Question

31 a special disc can be used to find out how polluted water is. polluted water has dirt in it. the disc is tied to a rope and lowered into water until it can no longer be seen.

yunxi used the same disc to measure the depth at which it can no longer be seen in three different lakes. the results are as shown.

\

$$\begin{array}{|c|c|c|c|} \\hline \\text{lake} & \\multicolumn{3}{c|}{\\text{depth at which disc can no longer be seen (cm)}} \\\\ \\cline{2-4} & 1^{\\text{st}} \\text{ measurement} & 2^{\\text{nd}} \\text{ measurement} & 3^{\\text{rd}} \\text{ measurement} \\\\ \\hline \\text{r} & 345 & 355 & 355 \\\\ \\hline \\text{s} & 750 & 745 & 755 \\\\ \\hline \\text{t} & 580 & 575 & 585 \\\\ \\hline \\end{array}$$

(a) state one variable that was changed.

(b) explain why the results were reliable.

(c) based on yunxis results, identify the lake that was least polluted. explain why.

Explanation:

Identify the changed variable

The independent variable changed by the experimenter is the location of the experiment, which is the different lakes (Lake R, S, and T).

Determine the reason for reliability

The experiment was repeated three times for each lake to ensure consistency and minimize random errors.

Identify the least polluted lake and explain

Lake S has the greatest depth at which the disc can still be seen (average around 750 cm). This indicates that light can penetrate deepest in Lake S, meaning its water is the clearest and therefore the least polluted.

Answer:

Question 31(a)

The lake where the measurement was taken (or the location of the experiment).

Question 31(b)

The measurements were repeated three times for each lake.

Question 31(c)

Lake S. It has the greatest depth at which the disc can still be seen, which means the water is the clearest and contains the least amount of suspended pollutants/dirt.