Density is a measure of how much mass a substance has in comparison to its size. Density is defined as the ratio of mass to the volume of a particular substance. The mass of a material is directly proportional to the volume. If mass versus volume for a particular substance were plotted on a graph the result would be a straight line. The slope of this line would be the density of the substance. When calculating the density of a sample, the mass and the volume of the sample are determined and the mass of the sample is then divided by the volume. The Système International d'Unités (S.I.) unit for density measurements is kg/m3, although it is often expressed in g/cm3 or g/mL.
Conceptually, density can be described as how close together the molecules in a substance are packed. Every substance has a specific density. The density of a particular substance is a property which can be used to identify the substance. It is an intensive physical property of a particular material and does not depend on the amount of material present. For example, a cube of rubber with a mass of 100 g and a volume of 350 mL would have a density of 0.29 g/mL (100 g divided by 350 mL). If this piece of rubber were cut in half so the mass was now 50 g and the volume was now 175 mL, the density would still be 0.29 g/mL (50 g divided by 175 mL). The density of this particular sample of rubber remains the same, regardless of how much rubber is present.
Because the density of a particular substance is constant, the density of an unknown substance can be used to determine its identity. The mass and volume of a sample of the material could be used to calculate the density, and this value can then be compared to the known densities of various materials. The density of a particular substance can also be used to set up a conversion factor for use in stoichiometry problems.
Another use for the density value of a particular substance is in determining whether this substance will sink or float. A material will float in a particular medium if the medium has a greater density than the material. The material will sink if its density is greater than that of the medium. For example, water has a density of 0.998 g/mL and cork has a density of 0.24 g/mL. Cork is less dense than water so it will float when placed in water.Lead has a density of 11.35 g/mL, therefore it would sink when placed in water.Helium is less dense than air, so a balloon filled with helium gas will "float" on the air. "Heavy" cream, which floats on top of milk, is actually less dense than the milk, despite its thicker consistency.
Even though the density of a particular material is a constant, physical property of that material, it does vary with temperature. Most materials expand with increasing temperature. When a material expands, it occupies a greater volume. The mass of the material remains the same, so when mass is divided by the greater volume, the density is actually lower with the increased temperature. It is because of this phenomenon that sidewalks are built with cracks or spaces between concrete squares and bridges are designed with spacers at various intervals. When the weather is hot, the concrete expands, and without the spaces would buckle and crack. Another example is the operation of hot air balloons. The fire underneath the opening of the balloon heats the air inside the balloon. As the temperature of the air increases, so does its volume, and the balloon inflates. As the volume of the air increases, its density decreases, so that the density of the hot air balloon is less than the air surrounding it. Because it is less dense, it "floats" on the air, rising off of the ground. When the air inside of the balloon is allowed to cool, the air contracts and becomes more dense, and the balloon begins to descend.
Because density varies with temperature, it follows that density also varies with the states of matter. A material in the liquid state has a different density than the same material in the solid or gaseous state. Density is generally greatest in the solid state, and continues to decrease as the material becomes liquid and then gaseous. A solid material is made up of particles which are closer together than either a liquid or a gas. As a solid moves to a liquid, the particles become farther apart, and as the liquid changes to a gas, the particles are even more widespread. The density of a material in the gaseous state is only approximately 0.1% as dense as it would be in the liquid or solid state. The density of a gas follows the Ideal Gas Law and varies inversely with temperature (measured in Kelvin) and directly with molar mass and pressure.
The difference in density between a material in the gaseous state and the same material in the liquid state is far greater than that between the material in the liquid state and the solid state. Most materials are only slightly denser as solids than as liquids. One exception to this rule is liquid versus solid water (ice). The density of ice is actually less than that of liquid water. This means that water expands when it freezes. It is because of this phenomenon that cracks in windshields seem to "grow" faster in the winter. Moisture gets inside of a crack, and as the temperature decreases, the water in the crack freezes and expands, causing the crack to expand as well.
Determining a material's density has many practical applications. When designing a boat, for example, the structure of the boat needs to be such that it is less dense than water, even when loaded with passengers and fuel. This is generally accomplished by including many large air pockets in the frame (if a boat were solid steel, it would sink rather quickly). "Lava lamps" utilize the changing density of paraffin wax with temperature to create the illusion of lava bubbling through the lamp. The temperature of the wax is increased by the heat from a light bulb at the base of the lamp. With the increase in temperature comes a decrease in density, and the paraffin floats up to the top of the lamp. Once it reaches the top, it is allowed to cool again and the density increases, causing the paraffin wax to sink back to the bottom of the lamp. Ships which travel through icy water need to be aware of the density difference between ice and water when approaching an iceberg. The density of ice is about 89% that of liquid water, which means 89% of a body of ice will remain below the surface of the water. When a seaman spots an iceberg, it is only 11% of the actual block of ice. What may appear to be a small iceberg is actually much larger.
Besides these practical applications, chemists utilize density values in many ways. They can be used to determine the identity of an unknown substance. They can also be used to create conversion factors for stoichiometry problems. The density of different fractions of petroleum is the basis behind petroleum refining. Density is a simple calculation that can be used in many aspects of chemistry.
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