A student measured the distance moved by different concentrations of citric acid solution through agar jelly. The agar contained Universal Indicator which changed colour in the presence of acid. The agar mixed with Universal Indicator was green at the beginning of the investigation.
Step 1 | Three test-tubes were labelled A, B and C. Three different concentrations of citric acid solution were made. |
Table 1 shows the volumes of 5% citric acid solution and distilled water that were used to make each solution.
Table 1
Step 2 | The base of a Petri dish containing agar and Universal Indicator was labelled A, B and C. |
Three holes were cut into the agar. This is shown in Fig.1
Fig. 1
Step 3 | The student was provided with one dropping pipette. Three drops of solution A were placed into the hole in section A of the Petri dish. |
Step 4 | Three drops of solution B were placed into the hole in section B of the Petri dish. |
Step 5 | Three drops of solution C were placed into the hole in section C of the Petri dish. |
Step 6 | A stop-clock was started. |
Step 7 | After 30 minutes the student observed the colour change in the agar around the hole in each section of the Petri dish. The colour change was caused by the diffusion of the citric acid solution through the agar. |
Step 8 | A ruler was used to measure the distance travelled by each concentration of citric acid solution through the agar. |
Fig. 2 shows the appearance of the Petri dish after 30 minutes.
Fig. 2
(i)
Measure the distance travelled by each concentration of citric acid solution after 30 minutes in Fig. 2 Rulers have been added to the image to assist you in taking these measurements.
Prepare a table and record these results.
You should include:
The concentration of the citric acid solutions
The distance travelled by the citric acid solutions.
[3]
(ii)
Describe how you decided where to measure the distance travelled by the citric acid solutions.
[1]
(iii)
State a conclusion for these results.
[1]
(iv)
The citric acid moves through the agar by diffusion. The diffusion coefficient is used to show the effect of concentration on diffusion.
The formula to calculate the diffusion coefficient is:
Calculate the diffusion coefficient, in mm2 per minute, for a 10 % solution of citric acid that travelled 14 mm in 30 minutes.
Give your answer to two significant figures.
[2]
(v)
Universal Indicator is used to estimate the pH value of substances.
Estimate the pH value for the green agar and the red agar.
[2]
(i)
State two variables that have been kept constant in this investigation.
[2]
(ii)
Identify one potential source of error in this investigation and suggest how the error could affect the results.
[2]
Describe how you could adapt this method to find the effect of temperature on the rate of diffusion. Agar melts at 70 °C.
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