A technician set up four test tubes to find out how quickly exhaled air changes the colour of hydrogencarbonate indicator solution. Different volumes of exh...
Question 1 Report
A technician set up four test tubes to find out how quickly exhaled air changes the colour of hydrogencarbonate indicator solution. Different volumes of exhaled air were bubbled gently through the same volume of indicator, and the time taken for the solution to turn yellow was recorded. The results are shown in Fig. 4.1 and in Table 4.1.
The diagram shows the apparatus used to bubble exhaled air through the indicator.
Table 4.1
| Volume of exhaled air / cm3 | Time to turn yellow / s | Rate of colour change (1000 / time) / per s |
|---|---|---|
| 20 | 50 | 20.0 |
| 40 | 28 | ______ |
| 60 | 18 | ______ |
| 80 | 12 | 83.3 |
(a) Name the colour that hydrogencarbonate indicator changes to when carbon dioxide is added. [1]
(b) Measure the height of the bar for 40 cm3 of exhaled air in Fig. 4.1, in cm. [1]
(c) Fig. 4.1 is drawn so that 1.0 cm represents 5 seconds. Use your measurement to state the time for the 40 cm3 tube. [1]
(d) Measure the difference in bar height, in cm, between the 20 cm3 tube and the 80 cm3 tube. [1]
(e) Complete Table 4.1 by calculating the rate of colour change for the 40 cm3 and 60 cm3 tubes. [2]
(f) Describe the relationship between the volume of exhaled air and the time taken for the solution to turn yellow. [3]
(g) State the independent variable and one variable that must be controlled. [2]
(h) Explain why exhaled air turns the indicator yellow. [2]
(i) A control tube had room air bubbled through for the same time. Predict and explain its colour. [2]
(j) Suggest two ways to make the timing of the colour change more reliable. [2]
(k) Plan how you would adapt this investigation to compare exhaled air before and after exercise. [3]
Answer Details
This question tests hydrogencarbonate indicator, ruler and scale readings, calculating a rate, describing an inverse relationship, and planning an extension.
(a) When carbon dioxide is added, hydrogencarbonate indicator turns yellow. [1]
(b) Line the ruler on the baseline and read up to the top of the 40 cm3 bar. Using the chart scale this bar corresponds to a time of 28 s, and since 1.0 cm represents 5 s, the bar is \(28 \div 5 = 5.6\) cm tall. Record about 5.6 cm. [1]
(c) Convert the measured height using the scale: \(5.6 \times 5 = 28\) s. [1]
(d) The 20 cm3 tube (50 s) is \(50\div5 = 10.0\) cm tall and the 80 cm3 tube (12 s) is \(12\div5 = 2.4\) cm tall, so the difference in bar height is \(10.0 - 2.4 = 7.6\) cm. [1]
(e) Rate of colour change is \(1000 \div \text{time}\): for 40 cm3, \(1000\div28 = 35.7\); for 60 cm3, \(1000\div18 = 55.6\). One mark each. Completed table: [2]
| Volume of exhaled air / cm3 | Time to turn yellow / s | Rate (1000 / time) / per s |
|---|---|---|
| 20 | 50 | 20.0 |
| 40 | 28 | 35.7 |
| 60 | 18 | 55.6 |
| 80 | 12 | 83.3 |
(f) As the volume of exhaled air increases, the time to turn yellow decreases [1]; quote data, for example 20 cm3 took 50 s but 80 cm3 took only 12 s [1]; so the rate of colour change increases and the relationship is inverse [1]. [3]
(g) Independent variable: the volume of exhaled air [1]. One controlled variable (any one): the same volume and concentration of indicator, the same temperature, or the same observer judging the colour [1]. [2]
(h) The carbon dioxide in exhaled air dissolves to form an acid (carbonic acid) [1], which lowers the pH of the indicator, and the fall in pH turns it yellow [1]. [2]
(i) A room-air control turns the indicator yellow much more slowly, or leaves it orange/red [1], because room air contains far less carbon dioxide than exhaled air [1]. [2]
(j) Any two ways to make the timing more reliable: use a colour reference to judge the end point, repeat and take a mean, use a colorimeter or light sensor, or keep the same observer. [2]
(k) To compare before and after exercise: [3]
- collect a set volume of exhaled air at rest and time the colour change [1];
- exercise, then repeat with the same volume of exhaled air and the same indicator [1];
- predict a faster colour change after exercise, because more respiration produces more carbon dioxide [1].
Exam tip: more carbon dioxide means faster colour change and shorter time, so time and rate move in opposite directions.
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