A student investigated the effect of acid concentration on the rate of diffusion in agar jelly. The student was given three test-tubes containing bromothymol blue indicator in agar jelly. Bromothymol blue is a blue indicator that turns yellow in the presence of an acid. The student used this method: Step 1 label three beakers A1, A2 and A3 Step 2 put $$\(20 \mathrm{~cm}^{3}\)$$ of 1 mol per $$\(\mathrm{dm}^{3}\)$$ hydrochloric acid solution into beaker $$\(\mathbf{A 1}\)$$ Step 3 put $$\(2 \mathrm{~cm}^{3}\)$$ of 1 mol per $$\(\mathrm{dm}^{3}\)$$ hydrochloric acid solution into beaker A2 Step 4 add $$\(18 \mathrm{~cm}^{3}\)$$ of distilled water to the hydrochloric acid solution in beaker A2 Step 5 use a clean syringe to transfer $$\(2 \mathrm{~cm}^{3}\)$$ of the solution from beaker A2 to beaker A3 Step 6 add $$\(18 \mathrm{~cm}^{3}\)$$ of distilled water to beaker $$\(\mathbf{A 3}\)$$ Table 1.1 shows the final concentrations of the hydrochloric acid solution in each beaker. Complete Table 1.1 by calculating the final volume of the solution in beaker A2. Write your answer in Table 1.1. Step 7 label the three test-tubes that contain the bromothymol blue indicator in agar jelly A1, A2 and A3 Step 8 use a syringe to transfer $$\(10 \mathrm{~cm}^{3}\)$$ of the hydrochloric acid solution from beaker $$\(\mathbf{A 1}\)$$ to test-tube A1 Step 9 use a syringe to transfer $$\(10 \mathrm{~cm}^{3}\)$$ of the hydrochloric acid solution from beaker $$\(\mathbf{A} 2\)$$ to test-tube A2 Step 10 use a syringe to transfer $$\(10 \mathrm{~cm}^{3}\)$$ of the hydrochloric acid solution from beaker $$\(\mathbf{A} 3\)$$ to test-tube A3 Step 11 place the three test-tubes into a water-bath at $$\(35^{\circ} \mathrm{C}\)$$ Step 12 start a stop-clock and leave the test-tubes in the water-bath for 20 minutes Step 13 after 20 minutes, remove the test-tubes from the water-bath Step 14 measure the temperature of the water in the water-bath. Fig. 1.1 shows the three test-tubes after they were removed from the water-bath in step 13. Fig. 1.1 (ii) Measure the height of the yellow agar jelly in each test-tube shown in Fig. 1.1. Prepare a table and record the results of the investigation. Include in your table: - your measurements from Fig. 1.1 - the final concentration of hydrochloric acid. (iii) State a conclusion for the investigation. ....................................................................................................................................... . ....................................................................................................................................... . ................................................................................................................................. (iv) Identify the independent variable and the dependent variable in this investigation. independent variable ..................................................................................................... ....................................................................................................................................... . dependent variable ........................................................................................................ ....................................................................................................................................... . (v) Fig. 1.2 shows the reading on the thermometer in step 14. Fig. 1.2 Record the temperature of the water in the water-bath at the end of the investigation, shown in Fig. 1.2. temperature ....................................................... $$\({ }^{\circ} \mathrm{C}\)$$ (vi) The student stated that the change in the temperature of the water-bath was not a significant source of error in the investigation. Explain why the student was correct to state this. ....................................................................................................................................... . ....................................................................................................................................... . ................................................................................................................................. (vii) Suggest a control experiment for this investigation. ....................................................................................................................................... . ....................................................................................................................................... . ................................................................................................................................. (viii) The student repeated the investigation but left the test-tubes in the water-bath for 30 minutes instead of 20 minutes. Suggest how the results for this second investigation would differ from the results for the first investigation. ....................................................................................................................................... . ....................................................................................................................................... . .................................................................................................................................

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