Finding Volume of Irregular Objects by Water Displacement
To find the density of an object, we must first find both the mass and the volume. Mass can be determined by using a balance. The volume of square or rectangular objects is easy to find by measuring the dimensions and multiplying length x width x height. But how do you find the volume of irregularly shaped objects?
We can determine the volume of irregular objects by using water displacement. The volume of the object is equal to the volume of water it displaces. Liquid volume can be translated into solid volume by the relationship 1 millilitre = 1 cubic centimeter, or 1 mL = 1 cm3. Measuring the volume of water displaced by an object will tell us the volume of the object. We can use the balance to determine its mass, and then calculate the density by dividing the mass by the volume.
Density Science Lab Procedure
100 mL graduated cylinders, one per group
Four small objects, such as rocks, marbles or fishing weights, for each group
Students will work in groups. If there is only one balance for the class, have it at the front of the classroom, and each group can take turns coming up and using the balance.
Determine the mass of each object to the nearest 0.1 gram.
Record the mass on the data sheet.
Fill the graduated cylinder about half full with tap water.
Read the volume by looking at the bottom of the meniscus.
Record the starting volume to the nearest 0.1 mL.
Place the first object into the graduated cylinder. Tilt the cylinder and slide the object in slowly so that it does not splash.
Record the ending volume to the nearest 0.1 mL
Remove the object by pouring the water into the sink, and dry it with a paper towel.
Add more water to the cylinder and repeat for the other three objects.
Calculate the volume of each object by using the following formula:
Ending volume – starting volume = volume of object
Calculate the density of each object using the formula: Density = Mass/volume.
The data sheet should contain the following information for each object:
Starting volume of water, mL
Ending volume of water, mL
Difference in volume of water, mL
Volume of object, cm3
Mass of object, grams
Density of object, g/cm3
Answer the following questions on the data sheet:
1. What are the two units we used to express volume? (mL and cm3)
2. What units did we use to express mass? (grams)
3. What is the formula used to calculate density? (mass/volume)
4. What is the method called to find the volume of irregularly shaped objects? (water displacement)
5. Which object had the greatest density? (answers will vary)
6. Which object had the lowest density? (answers will vary)
For homework help and practice problems, see the Density Study Guide.