Summary

In this lab, students will use the water displacement method to measure the volume of small objects, measure their mass, and calculate their density. They will also calculate the density of different liquids and predict which objects will float or sink in which liquids.

Grade Level

Elementary and Middle School

Objectives

By the end of this lab, students should be able to

• Measure mass and volume of liquids and solids.
• Calculate density.
• Compare density of liquids and solids.

• Understand how substances dissolved in a solution can affect the density of the solution and whether objects will sink/float in those solutions.

Chemistry Topics

• Density
• Mass
• Volume of liquids and solids
• Measurement

Time

Teacher Preparation: 15 minutes
Lesson: 50 minutes

Materials

Each group will need:

• 2- 250ml beakers
• 1- 100ml graduated cylinder
• ~300 ml tap water
• 1/4 cup salt (approx. 76 g of table salt)
• 4 or 5 small objects that can fit into the opening of a graduated cylinder. (Some options include a stone, a plastic, wood, or metal cube or irregularly shaped object, a piece of candle or crayon, an eraser, a cherry, grape, or other small fruit.
• Digital gram scale
• Paper towels for clean up

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Be careful with glass materials. Report any broken glass immediately to the teacher.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Students should wash their hands thoroughly before leaving the lab.

Teacher Notes

For this activity to work well for students, the objects chosen must be small enough to fit into the opening of a graduated cylinder but large enough to displace enough water so students can discern the change in volume of water using the water displacement method.

Also, if you choose an object that floats in water, you will need to instruct students to use a pencil or their finger to push the object down just under the surface so that the pencil or finger barely touches the surface of the water.

• Background:
  • Mass is the amount of matter in an object. It is measured in grams (g).
  • Volume is the amount of space an object takes up.
    • The volume of a liquid can be found using a beaker or graduated cylinder. It is measured in milliliters (ml).
    • The volume of a solid can be found using the water displacement method in a graduated cylinder. The volume of a solid is measured in cubic centimeters (cm³). The water displacement method is convenient because 1 ml = 1 cm³.
    • To use the water displacement method, add water to a graduated cylinder and note the original volume of the water. Then add the solid. Note the new volume. The new volume minus the original volume of water equals the volume of water displaced by the solid. This is also equal to the volume of the solid in cm³.
  • Density is the amount of mass in a given volume of a substance.
    • Density = mass / volume
    • Density is a physical property of an object and can be used to identify substances.
    • Objects which are more dense than water will sink in water; objects which are less dense than water will float in water.
    • Saltwater solutions are more dense than water because salt water has more mass in the same volume of liquid. 
• Helpful tips:
  • Have materials set up at lab tables when students arrive, including paper towels in case of spills.
• Ideal group size = 2-4 students
  • Groups of 4 can designate a member for each object to determine its mass, volume, density, and if it sinks or floats in water and/or salt water.
• Classroom management tips:
  • Elementary teachers may decide to have the mass of the beakers already labeled on the beakers and the water already poured into the beaker. To do this, place the beaker on the scale while it is empty. Write the mass of the beaker on a label on the beaker. Pour 100mL of water into each beaker and into the graduated cylinder.
  • Mark a line with a dry erase marker on each beaker and graduated cylinder pointing to 100 mL mark so that students can easily identify how much water is in each.
  • Students can sometimes forget that subtractions need to be made for volume of solids and mass of liquids inside the beaker. This can be remedied by setting up the subtraction problem for them on their lab report format or having a reminder taped next to the scale and graduated cylinder.
  • Pre-portion ¼ cup of salt (76g) for each group in a small plastic cup or a separate beaker or graduated cylinder to save time having to do this additional measurement.
• Differentiation:
  • Lower level students can have a lab report worksheet, which includes the measurements noted above and subtraction problem set up.
  • Higher level students can make the additional measurements.
  • Technology option: have students set up a lab report in PowerPoint with one step of the scientific method on each slide (Slide 1 = Purpose, Slide 2 = Research, Slide 3 = Hypothesis…)
  • Early finishers can examine a liquid density column model with a list of substances in the column and their density (out of order). These students can identify which substance is which by putting the density in order from least to greatest, then labeling the substance with the least density at the top and the substance with the greatest density at the bottom.
  • You can find a useful video animation on comparing Density on the AACT website.