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Students will begin their exploration in Investigation Two by applying what they have learned about qualitative and quantitative properties of matter and how to determine the quantitative property of mass with a new Procedural Tool, the triple beam balance.

INVESTIGATE

1. Remember that safety goggles are to be worn while handling chemicals.

2. Each student group should obtain the following necessary materials from the distribution point: one (1) woodblock, one (1) metal cube, one (1) 100 ml beaker of water, one (1) 100 ml beaker of baking soda (sodium bicarbonate), and one (1) empty 100 ml beaker.

 

Trial 1

Students ended Investigation One by completing steps to answer the question “What are the masses of the five samples?” The five samples that you were given were: a woodblock, a metal cube, water, baking soda, and the large gram bear. During the last trial of Investigation One, they used the triple beam balance to determine the mass of the woodblock, large gram cube, and metal cube.

In this Trial, students will gain additional practice with the triple beam balance and learn to account for the mass of a container when determining the mass of an object or substance. They will use a weigh dish while finding the masses of the metal cube and the woodblock and compare the masses obtained using the weigh dish to those obtained without a weigh dish. Through comparison, students should learn that a container can be used when determining the mass of the object, but its own mass must be subtracted from the total mass in order to accurately measure the mass of the object.

As students begin the lab portion of Investigation Two, they should remember the question from Investigation One that they will continue to investigate: What are the masses of the five samples?

b. Before students begin investigating the mass of the water and baking soda samples, they will explore using a container when determining mass rather than placing the objects directly onto the platform of the triple beam balance. As students conduct this Trial they should think about the following question: How does using a container when determining mass affect the measurement of mass indicated on the triple beam balance?

c. Encourage students to first record the mass of the woodblock and metal cube from Investigation 1 in Problem 1a of their Scientist Data Record.

d. Direct students to locate the weigh dish on their work table.

e. Guide students through obtaining the mass of the woodblock while using the weigh dish as follows:

• • Direct students to add the metal cube to the weigh dish and determine the mass. You should record the combined mass in Problem 1b.
  • Direct students to remove the metal cube and add the woodblock to the weigh dish, then record the mass in Problem 1c.

f. Encourage students to compare the masses obtained for the woodblock and metal cube when using the weigh dish to that without the weigh dish. Ask students: Why were the two masses different? Students should suggest that the masses were different because when the woodblock and metal cube were placed in the container they were measuring the mass of either object plus that of the container.

g. Ask students: If you had to use the weigh dish, but only wanted to know the mass of the metal cube, what could you do? Students should suggest obtaining the mass of the weigh dish and subtracting it from the total combined mass:

Combined mass of metal cube and weigh dish – mass of weigh dish = mass of metal cube

h. Tell students to try the solution they suggested, by determining the mass of the weigh dish alone.

• • Students should record the mass in Problem 1d.
  • Next direct students to subtract the mass of the weigh dish from the combined masses to obtain the mass of the woodblock and metal cube and record their answers in Problem 1e.
  • Encourage students to compare these measurements to those in problem 1a. Ask students: Is there a difference in mass? Why not? Student answers may vary. Students should indicate that there is no difference in mass because the mass of the container was independent of the mass of the object. However, some students may experience a slight difference in mass due to slight differences between wood blocks or between metal cubes if the cubes and blocks used by the groups in Investigation 2 are different from those used to obtain the original mass in Investigation 1. This may lead them to the conclusion that using a container does have an effect on the mass of the object. To minimize this problem, provide groups access to the same set of objects they used in Investigation 1 for Investigation 2 if possible. If this is not possible, suggest that students obtain the masses of the woodblock and metal cube without the weigh dish and compare these masses to ones obtained when using the weigh dish.

