 21.1 Water  21.2 Solutions  21.3 Acids, Bases, and pH

 We live on a watery planet.  All life on Earth depends on this combination of hydrogen and oxygen atoms.  What are the properties of water that make it so valuable?

 When two hydrogen atoms each share one electron with an oxygen atom, a neutral molecule is formed.

 Because negative charges repel, the electrons pairs around the oxygen atom are located where they are the farthest apart.  This results in a geometric shape called a tetrahedron.

 A water molecule has a negative end (pole) and a positive end.  A molecule (like water) with a charge separation is called a polar molecule.

 Ammonia, NH 3, is another polar molecule. With one lone pair and three bonding pairs of electrons.  This gives the ammonia molecule a pyramid shape.

 Methane, CH 4, is a nonpolar molecule.  Since there are no lone pairs of electrons, the electrons are shared equally between atoms.

 A hydrogen bond is a bond between the hydrogen on one molecule to another atom on another molecule.  Hydrogen bonds are relatively weak so they constantly break and re-form.

 Frozen water (or ice) has an organized structure that resembles a honeycomb because each water molecule can form hydrogen bonds with four other water molecules.

 The attraction between water molecules, called Cohesion, helps water travel from roots to stems and leaves.

 Water has a high specific heat value because of hydrogen bonds.  In order for water to boil, enough energy must be added to separate the hydrogen bonds.

 Water dissolves sodium chloride (salt) to form a solution of sodium (+) and chlorine (-) ions. Called dissosociation.

As water molecules collide with sugar crystals, attractions develop between the water molecules and sugar molecules at the surface of the solid. Dissolving

 In general, like dissolves like: ◦ water dissolves polar substances ◦ non-polar solvents dissolve non-polar substances