Understanding Density

Understanding Density Focus Questions: 1. What is the difference between mass, volume and density? 2. How do you find the volume of a substance? 3. How do you find the mass of an object? 4. How do you find the density of an object? 5. How does the density of an object affect its ability to float? • Volume is how much space an object (or amount of matter) takes up. • Mass is a little more complicated. Mass is the weight of an object on Earth (where gravity is the same everywhere on the planet). Mass is actually more specific than weight. Mass is really the “quantity of matter” that makes up an object (so it is the same on Earth or on the moon or anywhere in the Universe), but it would weigh less on the moon than on Earth because the moon has less gravity. So, again, mass is really the “quantity of matter” that makes up an object no matter what the gravity. • Density = Mass/Volume, which means that density is how heavy an object is compared to its size – so how closely its molecules are “packed” together. For instance, generally, a rubber ball is less dense than a baseball of the same size. Let’s look at what that looks like below. Each ball has the same volume. The baseball, however, is made up of more “matter” so has more mass per unit of volume. The baseball is denser than the rubber ball – it has more mass per unit of volume than the rubber ball. It has a higher density. baseball rubber ball Density = Mass/Volume also means that the larger the volume of an object compared to its mass, the less dense it is. For example, if a marble weighs the same as a much larger foam ball, the marble has a higher density than the foam ball, despite its smaller size. Let’s look at what that looks like below. marble foam ball Density and Water Scientists often equate the density of an object by how it compares to the density of water. Here are some important facts about density and water: 1) When an object is put in a body of water, it displaces its volume (size) of water. 2) If an object floats, it is less dense than water. 3) If the object sinks, it is more dense than water. That is why a life raft floats (it is less dense than water) and an anchor sinks (it is more dense than water). Finding the Density of Water Activity Do this simple activity to find the density of water. Keep this equation in mind: Density = Mass/Volume Materials: scale, beaker, water Procedure: 1) Weigh an empty beaker to find out its mass. (In this example, the beaker will be about 20 gm.) 2) Fill the beaker with 100ml of water. (100ml is the weater’s volume.) 3) Then reweigh the beaker with the water in it. (It will be about 120 gm.) 4) Subtract the mass of the empty beaker from the mass of the full beaker and you have the mass of the water. 120 gm – 20 gm = 100 gm. So you now have the mass of the water and the volume of the water. To find the density of water use the equation: Density = Mass/Volume: Density = 100gm/100 ml – So water has the density of 1 gram per milliliter (1gm/ml). Things with a density of less than 1gm/ml will float and those with a density of more than 1gm/ml will sink. Density Saving Your Life... We are made mostly of water, so our density is close to that of water, which means that we do float, but so close to water level that usually not enough of the head will be out to breathe. That is why life jackets are required for boaters. How does wearing a life jacket allow us to float? Life jackets are made to have very low density, with material that traps air inside. This allows the life jacket, and the people in them, to float. Buoyancy and Displacement – An Experiment Focus Questions: 1) When you put an object in water, how much water dies it displace? 2) Do two objects that weigh the same, but have different shapes, displace the same amount of water? 3) Do floating objects weigh as much as the water they displace? 4) Why would one object, that is shaped like a boat, float when the other object, shaped like a ball, sinks? Materials: • two plum-sized balls of clay • two pie pans • two beakers • two measuring cups • roll of paper towels • water • scale Procedures (see illustrated procedures on next page): 1) Take two balls of clay about the same size. Mold one into a boat shape. 2) Place the clay boat and ball into two beakers of water to see if they float. 3) Place each beaker in the center of each pie pan. 4) Carefully fill each beaker full of water to the very top. Don’t overflow into pie pan. 5) Place the clay ball into one beaker letting the water it displaces flow over into the pie pan. 6) Place the clay boat into the other beaker letting the water it displaces flow over into the pie pan. 7) Move the beakers out of the pie pans onto paper towels and carefully pour the water from each pie pan into the two separate measuring cups. 8) Compare the two measuring cups. Is the water level about the same? Critical Thinking Before the Experiment: 1) Do you think the amount of water in the measuring cup will be the same for the clay ball and the boat? ________ why? _____________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ Buoyancy and Displacement Experiment - Procedures: 1) Take two balls of clay about the same size. Mold one into a boat shape. 2) Place the clay boat and ball into two beakers of water to see if they float. 3) Place each beaker in the center of each pie pan. 4) Carefully fill each beaker full of water to the very top. Don’t overflow into pie pan. 5) Place the clay ball into one beaker letting the water it displaces flow over into the pie pan. 6) Place the clay boat into the other beaker letting the water it displaces flow over into the pie pan. 7) Move the beakers out of the pie pans onto paper towels and carefully pour the water from each pie pan into the two separate measuring cups. 8) Compare the two measuring cups. Is the water level about the same? This shows us that two objects that weigh the same will displace the same amount of water – even though they have different shapes. 10) Weigh the measuring cup of displaced water. Record its weight. 11) Empty the water and weigh the empty measuring cup. Subtract the weight of the measuring cup from the weight of the measuring cup full of water to find the weight of the displaced water. How does it compare to the weight of the clay ball? 12) Complete the questions the data sheet. 9) Weigh the ball of clay. Record its weight.