You are required to investigate the effect of concentration of hydrochloric acid (independent variable) on the rate of diffusion. The investigation involves placing agar cubes containing an indicator into dilute hydrochloric acid. As the acid diffuses into the agar cubes the indicator changes colour. You are provided with the materials shown in Table 1.1. Table 1.1 labelled, contents, hazard, volume/cm³ H, 1.0mol dm⁻³ hydrochloric acid, irritant, 100 A, agar, none, – W, distilled water, none, 60 It is recommended that you wear suitable eye protection. If H comes into contact with your skin, wash it off immediately under cold water. You must not touch the agar with your hands. Use the blunt forceps and paper towel to handle the agar. You will need to carry out a trial test (step 1 to step 4) before you start your investigation. Read step 1 to step 4 before proceeding. 1. Cut 3 cubes from the agar, A, as shown in Fig. 1.1. Each cube should be approximately 5mm x 5mm x 5mm but they must all have the same dimensions. Cut the cubes from A on the white tile provided. [Figure 1.1] 2. Put 10 cm³ of 1.0moldm⁻³ hydrochloric acid, H, into a beaker. 3. Put the 3 cubes you cut in step 1 into the beaker containing H, using blunt forceps. Start timing. As the acid diffuses into the agar cubes they change colour. The end-point is reached when the blue-green colour disappears and the whole cube has changed colour from blue-green to pink. Putting the beaker on the white card provided may help you to see the colour changes more clearly. 4. Measure the time taken for each cube to reach the end-point and record the times in (a)(i). If any cube remains blue-green after 3 minutes record as 'more than 180'.

1. (a)(i)Record your results in an appropriate table.[2]
2. (a)(ii)Calculate the mean time taken for the cubes to reach the end-point. mean time = ..........................................................................[1][1]
3. (a)(iii)A student carried out this trial experiment and recorded three results which were not consistent. Identify one error in this trial procedure that could have caused repeated results to be inconsistent. ................................................................................................................................................................................[1][1]
4. (a)(iv)State two variables that you will need to standardise during your investigation. 1 ................................................................................................................................................................................ 2 ................................................................................................................................................................................[2][2]
5. (a)(v)Complete Table 1.2 to show how you will prepare these concentrations. Table 1.2 volume of 1.0 moldm⁻³ hydrochloric acid /cm³, volume of distilled water, W /cm³, final concentration of hydrochloric acid /mol dm⁻³ 10, 0, 1.0 , , 0.8 , , 0.6 , , 0.4 , , 0.2 [2][2]
6. (a)(vi)Record your results and mean times in an appropriate table. [3][3]
7. (a)(vii)Using your mean times, calculate the rate of diffusion when the concentration of hydrochloric acid is 1.0moldm⁻³ and in the lowest concentration of hydrochloric acid you have investigated. rate in 1.0moldm⁻³ hydrochloric acid ................................ s⁻¹ rate in the lowest concentration ..................................... s⁻¹ [1][1]
8. (a)(viii)State whether you support or reject this hypothesis. Explain how your results provide evidence for this decision. support or reject ................................................................................................................................................................................ explanation ................................................................................................................................................................................ ................................................................................................................................................................................ ................................................................................................................................................................................[2][2]
9. (a)(ix)This procedure investigated the effect of the concentration of hydrochloric acid on the rate of diffusion. To modify this procedure for investigating a different variable, the concentration of hydrochloric acid would need to be standardised. Suggest the concentration of hydrochloric acid that should be used. Give a reason for your answer. concentration ................................................................................................................................................................................ reason ................................................................................................................................................................................ [1][1]
10. (a)(x)Think about how you could modify this procedure to investigate the effect of surface area to volume ratio on the rate of diffusion. Describe the modifications needed to investigate the effect of surface area to volume ratio. ................................................................................................................................................................................ ................................................................................................................................................................................ ................................................................................................................................................................................ ................................................................................................................................................................................[2][2]
11. (b)(i)Draw a bar chart of the data in Table 1.3 on the grid in Fig. 1.3. Each bar should be separated for each type of pathogenic bacterium. Use a sharp pencil for drawing bar charts. [Figure 1.3] [4][4]
12. (b)(ii)A scientist studied the results in Table 1.3 and noticed that one type was most affected by chemical C. The scientist decided to carry out further experiments to learn more about the effects of chemical C on this type of bacterium. State which type of bacterium in Table 1.3 the scientist should use for further experiments. Give a reason for your answer. type of bacterium ................................................................................................................................................................................ reason ................................................................................................................................................................................ [1][1]

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