titration due in 6 hours, 1.5 pages

101/114 EXAMPLE GROUP PROJECT PAPER “What’s in the Flask?”

Often scientists spend time determining the identity of unknown substances

through experiments designed to help them do so. One important physical property for

identifying different substances is density. Density is an intensive property, meaning it is

independent of the amount of the substance. Density is defined as mass divided by

volume, so density can be calculated by measuring the mass and volume of a given

sample. In the “What’s in the Flask?” investigation, one goal was to determine what is in

the flask by addressing physical and chemical properties (5). Physical properties

discussed were color, physical state (s, l, or g), odor, and density. Chemical properties

discussed were if a reaction was produced (formation of bubbles and/or color change).

The next goal was to determine the density of hot and cold water by measuring the

mass and volume of 3 samples of hot water and 3 samples of cold water, recording the

temperature, and calculating the density. The class discussed that heating a substance

causes the molecules to speed up and move further apart, occupying more volume

resulting in a lower density. Because of this, it was expected for the results of the

density of hot water to be lower than those for the cold water.

A common real-world application using density is the construction of ships. Cargo

ships need to be durable but still must be able to float. In history, this resulted in most

ships being constructed of wood. By taking advantage of both density and buoyancy,

ships can now be constructed out of steel but still float on the water. The density of

water is 1.00g/mL, so if an object's density is less than that of water, it will float; if its

density is more than that of water, it will sink (1). The density of steel is about 8 times

larger than that of water, yet a steel ship will float (2). The reason for this phenomenon

involves both density and buoyancy.

Bouyancy is based on Archimedes' principle (4). Archimedes was the most

famous mathematician and inventor in ancient Greece. He is best known for his

discovery of the relationship between the surface and volume of a sphere, and is also

known for his formulation of a hydrostatic principle called Archimedes’ principle (4). As

an object is submerged in water, it moves, or displaces, water according to how much it

weighs. Archimedes found that the water will push upward against the object with a

force equal to the weight of water that is displaced. How much water is displaced is

determined by the density of the object (2). A bowling ball and a beach ball may have

the same volume, but the bowling ball weighs much more, and is much denser, than the

beach ball. If the object weighs more than the water it is displacing, it will sink, much like

the bowling ball. The beach ball floats because the weight of the beach ball (mostly air)

is less than the weight of the water it displaces.

One would think that a steel ship would sink because steel is denser than water.

Steel ships can float because they have ballast tanks that hold air. These tanks provide

large volumes of air containing very little mass (1). This decreases, the overall density

of the ship, enabling the ship to float (1). The trapped air weighs much less than the

weight of the water it displaces, so the water pushes up harder than the ship pushes

down, allowing the ship to remain buoyant (4).

Submarines are another example of Archimedes’ principle. Archimedes

determined that a body which is completely or partially submerged in a fluid experiences

an upward force called the buoyant force, B, which is equal in magnitude to the weight

of the fluid displaces by the object (4). Submarines dive below the surface of the water

by emptying their ballast tanks and filling them with water, which is considered negative

buoyancy. To come up to the surface, compressed air is simply blown into the tanks

forcing the water out, which is considered positive buoyancy.

In conclusion, density is a physical property used to identify an unknown

substance by comparing it to an accepted value. In the “What’s in the Flask?”

investigation various physical and chemical properties were discussed, but used density

(m/V) to identify the liquid in the flask. It was determined that the higher the

temperature of water, the lower the density (3). In the real world application, boats float

because they are less dense than water and due to Archimedes Principle. Submarines

are able to dive and surface due to the same principle that a body that is completely or

partially submerged in a fluid exerts equal force (buoyant force) to the weight of the fluid

displaced by the object (2).

References

(1) Global Post Home Page. http://everydaylife.globalpost.com/can-study-density- used-real-world-41598.html (accessed April 21, 2015).

(2) "Why can boats made of steel float on water when a bar of steel sinks?" 01 April 2000. http://science.howstuffworks.com/science-vs-myth/everyday- myths/question254.htm (accessed April 21, 2015).

(3) Shapley, P. Temperature Effects on Density http://butane.chem.uiuc.edu/pshapley/GenChem1/L21/2.html (accessed April 21, 2015).

(4) Buoyant Forces Home Page. http://theory.uwinnipeg.ca/physics/fluids/node10.html (accessed April 21, 2015).

(5) Bauer, R., Birk, J., Sawyer, D. Laboratory Inquiry in Chemistry, 3rd ed.; Brooks/Cole Cengage Learning: Belmont, 2009, pp 19-21.