Density of Water

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Objective

SWBAT gather the data they need to calculate the density of water and identify the source of the units gram and centimeter cubed.

Big Idea

Understanding that a centimeter cube will displace one milliliter of water builds the foundation to understand how the volume of an object can be determined by the displacement of water.

The Need for This Lab

In the Float, Suspend, Sink lesson students predicted that the container suspended in water must be at or near the density of water. In fact the student calculation of the density for the suspending container was actually at or near the density of water.

In this lab students confirm that the density of water is 1 g/cm3.

Using this information, once students know the density of an object they will be able to predict if an object will float, suspend or sink in water. 

Additionally students have a fundamental understanding that will help them later determine the volume of an object based on the displacement of water. The amount of water displaced in milliliters is equivalent to the same number of centimeter cubes.

Investigation Preparation & Summary

5 minutes

Investigation Summary and Standards

In this lab, students collect the data needed to determine the density of 1 ml of water. Students also discover that a one centimeter cube displaces one milliliter of water. This is a big idea, needed for students to find the density of an irregular object. The amount of water displaced in milliliters is equal to the volume of the object in centimeter cubes. Students are also applying two important Mathematical Practices - using the appropriate tool strategically (MP5) and using precise language (e.g. units of measure) as well as attending to precision in measurement (MP6). The ability to follow precisely step-by-step instructions is a desired outcome for students. 

They will also continue to develop habits of work as a scientist by accurately recording their observations (RST.6-8.3 Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks) and respond to a prompt at the end of the lesson design for their reflection. Reflection about the learning helps students think deeply about what they are learning. 

Also in this lesson, students will demonstrate an understanding of mathematics and computational thinking (SP5) by using mathematical representations to describe and/or support scientific conclusions. They will understand the equivalent relationship of 1 milliliter and 1 centimeter cubed. This knowledge will allow them towards constructing a method to determine the volume of any object by the amount of water displaced.

Graduated Cylinders and Triple Beam Balances

As with most labs, I spend a few minutes reviewing how to use the equipment we need for this lab. (A materials list is included in the resources.)

First, I will remind students to read the water level at the center of the meniscus.  (Meniscus is from the Greek, meaning crescent shaped or diminutive moon, so it is one of those many science vocabulary words I can make "visible" for students by sharing its roots.) I share that a concave meniscus is created when the liquid (water) molecules are more strongly attached to the side of the cylinder than to each other. Students need to be at eye level with the meniscus to measure it properly.

I also review with students how to use the triple-beam balance. My students usually need to be reminded about which slide to move first, how to add the numbers from each beam to find the total mass and what to do if the final slide rests between two numbers.

Students in Action

25 minutes

Students in Action

Before placing any water into the graduated cylinder students should determine the mass using their balance. Since we are fortunate enough to have a matching class set of graduated cylinders I have a target mass in mind so I circulate around the room verifying that students are finding a mass for their cylinder that is very close to the actual mass.

Students follow the lab procedure, outlined in the Density of Water Lab. They fill their cylinders to 30 mL with water, using the pipette to add or subtract a few drops as precise measurements are very important. Next, students determine the mass of the graduated cylinder with 30 mL of water. Subtracting the mass of the cylinder from the mass of the cylinder with water students will be able to calculate the mass of the water. Students will find that the average mass of the 30 ml of water is 30 grams.

We could calculate the density of water at this point. Our units would be grams per milliliter. We do have the mass of water in grams and the volume of water in milliliters but we need to go further to be able to calculate the volume of an irregular object. 

So, next students discover what happens to the water level when gram centimeter cubes are added one at a time.

In this short video, I share how students should slide the centimeter cubes into the graduated cylinder. Because of the adhesive property of water - water likes to adhere to other surfaces. If students drop the centimeter cubes into the graduated cylinder with a splash, water droplets will adhere to the side of the cylinder. When students attempt to read the water level for the displacement to determine volume, the water adhering to the side of the cylinder could give them an incorrect reading. 

The table below shows the data I expect to see from students.  Students record the change in volume for 10 centimeter cubes.

Centimeter Cubes

 

ml

0

30

1

31

2

32

3

33

4

34

Connecting the Learning

15 minutes

So what is the density?

I lead students through this calculation since I want to make sure they understand the 1 gram per milliliter is the same as 1 gram per centimeter cubed. This makes an important connection to the lab they will develop next - Density of Irregular Objects.

First as a group, we calculate the density of water from data derived in the first part of the lab. We know the volume of water - 30 ml -- and the mass of 30 ml of water -- which is approximately 30 grams. I ask several student groups for their mass data. We may have to find an average or round to the nearest whole number, but the calculation should be pretty close to this - Density = mass / volume so 30 grams / 30 ml = 1 gram / 1 ml.

Next we will discuss what happened when we put the centimeter cubes into the graduated cylinder. Student results should show that each cube displaced 1 ml of water in the cylinder. I am very clear about using the term displaced. This is usually a new vocabulary word for students.

I walk the students through the logic to help them understand that 1 mL is equivalent to 1 centimeter cubed.

We agree that the density of water can also be expressed as 1 gram / 1 centimeter cubed.

Students cut and paste their data collection sheets into their science journals, then add both calculations for density.