Trials 2 and 3

In these Trials, students will work to determine the mass of the remainder of the five samples; baking soda and water. In doing so they will continue to practice using a container when determining the mass of an object or substance. Students will be asked the following questions at the conclusion of the Trials:

a. Direct students to locate the beakers of baking soda and water on their lab table. Ask students: How would you find the mass of the baking soda or water without also finding the mass of the beaker? Student answers may vary. If students poured the baking soda or water directly into the pan of the pan balance during Investigation 1, they may suggest that the baking soda should be poured directly on the pan. Other students may suggest using a weigh dish and subtracting the mass of the weigh dish. Other students may suggest keeping the samples in the beakers and then subtracting the masses of the beakers.

b. Encourage students to think about whether the weigh dish or beaker would be suitable for both samples by posing the following question: Why would it be better to use the beaker or the weigh dish for the baking soda and water? Student answers may vary. Students should suggest that it might be difficult to recover the full amount of baking soda from the platform of the triple beam balance, that the baking soda might damage the platform or would roll off the platform before its mass could be determined. Students should suggest that a container is a better way to measure a liquid because without a container the full amount of liquid would not stay on the platform, therefore the mass when determined would not be accurate. Students should also suggest that it might not be desirable to have some liquids in direct contact with the platform because they might damage the platform and that having such liquids spill on the work surface might also be hazardous.

c. Would it be better to use a beaker or a weigh dish for the baking soda? Either the beaker or weigh dish would be appropriate for determining the mass of the baking soda sample. If the beaker is used, the student would obtain the mass, pour the baking soda into another container, obtain the mass of the beaker alone and subtract the mass of the beaker from the combined mass. However, one drawback of using the beaker in which the baking soda was contained is that some baking soda may still cling to the beaker after it is emptied. Although the amount remaining in the beaker would be minimal, the mass technically would not represent the beaker’s mass. It is important for students to understand this concept. An alternative would be to use a paper towel to wipe the beaker clean or choose a clean beaker, obtain its mass first, add the baking soda, obtain the combined mass and then subtract the mass of the clean beaker.

d. Would it be better to use a beaker or weigh dish for the water? Student answers may vary. Some students may opt to choose the weigh dish because that is what they used for the woodblock, metal cube, and baking soda. Other students may choose the beaker, possibly by associating a beaker with a household measuring cup, or by recalling from experience that wide, relatively flat containers are not suitable for containing large amounts of liquids. Accept all answers.

• • Direct students to pour the water into the weigh dish. Tell them to pick up the weigh dish and water with two hands, holding opposite sides. Ask students: What happens to the water when you move the weigh dish? Students should indicate that the water shifts too easily within the weigh dish. Students should also notice that the weigh dish was somewhat unstable as the liquid moved.
  • Direct students to pour the water carefully back into its beaker and study how the water moves in the beaker when it is moved. Ask students: Why might it be better to use a beaker when measuring a liquid? Student answers will vary. Students should suggest that a taller, stiffer container such as a beaker is a better choice because it is more stable, and the liquid is less likely to spill when the beaker containing liquid is removed from the platform.

e. Direct students to determine the mass of the baking soda and water and record their results in Problem 2. If necessary, help students recall the steps for using a container when determining mass.

Trial 4

In this trial, students will explore the concept that mass is dependent on the size of the sample. Students will test how decreasing or increasing the amount of matter decreases or increases the mass obtained by massing a reduced or increased quantity of baking soda and a reduced or increased quantity of water.

a. Initiate this part of the investigation by asking the students to make a prediction based on the following questions: Does a sample of baking soda always have the same mass? Does a sample of water always have the same mass?

b. Ask students: How could you test your prediction?

c. Provide time for students to develop and record a plan for testing their prediction in Problem 4b:

• • Provide access to additional baking soda and water if students choose to test by determining the mass of a larger quantity of each sample

OR

• • additional weigh dishes or beakers if students choose to remove some baking soda and water from their samples and determine the mass of the smaller quantities.

d. If students have difficulty in formulating a plan, suggest the following:

• • Determine the mass of half of the baking soda.
  • Determine the mass of 30 ml of water

e. Direct students to record the approximate quantities and mass of the quantities they chose to test in Problem 4.