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What is Density? Density is defined as mass per volume. You can think of it as how many particles of a substance are packed into a certain amount of space. If the particles are packed tightly together, the density would be greater than if they are loosely packed with a lot of empty space around them. Here are some examples: Two people get into an elevator on the 10th floor. As the elevator goes down, it stops at each floor and two more people get in. The density of people in the elevator increases as it descends, because there are more people crowded together in the same amount of space. On the 10th floor, there is a lot of space between the people and not very much mass in the elevator. As more and more people get in, the mass increases and the people are crowded closer and closer together, but the volume (amount of space inside the elevator) remains the same. Therefore, the density is much greater when the elevator reaches the ground floor than it was at the 10th floor. If you put 10 Styrofoam peanuts into a shoe box, there is a lot of space between them, and the density is low. If you cram 1,000 Styrofoam peanuts into the shoe box, the mass will increase but the volume will remain the same, so the density will increase. Name: ___________________ 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 milliliter = 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. Procedure: Materials: Balance 100 mL graduated cylinders, one per group Four small objects, such as rocks, marbles or fishing weights, for each group Water Procedure: 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. 1. 2. 3. 4. 5. 6. 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. 7. Record the ending volume to the nearest 0.1 mL 8. Remove the object by pouring the water into the sink, and dry it with a paper towel. 9. Add more water to the cylinder and repeat for the other three objects 10. Calculate the volume of each object by using the following formula: 11. Ending volume - starting volume = volume of object 12. Calculate the density of each object using the formula: Density = Mass/volume Results: Create a data table containing the following information for each object: 1. 2. 3. 4. 5. 6. Starting volume of water, (mL) Ending volume of water, (mL) Difference in volume of water, (mL) Volume of object, (cm3) Mass of object, (g) Density of object, (g/cm3) Conclusion: Answer the following questions on the data sheet: 1. What are the two units we used to express volume? (mL and cm3) 2. What unit did we use to express mass? (grams) 3. What is the formula used to calculate density? (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) Name: _____________________ Practice Problems The formula for calculating density is D = M/V Show all work including formula! Include all units. 1. A rock has a mass of 20 grams and a volume of 4 cubic centimeters. What is its density? Solution: 20 g divided by 4 cm3 = 5 g/cm3 2. A piece of aluminum has a volume of 6 cm3 and a mass of 16.2 g. What is its density? Solution: 16.2 g divided by 6 cm3 = 2.7 g/cm3 3. A gold nugget has a density of 19.3 g/cm3 and a mass of 135.1. What is its volume? Solution: In this problem, you know the density and the mass, but not the volume. if you know two of the values, you can solve for the third by rearranging your formula, D= M/V, to solve for volume. The new formula is V = M/D. 135.1 g divided by 19.3 g/cm3 = 7 cm3 4. A brick has a density of 6 g/cm3 and a volume of 150 cm3. What is its mass? Solution: Here again you have two of the values, and you can solve for the third by rearranging your formula to solve for mass. The new formula is M = D x V. 6 g/cm3 x 130 cm3 = 900 g. Read more: http://www.brighthub.com/education/homeworktips/articles/52127.aspx#ixzz0VFSVGkDm For homework help and practice problems, see the Density Study Guide. Read more: http://www.brighthub.com/education/k12/articles/53612.aspx#ixzz0VFPvTFPe