Density Density is one of the most important measurements in science. It is also a very simple calculation that involves measurements of mass and volume. Q. What are you actually measuring when you calculate density? The amount of matter in an object for its space or volume. Example: Take two cubes that are exactly the same volume. (50cm3) One cube is made of metal and the other is made of wood. metal wood Q. Which cube has greater density? Or which has more mass packed into the cube? A. Of course, the metal cube has greater density. Why?

Density Because if you could look inside each cube at how closely packed the molecules (little particles that make up each cube) are spaced you would see a big difference. (metal) (wood) The particles in the metal cube are packed much more tightly. In the wood cube the particles have a lot of empty spaces between them. Q. So how do we find out how much stuff is packed into an object? A. We measure its mass on the balance!

Density By placing each cube on the balance we can determine their mass and then calculate the density of each cube. Remember they each have the same volume of 50 cm3. 50cm3 50cm3 Mass = 240 grams Mass = 35 grams So how do we calculate the density? Density = Mass divided by volume or (mass goes in the calculator first) M V D =

Density Metal Wood 240g 35g Density = mass volume 240 g 35 g 50cm3 50cm3 240g 35g Density = mass volume 240 g 35 g = 4.8 g/cm3 = .7 g/cm3 50 cm3 50 cm3 The metal cube has a much higher density than the wood cube. Even though they are the same volume (size) the metal has much more mass packed into its space. Remember, density is not how heavy or how big. It is the stuff that is packed inside of an object.

Practice Problems A. 300 g What is the mass? What is volume? What is the density? 4 cm 4 cm 8 cm B. R = 2 cm 250 g What is mass? What is volume? What is density? = 8 cm

Density When you measure density you must compare it to something. Since all substances have mass and volume, therefore all substances have density. When measuring the density of a substance, it is always compared to the density of water. The density of water = 1.0 g/cm3 This means that any substance with a density of greater than 1.0 g/cm3 will sink in water. Any substance with a density of less than 1.0 g/cm3 will float in water. So let’s look at our metal and wood cubes from earlier. metal wood D = 4.8 g/cm3 D = .7 g/cm3

Density Water = 1.0 g/cm3 wood metal 4.8 g/cm3 .7 g/cm3 The metal cube sinks because its density is greater than water. The wood cube floats because its density is less than water.

Density Ice has a density of .9 g/cm3. Does ice sink or float in water? Draw a simple diagram to show what would happen when ice is placed in water. ice