Biology Practical Diffusion

Title:                                               Diffusion in Gases

Aim: To observe and study the rate of diffusion of a substance in air.

Apparatus:

Beaker, glass tube, scissors, forceps,  rubber bungs, ruler, felt pen, stop watch, cotton wool, needle, red litmus paper, water, ammonia solution, dilute acid.

Method:

1.           The glass tube was marked at 5 cm intervals using a ruler and felt pen. The markings were arranged such that the zero mark coincided with the bottom of the rubber bungs.

2.           Strips of litmus paper were cut.

3.           A beaker was half filled with water. Using forceps  a piece of litmus paper was dipped in the beaker. The excess water was shaken off and then the paper was placed inside the glass tube at one of the markings.

4.           Step 3 was repeated for the other litmus strips.

5.           One end of the tube was sealed using a rubber bung.

6.           The needle was used to stick the small piece of cotton wool to the lower surface of one of the rubber bungs.

7.           6 drops of ammonia solution were dropped on the cotton wool. The rubber bung was inserted in the glass tube and simultaneously the stop clock was started.

8.           The time at which the piece of litmus started turning from red to blue was recorded. In turn, the time taken for each piece of litmus to turn blue was recorded.

9.           After a complete set of readings were taken, the tube was opened and the cotton wool and the litmus paper were removed. The glass tube was washed thoroughly with tap water then distilled water to remove any traces of ammonia.

10.     The experiment was repeated but instead 12 drops, 18 drops and 24 drops of ammonia solution were dropped on the cotton wool.

11.     The results of ammonia were tabulated as shown below.

12.     Graph of time taken by ammonia to travel specific distances along the glass tube

Results:

Discussion:

The effect of ammonia on moist red litmus paper was due to the reaction of ammonia with water forming ammonium hydroxide. This substance is alkaline in nature and caused a change in the colour of the litmus. Such  a change happened because the ammonia vapour evolved from the piece of cotton wool, moved through the glass tube until it reached the various pieces of litmus paper.

Diffusion is the name given of a process which occurs spontaneously or naturally. It involves the movement of a substance from an area of high concentration to an area of low concentration.Such a movement takes place in the alveoli in lungs. The blood in the capillaries around the alveoli has very little oxygen while the alveoli are saturated with the gas. So, oxygen diffuses from the alveoli into the capillaries and is carried all over the body where it could be used for respiration. On the other hand, while the blood in the capillaries is saturated with CO2,  the concentration of this gas in the alveoli is low. So CO2 diffuses from the capillaries into the alveoli. The gas is then expelled out of the body through the mouth.

From the results obtained and the graphs plotted it could easily be seen that the higher the concentration the shorter the time taken for the ammonia to travel to the other end of the tube. This happened because an increase in the concentration of ammonia increased the diffusion gradient and the higher the diffusion gradient the faster the rate of diffusion and so the faster the litmus paper turned blue.