Text and flow chart for Exercise 3 & 4
Water may contain bacteria. Bacteria produce substances which can be traced in water.
If the concentration of these substances
is increased, one can draw the conclusion that the water contains bacteria.
The bacteria Ec and Kp produce indol (I). The bacteria Cf and Ec produce acid (S).
The bacteria Ea and Kp produce acetoin (A).
If one has a water sample which contains exactly one type of bacteria, of this bacteria type can be detected by
conducting a test. The flow chart shows how the test is conducted.
Example 3
2. Which of the following statements is/are correct?
I. If decision step Y is labeled “Increase in concentration of S?”, the flow chart may be correct.
II. If decision step X is labeled “Concentration of A normal?”, the flow chart may be correct.
Degree of difficulty: medium
Solution
The correct solution is (A).
| Bacteria in water: | I | S | A |
| Ec | Increased | Increased | Normal |
| Kp | Increased | Normal | Increased |
| Cf | Normal | Increased | Normal |
| Ea | Normal | Normal | Increased |
Statement I is correct:
If I is increased, then you come to decision step Y, since you answer the first question “YES”.
If decision step Y is labelled “Increase in concentration of S?”, an increase in S leads to the decision
in the flow chart that the water sample contains Ec. This is correct, for if the water contains Ec, I and
S are increased. However, if S is normal, then the question in decision step Y is answered “NO” and you
come to the decision “Kp in water”.
This is correct, for if the water contains Kp, I is increased and S is normal.
If I is not increased, then you come to decision step X and the question posed
there decides whether the flow chart is correct. Hence the flow chart may be correct.
Statement II is incorrect:
If decision step X is labelled “Concentration of A normal?”, the flow chart leads to the wrong decision
in two cases: if Ea are in the water or if Cf are in the water.
If Ea are in the water, then I is normal and A is increased. The question in the first decision
step must therefore be answered “NO” and you come to decision step X. If decision step X is
labelled “Concentration of A normal?”, then
the question in decision step X must be answered “NO” and you come to the wrong decision “Cf in water”.
If Cf are in the water, then I is normal and A is normal. If decision step X is labelled “Concentration
of A normal?”, then the question
in decision step X must be answered “YES” and you come to the wrong decision “Ea in water”.
Example 4
2. Which of the following statements is/are correct?
I. If decision stage Y is labelled "Increase in concentration of A?", the flow chart may be correct.
II. If decision stage X is labelled "Concentration of S normal?", the flow chart may be correct.
Degree of difficulty: medium to difficult
Solution
The correct solution is (B).
| Bacteria in water: | I | S | A |
| Ec | Increased | Increased | Normal |
| Kp | Increased | Normal | Increased |
| Cf | Normal | Increased | Normal |
| Ea | Normal | Normal | Increased |
Statement I is incorrect:
If the concentration of I is increased, then you come to decision step Y, since you answer the first
question "YES". If decision step Y is labelled "Increase in concentration of A?", an increase in
A leads to the decision in the flow chart that the water sample contains Ec. This is incorrect,
for if I and A are increased, the water contains Kp.
Statement II is correct:
If I is not increased, you come to decision step X. If decision step X is labelled "Concentration of S normal?",
an increase in S leads to the decision Cf in the flow chart, because the question is answered "NO".
This is correct because the combination of "I normal and S increased" means that the water contains Cf.
If S had a normal concentration level, the flow chart would lead to the decision Ea.
This is correct because the combination of "I normal and S normal" means that the water contains Ea.
