Water Cycle - Water (3)
Lab 

   
OBJECTIVES:
  • Contrasting water with hydrogen peroxide.
  • Exploring the chemistry of water. 

VOCABULARY:

  • electron
  • hydrogen peroxide
  • hydrogen
  • neutron
  • oxygen
  • proton
  • water
MATERIALS:

Students compare water with hydrogen peroxide.

BACKGROUND:

Water is a transparent, odorless, tasteless liquid composed of the elements hydrogen and oxygen. It is a very good solvent, meaning that many substances can dissolve in it easily.  Water is important to our lives, and without it we could not live.  In fact, there are no living creatures that can live without water.   It illustrates the three states of matter:  solid (ice), gas (steam), and liquid (water).  

The forms water take, depends upon the temperature.  At low temperatures, the molecules do not move around as much and form a crystalline structure that is rigid (ice).  In the liquid state, water molecules move more freely.  Water molecules in the form of steam are moving very fast with large spaces between the molecules.  Although ice is crystalline, it tends to have the molecules in a rigid structure that is spaced farther than the molecules of liquid water and this is quite important, for if ice were denser, it would sink in water.  Imagine what would happen if icebergs grew from the bottom of the ocean instead of floating on the surface. 

Water and hydrogen peroxide are made of the same elements:  oxygen and hydrogen. However, hydrogen peroxide  (H2O2) has 1 more oxygen than water (H2O). 

PROCEDURE:
  1. Give students a periodic table placemat and have them look up hydrogen and oxygen. Notice that hydrogen and oxygen are both gases, but yet they produce water which is a liquid under normal conditions.  They should then transfer the information in each of the boxes on the periodic table to the appropriate place on the lab sheet.  Make sure you point out that hydrogen has 1 proton, 1 neutron, and 1 electron.  This may be their first introduction to neutrons, protons, and electrons, so go slowly.  The students can understand individual examples if you go over them carefully and make sure you explain as you go over.
      
  2.  Introduce that water and hydrogen peroxide are made of the same elements: hydrogen and oxygen.  Water has 2 hydrogens and 1 oxygen, and hydrogen peroxide has 2 hydrogens and 2 oxygens.  We would like the students to experiment with the properties of each of the liquids and then have them construct an "atomic" model to see if they can see why there is a difference.  They will not be able to figure this out why the different number causes the difference, just to learn that there is a difference.  
      
  3. In the second part of this exercise have students look at water and hydrogen peroxide.  Have them answer the questions on the lab sheet.  Remember hydrogen peroxide smells different than water.  Students should smell the two liquids by using a cupped hand to bring the odor to their nose.  Do not have students put their nose over the liquid.  
      
  4. It also tastes different, but we don't recommend that you do this part.  Also, put a  drop of water, then hydrogen peroxide on a small piece of hamburger meat.  Have the students notice that hydrogen peroxide reacts with the meat and causes fizzing.  Students may not realize that hydrogen peroxide is used when they get a scratch or cut.  What the hydrogen peroxide does is take out the oxygen in the meat, causing any bacteria (that needs oxygen) to die.
      
  5. In Exercise 3 have the students cut out the hydrogen and oxygen atom.  The black dots represent the number of electrons.  The first shell can only hold 2 electrons.  The second shell can only hold eight electrons.    Tell the students that the outer shell of the oxygen must have 8 electrons. Have the students try and figure out how to put the atoms together to make the molecule satisfied.  The clue on the student's lab sheet is actually the answer, x  belongs to hydrogen and belongs to oxygen.   The students will have a little trouble with this until they get the idea that the outer shell needs to be filled with the maximum amount of electrons.

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