In the case of copper and aluminum, their atoms are arranged similarly, but copper atoms are smaller and have more mass than aluminum atoms, giving it a higher density. Density, Sinking and Floating Why do some things float, while others sink? You might expect heavier objects to sink and lighter ones to float, but sometimes the opposite is true. The relative densities of an object and the liquid it is placed in determine whether that object will sink or float. You can really see relative densities at work when you look at a heavy object floating and a lighter one sinking.
For example, imagine putting a small piece of clay and a large, heavy wax candle in a tub of water. Sinking and floating applies to liquids too.
For example, if you add vegetable oil to water, the oil floats on top of the water because the oil has a lower density than the water. The water pushes upward against the object with a force buoyancy equal to the weight of water that is displaced. When the ball is submerged in the water, it displaces its volume in water.
However, the weight of the ball is more like 55 N. A beach ball may have the same volume as a bowling ball, but it has a much smaller mass. When you a beach ball in a tub of water, it displaces the mass of water equal to its own mass—about 0.
If you were to try to push the beach ball down and displace more water, the water would push back with a force greater than the weight of the beach ball. The push of the water keeps the beach ball afloat.
Buoyancy is the upward force we need from the water to stay afloat. Buoyant forces are why we feel so much lighter when we are in a swimming pool. Our bodies are mostly water, so our density is fairly close to that of water. Because of this, an average person needs only a little bit extra buoyancy to float. A life jacket provides this extra lift.
Changing Density You can change the density of a substance by heating it, cooling it, or by adding something to it. There are two possible ways to make that object float, however:.
Archimedes : Greek mathematician, physicist, engineer, inventor and astronomer c. In other words, the buoyancy is equal to the weight of the displaced fluid. For example, when an object goes into water, it displaces the water. EDinformatics Mass, Volume, Density. Density plays a part in why some things float and some sink. Objects that are more dense than water sink and those less dense float.
Hollow things often float too as air is less dense than water. This is partly why huge heavy ships float. Another thing to consider is the shape of an object. Generally the more of the outside of an object that is touching the water the more buoyant it is. Water pushes back up against objects so the more surface area an object has the more water pushes back against it helping it to float. When an object floats, it pushes water out of the way displacement.
Have you ever noticed that when you climb into a bath the water level rises? This easy activity demonstrates how reducing the density of a heavy object allows it to float. Place the bubble wrapped ball on the surface of the water, you should find it now floats. Although the bubble wrap makes the ball weigh a little more, it also displaces extra water making the ball more buoyant. The pockets of air in the bubble wrap mean that the ball and bubble wrap together are less dense than the water, which means the ball floats!
Can you think of a different way to make the ball float? You may use a dropper to add the last bit of water and prevent spilling. The goal is to compare the mass of equal volumes of wax and water. The water has a greater mass than an equal volume of wax. So, the density of water must be greater than the density of wax. If students have trouble understanding this relationship between the mass and density of equal volumes, have them think about the demonstration from Chapter 3, Lesson 1 with the aluminum and copper cubes.
Both had the same volume, but the copper cube weighed more. Because the copper had more mass, it also had a greater density. The clay has a greater mass than an equal volume of water. So, the density of clay is greater than the density of water.
Wax is made of carbon and hydrogen atoms connected together in long chains. These long chains are tangled and intertwined and packed together to make the wax. Even though they both have lots of hydrogen atoms, water is more dense than wax because the oxygen in water is heavier and smaller than the carbon in the wax. Also, the long chains of the wax do not pack as efficiently as the small water molecules. Clay has oxygen atoms like water, but it also has heavier atoms like silicon and aluminum.
The oxygen atoms are bonded to the silicon and aluminum to make molecules with a lot of mass. These are packed closely together, which makes the clay more dense than water. Students should realize that if an object weighs more than an equal volume of water, it is more dense and will sink, and if it weighs less than an equal volume of water, it is less dense and will float.
Predict whether the following objects will sink or float. Read more about sinking and floating in the teacher background section. Note : Students may wonder why boats made out of dense material like steel can be made to float. This is a good question and there are several ways of answering it.
A key to understanding this phenomenon is that the density of the material and the density of an object made of that material are not necessarily the same. If a solid ball or cube of steel is placed in water, it sinks. But if that same steel is pounded and flattened thin and formed into a big bowl-like shape, the overall volume of the bowl is much greater than the volume of the steel cube. The mass of the steel is the same but the big increase in volume makes the density of the bowl less than the density of water so the bowl floats.
This is the same reason why a steel ship is able to float. The material is shaped in such a way so that the density of the ship is less than the density of water. The American Chemical Society is dedicated to improving lives through Chemistry. Skip Navigation. Lesson 3. Engage Do a demonstration to show that the wax is heavier than the clay but that the wax floats and the clay sinks.
Materials for the demonstration 1 tea light candle Clay Clear plastic container Water Large balance Teacher preparation Use a small enough piece of clay so that you are sure that the candle weighs more than the clay.
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