Read a biology experiment: variables, controls, repeated trials

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. Read this first

   Two classes grew the same plant (example data)

   class	plants	mean height (cm)
   A	8	18
   B	24	14

2. 1.

   Esme combines the two classes into one data set. What is the mean height of all the plants?

   • 32 cm
   • 14 cm
   • 16 cm
   • 15 cm

3. Result (example data)

   The plants given plant food sit by the window, and they are taller, 18 cm against 15 cm. Explanation 1: the plant food helped. Explanation 2: the window light helped.

4. 2.

   Jun reads the result. Which extra test best separates the two explanations?

   • Add a third group with extra plant food by the window
   • Repeat the test with the same positions and many more plants in each
   • Measure the plants by the window more often, to be sure of their heights
   • Put some fed and some unfed plants both by the window and away from it

5. Four trays (example data)

   tray	treatment	mean height (cm)
   A	plant food	18
   B	plant food	15
   C	no plant food	14
   D	no plant food	12

6. 3.

   Kira had two trays for each treatment. What do the means show about a tray effect?

   • Trays A and D show the effect clearly: 18 cm against 12 cm, a 6 cm gap
   • Trays given the same treatment differ by 2 to 3 cm, so plant food is unclear
   • Fed plants are clearly taller on average: 16.5 cm against 13 cm
   • Tray effects do not matter, since each tray held many plants to average

7. Six lakes (model data)

   lake	phosphorus (µg per L)	algae, as chlorophyll (µg per L)
   1	10	3
   2	20	6
   3	35	9
   4	50	14
   5	70	20
   6	90	27

8. 4.

   Luz sees both numbers rise together across lakes. Does this show phosphorus causes more algae?

   • Yes: the algae rise as phosphorus rises in each of the six lakes studied
   • No: sunlight or warmth could drive both; adding phosphorus would test it
   • Yes: the pattern is almost a straight line, so one must cause the other one
   • No: six lakes are too few to show any pattern in the data
9. 5.

   The first 10 minutes gave 3 mL per minute. Milo predicted 90 mL at 30 minutes from the first 10 minutes. The reading was 62 mL. What does this show?

   • The straight-line model fits the early readings, but the rate slowed later
   • The 62 mL reading must be a measuring error, since the model said 90 mL
   • The yeast stopped making gas at minute 10, when the rate was highest
   • The model is right, since gas is still rising at 30 minutes, just as it predicted
10. 6.

    Noah uses the model for predictions. Which prediction is least trustworthy?

    • No prediction is doubtful, since a model that fits days 2 to 6 fits the later days too
    • Day 60: 94 cm, far outside the days from 2 to 6
    • Day 3: 8.5 cm
    • Day 5: 11.5 cm
11. 7.

    Seedling A, height = 4 + 1.5 × day; seedling B, height = 1 + 2.5 × day. Pia wants to know when seedling B is as tall as seedling A. On which day are they the same height?

    • At no point, since seedling A starts taller than seedling B
    • Day 1.2, since 3 ÷ 2.5 = 1.2
    • Day 3, when both are 8.5 cm tall
    • Day 2, since 4 − 1 = 3 and 3 ÷ 1.5 = 2

12. Bean seeds sprouted of 50 per dish after 7 days (example data)

    salt (g per L)	seeds sprouted
    0	46
    2	40
    4	21

13. 8.

    Ren writes up the results. Which claim, evidence and reasoning is best?

    • Salt lowers sprouting. Evidence: 46, 40, then 21 of 50 as salt rose. Reasoning: salt pulls water out of seeds.
    • Salt at 2 g helps. Evidence: 40 sprouted at 2 g, which is many. Reasoning: seeds need minerals to sprout.
    • Salt ruins seeds. Evidence: 21 of 50 sprouted at 4 g. Reasoning: seeds need water, and salt is a chemical.
    • Salt lowers sprouting. Evidence: seeds are alive. Reasoning: living things dislike salt and avoid it.
14. 9.

    Working explanation: warmer water speeds up the yeast. Tala wants new data to test the explanation. Which new result would most challenge it?

    • Less gas at 50 °C than at 40 °C, which would show that heat can harm yeast
    • Bubbles forming faster at 50 °C, which would fit the pattern
    • Three bottles at 40 °C giving 21, 22 and 23 mL, close to the mean
    • More gas at 35 °C than at 30 °C, which would fit the pattern in the table

15. Two yeast studies (example data)

    study	bottles per temperature	yeast batches	how gas was measured
    A	4	1	syringe volume
    B	1	3	bubbles counted by eye

16. 10.

    Both studies found more gas at 40 °C. Zee compares them. Which statement is best?

    • A is stronger: replicates, one yeast batch and a measured volume cut chance
    • B is stronger, since it used three yeast batches against one for A
    • B is stronger, since counting bubbles is a more direct measure of gas
    • They are equally strong, since both found more gas at the same temperature