Chemistry

Experiment A-5

In Experiment A-4 you hopefully found that each different substance had a unique straight line pattern relating the mass and volume of multiple samples.  If you are familiar with graph properties, you may have noticed that the slope of each substances line is what is more commonly called the density of that substance.  Density is the ratio of mass / volume for any sample of a substance.

In Experiment A-2 you created a classification system for a set of chemical substances.  Your system might have been as simple as putting the substances into alphabetical order!  But other systems might be more useful.  One system of classification commonly used by chemists is based on attempts to separate the material into separate, distinct substances.  Physical methods of separation include filtering, sifting, distilling, panning, and many other methods of sorting.

• A substance that can be separated by physical methods into two or more distinct substances is called a mixture.
  
  
• A substance that all physical attempts to separate only finds a single material is called a pure substance.  Note this is different from the common meaning of pure.  For example pure blood is a mixture while poisonous arsenic is a pure substance.
  
  There are harsher chemical methods of separation, including strongly heating, chemical reaction and electrolysis, which can be used on pure substances:
  
  
  • A pure substance that can be separated by chemical methods into two or more distinct substances (but not by physical methods) is called a compound.
    
    
  • A pure substance that, after all chemical attempts to separate, leaves a single material is called an element.  You may have heard that pure water is a pure substance because it can't be separated by any physical method.  But is it also a compound because it can be separated into the elements of hydrogen and oxygen by electrolysis.

This system of classify is very useful for understanding the composition of materials.

The process of taking apart something to study its composition is known as analysis.  This is a common process used by chemists as well as many other occupations.  So it will be of value to develop this skill.

Experiment

Analyzing Cereal for Iron:  harder

1. Obtain a box of breakfast cereal which claims to contain iron.  (Generally the iron is in the form of a fine powder mixed with the other ingredients.)  The amount of cereal needed will depend on the sensitivity of your balance.  expect only tiny amounts of iron.
   
   
2. Determine the mass of the cereal and perhaps the magnet if it is light weight.
   
   
3. Use a magnet to stir the cereal with enough water to cause the solid cereal to form a slushy slurry.  Anticipate it will take a long period of stirring for the magnet to collect nearly all the iron.  You might inspect the magnet periodically noticing if any iron is collecting.
   
   
4. Dry then weigh the iron either by subtracting the mass of the magnet or by removing and weighing the iron alone.
   
   
5. Calculate the percent composition of iron compared to the total cereal mass using the formula above.
   
   
6. Consider possible sources of error in you measurements and calculations then estimate how much your percent composition might be off.

Analyzing Mixed Food:  easier

1. Obtain a package of trail mix, a mixture of dried fruit or a mixture of nuts.
   
   
2. Determine the mass of the total mix.
   
   
3. Sort out at least one kind of fruit or nut.
   
   
4. Determine the mass of the selected fruit or nut.
   
   
5. Calculate the percent composition of fruit or nut compared to the total mass of the mixture using the formula above.
   
   
6. Consider possible sources of error in you measurements and calculations then estimate how much your percent composition might be off.

If you need course credit, use your observations recorded in your journal to construct a formal report.