Scientific Investigation

Sample questions

1. Ocean Temperature and Salinity Profiles

   Research ships lowered an instrument package into the ocean at two sites. As it sank to a depth of 1,000 m, the package recorded temperature and salinity, the mass of dissolved salt in grams per kilogram of seawater (written ppt). Site X lies in a tropical sea. Site Y lies in a subpolar sea.

   Before the first cast at each site, the temperature sensor was held for 5 minutes in a tank of well-stirred ice water. It read 0.0 °C each time. At Site X, the cast was repeated 3 hours after the first. The two sets of readings differed by no more than 0.1 °C at every depth, and only the first cast is reported here.

   Table 1: Temperature

   Depth (m)	Site X (°C)	Site Y (°C)
   0	27.5	4.1
   100	24.0	4.3
   200	15.2	4.0
   400	9.5	3.7
   600	6.8	3.4
   800	5.2	3.0
   1000	4.4	2.8

   Table 2: Salinity

   Depth (m)	Site X (ppt)	Site Y (ppt)
   0	35.2	33.6
   100	35.6	33.8
   200	35.2	34.2
   400	34.6	34.5
   600	34.4	34.6
   800	34.5	34.7
   1000	34.6	34.7

   Question 1

   Why did the researchers hold the temperature sensor in ice water before the first cast at each site?

   • A.

     To find out how the salinity of the water changes with temperature

   • B.

     To determine whether the sensor's readings depend on depth

   • C.

     To increase the number of temperature readings collected

   • D.

     To confirm that the sensor read correctly at a known temperature

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2. Question 2

   At Site X the cast was repeated 3 hours after the first one. The most likely reason was to:

   • A.

     measure how quickly the instrument package sinks on a second trip

   • B.

     test whether salinity changes the temperature readings that the sensor gives

   • C.

     check that the readings could be reproduced and were not a brief change

   • D.

     collect data at depths below 1,000 m, where the water is colder

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3. The Green Sahara

   From about 11,000 to 5,000 years ago, much of the Sahara held lakes and grassland. Two scientists explain this wet period using the same lake-sediment cores. Each core layer was dated by measuring radioactive carbon in tiny plant fragments, and pollen and wind-blown dust were counted in a sample from the same layer. Table 1 gives summer sunlight over northern Africa and rainfall estimated from lake levels. Table 2 gives grass pollen and dust in the sediment.

   Scientist 1 Slow changes in Earth's orbit increased summer sunlight over northern Africa. The extra sunlight heated the land more than the nearby ocean, which pulled moist air inland and strengthened the monsoon. Rainfall therefore rose and fell with the sunlight, and plants simply responded to the rain. Scientist 1 cites sunlight, rainfall and dust.

   Scientist 2 The orbital change gave only a modest push. Plants made it larger: grass-covered land releases more water vapor than bare sand does, so more rain fell, which supported more plants, which released still more vapor. A small change in sunlight could therefore produce a large change in rainfall, and the region could dry abruptly once plant cover fell below a critical amount. Scientist 2 cites rainfall and grass pollen.

   Table 1

   Age (thousand years ago)	Summer sunlight (% above today)	Estimated rainfall (mm per year)
   12	7	200
   10	8	450
   8	6	600
   6	4	450
   4	2	300
   2	1	150

   Table 2

   Age (thousand years ago)	Grass pollen (% of grains)	Dust (% of sediment)
   12	5	70
   10	40	22
   8	62	8
   6	45	18
   4	10	55
   2	3	75

   Question 3

   Why did the researchers count pollen and dust in a sample taken from the same layer that was dated?

   • A.

     So the pollen grains and dust grains would be the same size

   • B.

     So they could show that sunlight caused the dust to blow

   • C.

     So the pollen and dust counts could be assigned to the age of that layer

   • D.

     So they would have more layers to date

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4. Question 4

   Scientist 2's argument draws on one measurement from the cores that Scientist 1's argument does not. That measurement is the:

   • A.

     summer sunlight

   • B.

     percent of dust

   • C.

     estimated rainfall

   • D.

     percent of grass pollen

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5. Bleach and Fading Dye

   Bleach clears a colored dye by reacting with the dye's molecules and destroying them. In every trial below, 20 mL of dye solution at 22 °C was stirred in a clear beaker, and drops of bleach of identical strength were added. The bleach was always in large excess over the dye. A timer was started when the bleach was added, and the time for the solution to turn completely clear was recorded. The rate of fading is the amount of dye destroyed per second. Each trial was done once the beaker had been rinsed with distilled water and the solution had been brought to 22 °C in a water bath, so that the starting conditions matched.

   Experiment 1 The dye solution had a relative concentration of 1.0 in every trial. Different numbers of bleach drops were added, and the time to clear was recorded for each number.

   Table 1

   Bleach (drops)	Time to clear (s)
   1	240
   2	120
   4	60
   8	30

   Experiment 2 Two drops of bleach were added to dye solutions of different relative concentrations. A concentration of 1.0 is the one used in Experiment 1, and a concentration of 2.0 has twice as many dye molecules in the same volume.

   Table 2

   Dye (relative concentration)	Time to clear (s)
   0.5	121
   1.0	120
   2.0	119
   4.0	120

   Question 5

   Experiment 2 was designed to determine whether:

   • A.

     the amount of dye in the solution affects the time needed for two drops of bleach to clear it

   • B.

     adding more drops of bleach shortens the time needed to clear the dye

   • C.

     the temperature of the dye solution affects the time needed to clear it

   • D.

     the dye or the bleach is used up first in the reaction

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