (mm)

000

500

250

25

62.5

SA:V Ratio0

3:5

4:5

6:5

3:2.5

4:2.5

Time it took for acid to penetrate.

9min 52secs

3mins 08secs

6mins 37secs

4mins 15secs

2mins 41secs

After completion of the test, and comparing the times of each block, we had noticed a trend, the largest gelatine block had taken longer to diffuse than the smaller gelatine block, this is as I stated in my prediction, the smaller the surface area to its volume, the quicker it will diffuse. We also noticed that block 3 and 4 were at similar times, this is mainly because the surface area to volume ratio is very similar.

I feel that it would have been a fairer test it all the cubes were cubes, and not cuboids, as this could affect the time in diffusion. I also feel that we should of would in teams of five, so that all blocks could be placed in at the same time, or that we completed the test one block at a time, therefore the gelatine blocks would have been timed more accurately. I also think that the test should have been carried out numerous of times, to eliminate any anomalies.

H.B.Benson