Physical Science Lab #8

Atomic Matter: Substances and Mixtures

Theory

In coming up with the macroscopic (visible) and microscopic (not visible since on the atomic scale) properties of different classes of atomic matter you have considered the macroscopic differences between pure substances and mixtures using two basic ideas:

Idea one (consistency of properties)

Pure substances: Different samples of which must have the same boiling point, melting point and mass density (among other things).
Mixtures: Different samples of which will not necessarily have the same boiling point, melting point and mass density (among other things).

Idea two (divisibility)

Mixtures: Which can be separated into two or more pure substances by mechanical (non-chemical) methods.  E.g. in the classroom you saw how I could separate a mixture of iron powder and sulfur (each of which happen to be elements – made of a single type of atom) by means of a magnet.
Pure substances: Which cannot be separated by mechanical (non-chemical) methods.  E.g. in the classroom you saw how I could not separate iron (II) sulfide (an ionic compound – and hence a pure substance – composed of the elements iron and sulfur) with the magnet.

Procedure
Much of this you have already done.  I am writing it down here to remind you. You will start at step 9 on Tuesday.

1. Half fill a 250 mL beaker with tap water and place it on the wire screen and iron ring over a Bunsen burner. 
2. Note the temperature of the water every 30 s and continue noting the data until the water has reached a stable (constant) temperature.
3. Graph the results and note the temperature of your group and the other groups in the class.
4. What did you notice about the temperatures of the various groups?  Is water a pure substance or a mixture?  Explain. Sketch what you think the graph would look like if you included temperatures from -273°C (absolute zero) to well above the plasma point (the point at which gas turns to plasma).  What is happening when the graph is climbing?  How about when the graph goes flat?
5. I will give you each a different sample of saltwater.  Note the boiling point for your sample and compare your result to the results of your classmates.  What does this tell you about saltwater?  Is it a mixture or a pure substance?  Explain?
6. Find the three ice-water samples – one of pure water, one with a little salt and the last with a lot of salt.  If the ice-water has been sitting awhile what can you say about its temperature?  Note the temperature of each sample.  What does this tell you about the nature (pure substance or mixture) for water compared to salt water.
7. Place an empty 100mL graduated cylinder on the digital scale and set the mass to zero.  Fill the cylinder to about 100mL and note the exact volume (measure to the bottom of the meniscus).  Weigh the water-filled cylinder on the scale and note the mass (this will be the mass of the water added).  The mass divided by the volume will give the density of pure water.  Note the result and compare to the results of your classmates.  What does this tell you about the nature of pure water?
8. Redo step 7 but using the saltwater samples from step 5.  Discuss what this says about saltwater.
9. Make sure the digital scale is zeroed.  Add a small handful (about a spoonful) of crushed salt to a crucible and weigh it.  Note the mass. 
10. Add water to the salt until the crucible is about one third filled.  Make sure all the salt dissolves.  Describe what you see.
11. Place the crucible over a Bunsen flame and allow all the water to boil away.  Let the crucible cool for 5 to 10 minutes until it is safe to handle.  Describe what you see.
12. Weigh the crucible again.  Compare this mass to the previous one.  What do you notice?  What does this say about the nature of saltwater?  Where did the water go?  Was there any salt in the water after it left?