Chemistry                                                               Name_____________________________________

Density Investigation

Prelab Discussion: 

      Many items made of plastic look very different from each other.  Some are colorless and transparent, others are translucent, while others are opaque.  From experience you know that some plastics are rigid and brittle and others are softer and more flexible.  You also are aware that all plastics are not the same color.  Dyes that are added to help identify plastics do not alter the physical properties of the plastics.  You will be using three different plastic strips plus an unknown plastic strip in this lab:  PS is yellow;  HDPE is red; PVC is green; and the unknown plastic is white.

      Density is an intensive physical property of matter.  It does not depend upon the amount of substance present but only on what the substance is.  Within certain temperature constraints, the density of gold is the same no matter how large or small the piece.  Density is mass per unit volume.  Common units of density are grams per milliliter and grams per cubic centimeter.  Archimedes used the principle of density to determine if the king’s jeweler was cheating him out of the gold he was given to make a crown.  He discovered that the volume of a substance can displace an equal volume of water.  Combining this with the mass allowed him to calculate the density of the king’s crown and determine if it had the requisite amount of gold in it.  This applies to how buoyant a substance is in a liquid.  Liquids also have different

densities and can appear to change the buoyancy of a substance.

Purpose: 

·        To determine the identity of an unknown plastic strip

·        To determine the density of a known plastic strip

·        To extend the knowledge gained from this investigation to a societal application

Materials and Equipment:

      H2O           NaCl(aq)            C3H7OH          three vials and caps           graduated cylinder           

      13 PS plastic strips              3 HDPE plastic strips                  3 PVC plastic strips           

      3 unknown plastic strips

Procedure:

Part I.  Determination of Density by Buoyancy

1.  Fill each vial three-quarters full with the liquids shown below:

      vial S :  sodium chloride solution (NaCl(aq))

      vial +:  tap water (H2O)

      vial A:  2-propanol (C3H7OH)

2.  Drop one of each type of plastic in each of the vials.  You should have four pieces of plastic (each a       different color) in each vial.

3.  Cap and gently invert each vial several times to remove air bubbles.  You may need to tap the sides of       the vials to help dislodge bubbles “stuck” on the plastic strips.

4.  Determine whether each plastic sample floats, sinks, or remains suspended in the liquid.  Record your       observations in data Table 1.

Part II.  Determination of Density by Volume Displacement

1.  Measure the mass of 10 dry PS plastic strips.  Record the mass in Table 2.

2.  Fill the graduated cylinder to approximately 10 mL with tap water.  Record the exact volume of water    in data Table 2.

3.  Place the  10 dry PS plastic strips into the water in the graduated cylinder.  You may need to tap the side of the cylinder to help dislodge bubbles “stuck” on the plastic strips.  Record the volume of water    and PS plastic strips in data Table 2. 

Data  and Observations:

TABLE 1:  Buoyancy Data

Sample

NaCl

H2O

C3H7OH

PS (yellow)

 

 

 

HDPE (red)

 

 

 

PVC (green)

 

 

 

unknown (white)

 

 

 

 

TABLE 2:  Mass and Volume Data

mass of 10 PS plastic strips

volume of water

volume of water & 10 PS plastic strips

 

 

 

Calculations and Analysis:

1. In Table 3, list the three known plastic strips in order of density from most dense to least dense, and list the three liquids in order of density from the most dense to the least dense.

TABLE 3:  Density Determination by Buoyancy

PLASTICS

LIQUIDS

most dense

most dense

 

 

least dense

least dense

2.  Determine the density of one strip of PS plastic.  Show all your set-ups and calculations.

 

 

 

 

 

 

Questions and Conclusions:

1.  What evidence led to your conclusion as to the identity of the most dense plastic?

 

 

 

 

2.  What evidence led to your conclusion as to the identity of the least dense liquid?

 

 

 

 

3.  The unknown plastic strip (white) has the same chemical composition as one of the known strips.        What is the identity of the unknown plastic strip?  Justify your answer.

 

 

 

 

4.  Plastics must be sorted into types before they can be recycled.  Some recycling plants shred the plastic       mixtures into small strips (like in your lab) before separating.  Propose a method that can be used to       separate the shredded plastic mixture into separate recycling types.