Chapter 21.1 Learning Goals Describe water in terms of its polarity. Discuss properties of water related to hydrogen bonding. Explain why water is a good solvent.
Solubility Key Question: Investigation 21A What does it mean to dissolve?
21.1 Water 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?
21.1 The shape of a water molecule When two hydrogen atoms each share one electron with an oxygen atom, a neutral molecule is formed.
21.1 The shape of a water molecule 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.
21.1 Water is a polar molecule 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.
21.1 Another polar molecule Ammonia, NH3, is another polar molecule. With one lone pair and three bonding pairs of electrons. This gives the ammonia molecule a pyramid shape.
21.1 Methane is nonpolar Methane, CH4, is a nonpolar molecule. Since there are no lone pairs of electrons, the electrons are shared equally between atoms.
21.1 Hydrogen bonds 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.
21.1 Ice and hydrogen bonds 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.
21.1 Properties of water 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.
21.1 Plants and hydrogen bonds The attraction between water molecules helps water travel from roots to stems and leaves.
21.1 Water is a universal solvent Water dissolves sodium chloride (salt) to form a solution of sodium (+) and chlorine (-) ions.
21.1 Water is a universal solvent In general, like dissolves like: water dissolves polar substances non-polar solvents dissolve non-polar substances