2003 Virginia SOLs
Motivation for Learning
Remind the students that they have been actively engaged in the first step of the scientific method, observation, and that they are now ready to hypothesize. If someone argues that the ice cubes are different, you can take each cube out and put it in the opposite beaker. Someone, sooner or later, will guess that the beakers contain different liquids.
This would be a good time to point out that in the scientific process answering one question often leads to another. Once the discussion has come around to a density or weight difference in the liquid, you can extend your demonstration:
Density is a measure of how much mass per unit volume a certain substance or object has (d = m/V). This property is an intrinsic property to each particular substance. This means that it is a property of the substance and depends on the type of material as opposed to the amount. Extrinsic properties depend on the amount of the material. Some examples of other intrinsic properties include color, melting point, freezing point, and boiling point. Extrinsic properties include volume, mass, and temperature. This activity will help students differentiate between extrinsic properties such as volume or mass alone and intrinsic ones such as density. Because density is intrinsic, as you add more and more of a substance to the sample, the density does not change. The volume and the mass both change as you add more to a sample. This is the relation that will be explored fully in the following activity. The density scale is historically related to the density of water, which is 1.0 g/cm3 (or 1000 kg/m3) at 15°C. In this lab, students will measure volume in mL. 1mL = 1 cm3 .
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Density of Liquids
Show the students the three liquids and tell them what they are. Ask them to devise a way to compare the masses of the three liquids. They should eventually come to the conclusion that they must mass the same volume of each liquid in order to compare the masses fairly. Tell them that scientists and engineers use density to compare masses of different materials. Density (in units of g/cm3) is the mass of 1 cubic centimeter (cm3) of the substance. For liquids, 1 cm3 in volume is equal to 1 mL. The formula for density is:
Density = Mass / Volume
Find the mass of 10 mL of the alcohol and record in the table. If using a triple beam balance, mass the cylinder with the 10 mL of alcohol in it and record this value. Subtract the mass of the cylinder to obtain the mass of the alcohol. Find the density by dividing the mass of just the alcohol by 10 mL. Record the density in the table.
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Students can determine the relative density of several different liquids by placing a hydrometer in each. Hydrometers can be made by putting a penny or other small weight in the bottom of a 75 mL test tube. The liquids being tested can be placed in a 100 mL graduated cylinder. Place the hydrometer in the liquid. It will float, and the sides of the graduated cylinder will keep it from leaning over. Measure the length of the hydrometer that is sticking out of the liquid. Repeat the process with the other liquids. The lengths of the test tube (hydrometer) sticking out of the surface of the liquids can determine their relative densities.
Students can extend this activity by using excel to make this graph of mass vs. volume. The students will make a spreadsheet of the data and then make a graph using excel to show the relationship between mass and volume. For instructions on how to use excel, click here.
Students with Special Needs
Some students may have difficulty manipulating the different liquids in the activity. Click here for further information on laboratories with students with special needs.
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1. As the mass increases, what happens to the volume?
2. As the volume increases, what happens to the mass?
3. How many grams of alcohol (red liquid) would occupy 25 mL?
4. What volume would 65 grams of water (blue liquid) occupy?
5. How many grams of salt water (green liquid) would occupy 80 mL?
Answers to Assessment