UNIT SEVEN: Earth’s Water



Chapter 21 Water and Solutions



Chapter 22 Water Systems



Chapter 23 How Water Shapes the Land

Chapter Twenty-One: Water and Solutions



21.1 Water



21.2 Solutions



21.3 Acids, Bases, and pH

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.

Investigation 21A

Solubility 

Key Question:

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, NH 3 , 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, CH 4 , 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