Two variables in this investigation are location of trap and size of jar.

Transcription

1 An eighth-grade class wants to identify a representative sample of the crawling and flying insects living in the schoolyard. The students build and set five traps like the one shown below. They place the traps in the listed locations at the end of the school day and check them the following morning to see what they have caught. Two variables in this investigation are location of trap and size of jar. In your Answer Document, explain how each of these variables could affect the conclusions of the investigation. Explain why the students collecting procedures will not allow them to collect representatives of all insects living in the schoolyard. Describe one way the students could change their collecting procedures to correct for this weakness. (4 points)

2 What is Expected There are many ways the student can answer this question correctly. The size and location of the trap will affect the types of insects that can be captured. Small jars may not capture large insects. Jars placed on the ground may not capture flying insects. This would prevent students from gathering a representative sample of the insects in the schoolyard. To capture a more representative sample, students may need to use other types of traps, such as butterfly nets, to capture flying insects. Students may also need to use a variety of jar sizes to capture small and large insects.

3 Baking Soda and Vinegar Reaction Students perform a chemistry experiment by mixing baking soda with vinegar. They mix 4 grams of baking soda with 50 grams of vinegar in a 6-gram plastic cup as shown below. The cup is left uncovered. The students record the mass and temperature of the mixture every 5 seconds. They continue these observations until 10 seconds after they see the mixture stop bubbling. Their data are shown in the table below. Which graphs show the trends in the mass (M) and temperature (T) changes during the baking soda and vinegar reaction shown in the diagram?

4 A. B. C. D.

5 B.The mass decreased (went down) over time and the temperature decreased (went down) over time. May 01, 2012

6 Question 31 The data in the table show the temperature of the cup-baking soda-vinegar system. The temperature of the surrounding air was not measured. Assuming that the total amount of energy remains constant, what conclusion about energy transfer does the temperature data support? A. Thermal energy was lost from the surrounding air and the system. B. Thermal energy was gained by the system and the surrounding air. C. Thermal energy was transferred from the surrounding air to the system. D. Thermal energy was transferred from the system to the surrounding air.

7 D. Thermal energy was transferred from the system to the surrounding air. The system got colder; thermal energy decreased. But since energy must be conserved (stay the same) it had to be transferred somewhere. This means that if energy decreases somewhere, then it must increase somewhere else because energy cannot be lost. The only place for this extra energy was the air. So, the air must have warmed up, which is the same as increasing in thermal energy. May 01, 2012

8 Question 32 The data show that the mass of the cup and its contents decreases while the mixture is bubbling. In your Answer Document, explain why the mass of the cup and its contents at 25 seconds is less than the initial total mass of the cup, baking soda and vinegar. Include in your answer a valid scientific reason on which you based your explanation. (2 points)

9 What is Expected Because the mixture bubbles, it must be producing (making) gas. The new gas will escape the cup and mass will be removed from the cup. The student should then explain that the mass of the escaped gas will be equal to the difference between the initial (starting) mass and the final mass (6 grams). May 01, 2012

10 Question 7 A student studying rock densities needs to measure the volume of a small rock sample to the nearest milliliter (ml). The student knows that the rock sample has a volume of at least 5 ml. Which tool should the student use to get the most accurate measure of the volume of water displaced by the rock?

11 C. Students need to measure the rock volume to the nearest milliliter. The students would then need to pick a tool that has the smallest increments of measurement (smallest space between graduation marks). In this case the tool for option C has the smallest increments of measurement, down to the 1ml. May 01, 2012

12 Two students are studying the effect of insects on plant growth. They prevent insects from eating some plants by covering the plants with a transparent mesh cloth. The students leave the other plants uncovered. Both students use the same pots and the same type of soil for each plant. The table shows details of their investigations. Which statement describes a weakness in the investigations? A. Student X used an equal number of plants. B. Student X exposed the plants to varying sunlight. C. Student Y covered half the plants with mesh cloth. D. Student Y potted the plants in the same type of soil.

13 B. Student X exposed the plants to varying sunlight. Growing the plants in varying (different) conditions represents a weakness in the investigation, because more than one variable (factor) is being changed at a time. Usually, in an investigation, only one factor is changed (made different) at a time. May 01, 2012