Monday, December 9, 2013


The density of a gas will change to fill a container

Density is a measurement of how solid something is.  Specifically it is the mass per unit volume of a substance

Density is how much things are packed together. 

  • It tells you how much stuff (mass) is found in a certain amount of space or volume (D=m/V). 
  • Imagine a solid cube that is one centimeter tall, one centimeter wide, and one centimeter long. If that cube is filled with Styrofoam, it is light. If it is filled with lead (Pb), it is heavy. The lead is heavier because it has a higher densitythan Styrofoam.
  • If you have two objects of the exact same size (volume), the more dense object will weigh more than the less dense object. 
  • Because all objects are made out of molecules, it is possible to determine how tightly packed those molecules are. This is known as density
  • The more tightly packed the molecules of an object, liquid or gas are, the more dense we say they are.
  • The density of a solid object will remain the same no matter where we place the object. 
  • The density of a liquid will change only slightly. However, the density of a gas changes drastically.

  • A gas will expand to fill whatever space it is provided. Thus, if we take a certain amount of gas out of one container and place it into another container that is twice as large the gas will expand, filling the larger container. We still have the same number of gas molecules, but now, they are filling a much larger area. Thus, the gas is half as dense as it was, or in other words, there is twice as much space between the molecules as there was in the smaller container.
So there are two things contributing to density:

  1. The mass of the atoms or molecules that makes up the material.
  2. The volume or amount of space the material takes up. If the molecules or atoms are “packed” in more closely, it will be more dense.

  • Example:  styrofoam is a low density material. Even a large styrofoam container does not weigh much. The molecules in the styrofoam do not have much mass and there is a lot of space between them. 
  •  A brick, on the other hand, is much more dense. Even a moderate sized brick can be pretty heavy. This is because the molecules which make up the rock have more mass and are packed more closely together.
Density and Buoyancy are closely related. 
  • A less dense substance will float on a more dense substance.
Take a look at the two boxes below. Each box has the same volume. 
If each ball has the same mass, which box would weigh more? Why?

We can calculate density using the formula: 
Density= Mass/Volume
 The mass and volume of two blocks. 
Block I  
Mass = 79.4 grams Volume=29.8 cubic cm.
Block II  
Mass= 25.4 grams Volume=29.8 cubic cm.
Calculate the density for each block.
answer for Block I

________________grams/cubic cm. 
 answer for Block II: 

________________grams/cubic cm.

Using the table below it is now possible for you to determine what substance makes up each block.

The densities for some common substances are:

Block I is made of _________________
Block II is made of _________________

Buoyancy is the ability of an object to float in a liquid
  • The relation of the object's weight to the weight of the water displaced is what determines if the object will float; 
  • although the size and shape of the object do have an effect, they are not the primary reason why an object floats or sinks. 
  • If an object displaces more water than its weight, it will float. Buoyancy is an important factor in the design of many objects and in a number of water-based activities, such as boating or scuba diving.  

What is buoyancy?
 As you OR an object floats, the weight of you or the object presses down into the water; the water presses back, pushing you up.

When you get into the pool, your body displaces a volume of water (the "hole" in the water that your body fits into). As long as the water your body displaces weighs more than you do, you float.

This is basically Archimedes' Law.

You weigh less than the water you're in, because your lungs are full of air, like a balloon, and like a balloon, the air in your lungs lifts you to the surface naturally.
Why don't we float alike?
Everyone floats in the water at their own natural level.

Different factors contribute to how high — or low — in the water you float.

  • First, your body type has a lot to do with your buoyancy. 
    • Fat floats, as you've probably heard, while your bones and muscles, denser than fat, are not as willing to float.
    • Also, the relative size of your lungs to the rest of your body determines how high in the water your body will float.

  • Second, the density of the water is a factor. 
    • Saltier water weighs more per unit of volume, so you will float higher in saltier water (the Red Sea, for instance) than you would in fresh water.

  • Finally, there is a curious phenomenon of apparently greater buoyancy — for some people — in deep water.


Basics 1
When you place a block of wood in a pail of water, the block displaces some of the water, and the water level goes up. If you could weigh the water that the wood displaces, you would find that its weight equals the weight of the wood.

Basics 2
This doesn't mean that if you had a few blocks of wood that were exactly the same size and shape, they would each displace the same amount of water. A block of wood made of oak, for example, sits deeper in the water (and therefore displaces more of the water) than does a block of pine. The reason is that it's heavier for its size, or denser—in this case, the molecules that make it up are more closely packed together than the molecules that make up the pine.

Basics 3
If you could somehow keep increasing the density of the block, it would sink lower and lower into the water. When its density increased enough to displace an amount of water whose weight was equal to the weight of the block, it would, in a sense, become weightless in the water.

Making the block just slightly denser would cause it to sink to the bottom.
 _________________________________________Before watching the following video clips - have your vocab cards ready.  You can double check your understanding of the terms used by easily checking your vocab cards.




Buoyant Force - Why does a ship float?