1. Density in Nature and Design: Boats and ships
Large ships can change their density to be able to sail in all waters. To keep the ship steady on an even trim and the propeller below the water line, pumps are used to place seawater into ballast tanks within the ship. This makes the ship heavier and keeps it to the correct level in the water.  Ships have many small tanks for the storage of ballast water. This prevents water moving about, and upsetting the balance of the ship. Oil tankers have ballast tanks in the space between the inner and outer hull and bulk carriers have side ballast tanks. As these ships are loaded, ballast water is pumped out to allow the ship to float at a certain level. When tankers and bulk carriers unload or discharge their cargo, ballast water is pumped in, to allow the ship to float at the same level.
"Ships and Boats | How Do They Float? | History of Ships." Explain That Stuff. N.p., 17 Jan Web. 19 Mar
"Ships and the Marine Environment - Why Ships Need Ballast Water." Ships and the Marine Environment - Why Ships Need Ballast Water. N.p., n.d. Web. 19 Mar

2. Density in Nature and Design: Ice
Ice is unusual in that most materials become more dense when they move into their solid phase, yet ice is less dense than liquid water. Ice floats on water and lakes freeze from the top down to the bottom. This is important for animals that live on ice, because their habitats would be reduced or eliminated if ice sank. Also, fish and other pond life benefit from the layer of frozen water at the top of their habitat, because it provides some insulation by making a barrier between the cold air and the water below. This helps wildlife to survive in ponds over winter. The ice also acts as a barrier between organisms and UV radiation, nutrients/contaminants, atmospheric gases and precipitation. 
"The Amazing Properties of Water." The Amazing Properties of Water. N.p., n.d. Web. 19 Mar
"CCIN." Lake Ecology | CCIN. N.p., n.d. Web. 19 Mar

3. Density in Nature and Design: Hot Air Balloons
The hot air inside the envelope is less dense than the surrounding (cooler) air. This difference in density causes the hot air balloon to be lifted off the ground due to the buoyant force created by the surrounding air. This buoyant force must exceed the total weight of the heated air, the envelope, the gondola, and the passengers and equipment on board. To lower the hot air balloon, the balloon operator can either stop firing the burner, which causes the hot air in the envelope to cool (decreasing the buoyant force), or open a small vent at the top of the balloon envelope. This releases some of the hot air, which decreases the buoyant force, which also causes the balloon to descend. To maintain a steady altitude, the balloon operator intermittently fires and turns off the burner. To move the balloon horizontally, the balloon operator raises or lowers the hot air balloon to the altitude that matches a wind current.
"Hot Air Balloon Physics." Real World Physics Problems. N.p., n.d. Web. 19 Mar
"Properties of Matter Reading Selection: Just a Load of Hot Air." Properties of Matter Reading Selection: Just a Load of Hot Air. N.p., 15 Sept Web. 19 Mar

4. Density in Nature and Design: Blood Centrifuges
Centrifuges separate blood products by generating forces that cause the denser elements to move to the bottom of the test tube and the less dense ones to move to the top. Because red blood cells are heavy, it is easy to separate them from other components. Why separate blood into its components? Each part of the blood has a different function. Patients may need only specific parts of blood for treatment. By separating blood, we can use a single donation for several patients. For example, red blood cells help to increase the supply of oxygen to the body after a severe blood loss, or a disease like anemia. Plasma helps to stop or prevent bleeding that may occur because of surgery or treatment for a disease like cancer. Blood separation can also help to isolate parasites.
AboutKidsHealth, and "Blood Transfusions: What You Need to Know." Blood Transfusions: What You Need To Know - AboutKidsHealth. AboutKidsHealth, 06 Aug Web. 19 Mar
"How Does a Centrifuge Separate Blood Products?" Reference. N.p., n.d. Web. 19 Mar
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5. Density in Nature and Design: Swim Bladder
On Earth, an object immersed in a fluid (such as a fish or a person) experiences two major forces– the downward pull of gravity, and the upward push of buoyancy. Particles at the lower levels are pushed down by the weight of all the particles above them. The particles at the upper levels have less weight above them. Consequently, there is always greater pressure below an object than above it, so the fluid constantly pushes the object upward.  
To ascend in the water, a fish must reduce its overall density by increasing its volume without significantly increasing its mass. Most fish do this with a swim bladder, which is an expandable sac, like a human lung. To reduce its overall density, a fish uses its gills to fill the bladder with oxygen collected from the
surrounding water. When the bladder is filled, the fish has a greater volume, with almost the same mass. The result is lower density, and the fish rises in the water. When the bladder is completely deflated, the fish has minimum volume, higher density, and it sinks. To stay at a particular level, a fish fills its bladder to the point at which it displaces a volume of water that weighs what the fish weighs. In this case, the forces of buoyancy and gravity cancel each other out, and the fish stays at that level. In this case, it has neutral buoyancy.
"How Do Fish Rise and Sink in the Water?" HowStuffWorks. N.p., 04 May Web. 19 Mar  
The Editors of Encyclopædia Britannica. "Swim Bladder." Encyclopædia Britannica. Encyclopædia Britannica, Inc., 26 Aug Web. 19 Mar

6. Density in Nature and Design: Submarines
To control its buoyancy, the submarine has ballast tanks and auxiliary, or trim tanks, that can be alternately filled with water or air. When the submarine is on the surface, the ballast tanks are filled with air and the submarine's overall density is less than that of the surrounding water. As the submarine dives, the ballast tanks are flooded with water and the air in the ballast tanks is vented from the submarine until its overall density is greater than the surrounding water and the submarine begins to sink (negative buoyancy). A supply of compressed air is maintained aboard the submarine in air flasks for life support and for use with the ballast tanks.
Brain, Marshall, and Ph.D. Craig Freudenrich. "How Submarines Work." HowStuffWorks Science. HowStuffWorks, 17 Aug Web. 19 Mar
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