Studio_1_FINAL_F2006 - Chem 25: Studio #1 Using Physical...

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Using Physical Properties to Identify Unknowns NAME:___________________________ STUDIO:___________________________ Substances are identified by their physical and chemical properties. All this means is that how substances behave under certain conditions tells us something about what they are at the most fundamental molecular level. If we can quantify that behavior – hang a number on it – we can frequently discriminate among substances that otherwise seem very similar. We can also determine whether a substance we are working with is chemically pure – composed of a single type of molecule – or if it is a mixture of more than one type of molecule. Frequently we can use what we know about the properties of the substances in a mixture to separate that mixture. All of these issues have important commercial ramifications and touch our lives in a variety of ways. Let’s start with two simple physical properties, density and melting point, and explore what they tell us about matter. PART I: DENSITY, POLYMERS, AND WHY WE SEGREGATE PLASTICS FOR RECYCLING Here’s the issue: the modern world uses more plastics (by volume) than steel. Much of this polymeric material is used in packaging, which means it’s used once and then discarded. The accumulation of all this garbage has led modern societies to consider what can be done to make it possible for us to re-use some of this material to minimize the amount of garbage produced. Polymers are so popular because we know how to fine- tune their properties by altering their molecular structure to give us exactly the behavior we need in a material; the structure and composition of a material determine its properties . This means that we need to separate the mixtures of polymers that end up in the garbage so that the recycling yields materials of known composition that we can use again, as opposed to yielding a useless and highly variable mixture that doesn’t have the properties we need. Density is “mass per unit volume.” It is the ratio of the mass of a material vs. the volume it occupies. The typical units we’ll use for liquids and solids are g/mL or g/cm 3 . Water at 4 o C has a density of 1.00 g/mL. Density is a major factor that determines whether a solid floats or sinks in water. Motor oil is a homogeneous mixture of several hydrocarbons with a density of about 0.87 g/cm 3 , so it floats on water. Shape matters, too, for solids; steel has an average density of about 7.86 g/cm 3 , so you would expect it to sink in water, but if it is molded in a fashion to displace a mass of liquid larger than its own mass (like a ship) it floats. Shape isn’t an issue with the solid samples in this lab. In the lab:
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This note was uploaded on 02/26/2008 for the course CHEM 025 taught by Professor X during the Fall '06 term at Lehigh University .

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Studio_1_FINAL_F2006 - Chem 25: Studio #1 Using Physical...

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