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446939196 396035520 Density Lab SE Key pdf
Chemistry (CHEM253)
Harvard University
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Density Laboratory Answer Key
Vocabulary: buoyancy, density, graduated cylinder, mass, matter, scale, volume
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
[Note: The purpose of these questions is to activate prior knowledge and get students thinking. Students are not expected to know the answers to the Prior Knowledge Questions.]
Of the objects below, circle the ones you think would float in water. Predictions will vary. Correct answers are shown:
Why do some objects float, while others sink?
Answers will vary. [An object floats when the weight of the liquid it displaces is equal to the weight of the object. Objects sink when they are heavier than the equivalent volume of liquid. In other words, objects sink when they are denser than the liquid.]
Gizmo Warm-up The Density Laboratory Gizmo™ allows you to measure a variety of objects, then drop them in water (or other liquid) to see if they sink or float.
- An object’s mass is the amount of matter it contains. The mass of an object can be measured with a calibrated scale like the one shown in the Gizmo. Drag the first object onto the Scale. (This is object 1 .)
What is the mass of object 1? 19 grams (19 g)
- An object’s volume is the amount of space it takes up. The volume of an irregular object can be measured by how much water it displaces in a graduated cylinder. Place object 1 into the Graduated cylinder.
What is the volume of object 1? 14 cm 3 (equivalent to 14 mL)
Note: While milliliters (mL) are used to measure liquid volumes, the equivalent unit cubic centimeters (cm 3 ) are used for solids. Therefore, write the volume of object 1 in cm 3.
- Drag object 1 into the Beaker of liquid. Does it sink or float? It sinks.
Activity A:
Float or sink?
Get the Gizmo ready: Drag object 1 back to the shelf. Check that Liquid density is set to 1 g/mL.
Question: How can you predict whether an object will float or sink?
Observe: Experiment with the different objects in the Gizmo. Try to determine what the floating objects have in common and what the sinking objects have in common.
Form hypothesis: Compare the floating objects, then do the same for the sinking objects.
A. What do the floating objects have in common? Answers will vary.
B. What do the sinking objects have in common? Answers will vary.
- Collect data: Measure the mass and volume of objects 1 through 12, and record whether they float or sink in the table below. Leave the last column blank for now.
Object Mass (g) Volume (cm 3 ) Float or sink? Density (g/cm 3 )
1 19 g 14 cm 3 Sink 1 g/cm 3
2 11 g 9 cm 3 Sink 1 g/cm 3
3 4 g 5 cm 3 Float 0 g/cm 3
4 135 g 7 cm 3 Sink 19 g/cm 3
5 4 g 3 cm 3 Sink 1 g/cm 3
6 78 g 29 cm 3 Sink 2 g/cm 3
7 2 g 21 cm 3 Float 0 g/cm 3
8 24 g 26 cm 3 Float 0 g/cm 3
9 99 g 44 cm 3 Sink 2 g/cm 3
10 42 g 61 cm 3 Float 0 g/cm 3
11 65 g 40 cm 3 Sink 1 g/cm 3
12 104 g 114 cm 3 Float 0 g/cm 3
(Activity A continued on next page)
Activity B:
Liquid density
Get the Gizmo ready: Drag all the objects back onto the shelf. Check that the Liquid density is still 1 g/mL.
Question: How does liquid density affect whether objects float or sink?
- Observe: Place object 1 into the Beaker of liquid. Slowly move the Liquid density slider back and forth. What do you notice?
When the liquid density increases, object 1 starts to float. When the liquid density decreases, object 1 sinks again.
- Form a hypothesis: Buoyancy is the tendency to float. How do you think the liquid density
affects the buoyancy of objects placed in the liquid? Hypotheses will vary.
- Predict: In the table below, write the density of each object. Then predict whether the object will float or sink in each of the fluids. Write “Float” or “Sink” in each empty box of the table. Predictions will vary. Correct results are shown below.
Object Object density
Liquid density 0 g/mL 1 g/mL 2 g/mL
1 1 g/cm 3 Sink Sink Float 2 1 g/cm 3 Sink Sink Float 3 0 g/cm 3 Sink Float Float 4 19 g/cm 3 Sink Sink Sink 5 1 g/cm 3 Sink Sink Float
Test: Test your predictions using the Gizmo. Place a checkmark (/) next to each correct prediction, and an “X” next to each incorrect prediction.
Draw conclusions: What is the relationship between the object density, the liquid density, and the tendency of the object to float?
If an object is less dense than the surrounding liquid, it will float. If an object is denser than the liquid, it will sink.
Extension:
King Hieron’s crown
Get the Gizmo ready: Drag all the objects back onto the shelf. Set the Liquid density to 1 g/mL.
Introduction: In the third century B., King Hieron of Syracuse asked the famous mathematician Archimedes to determine if his crown was made of pure gold. This was a puzzling problem for Archimedes—he knew how to measure the weight of the crown, but how could he measure the volume?
Archimedes solved the problem when he got into his bath and noticed the water spilling over the sides of the tub. He realized that the volume of the displaced water must be equal to the volume of the object placed into the water. Archimedes was so excited by his discovery that he jumped out of the bath and ran through the streets shouting “Eureka!”
Question: How can you tell if a crown is made of solid gold?
- Think about it: Gold is one of the densest substances known, with a density of 19 g/cm 3. If the gold in the crown was mixed with a less-valuable metal like bronze or copper, how would that affect its density?
If the gold is mixed with other metals, the density of the crown will decrease.
- Observe: Drag each of the crowns into the liquid. Based on what you see, which crown do you think is densest? Explain why you think so.
Crown B sinks most quickly in the liquid, so crown B is probably the densest crown.
- Measure: Find the mass, volume, and density of each of the three crowns.
Crown Mass (g) Volume (cm 3 ) Density (g/cm 3 )
A 325 g 65 cm 3 5 g/cm 3
B 1250 g 65 cm 3 19 g/cm 3
C 306 g 65 cm 3 4 g/cm 3
- Draw conclusions: Which of the three crowns was made of gold? Crown 3
Explain: Only crown B had a density close to the density of pure gold, 19 g/cm 3_._
446939196 396035520 Density Lab SE Key pdf
Course: Chemistry (CHEM253)
University: Harvard University
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