Properties of Water

A compound is a pure substance made up of atoms of two or more elements The proportion of atoms are always fixed Chemical formula shows the kind and proportion of atoms of each element that occurs in a particular compound

Molecules are the simplest part of a substance that retains all of the properties of the substance and exists in a free state Some molecules are large and complex

Chemical Formulas Subscript after a symbol tell the number of atoms of each element H 2 0 has 2 atoms of hydrogen & 1 atom of oxygen Coefficients before a formula tell the number of molecules 3O 2 represents 3 molecules of oxygen or (3x2) or 6 atoms of oxygen

The extraordinary properties of Water At sea level, pure water boils at 100 °C and freezes at 0 °C. The boiling temperature of water decreases at higher elevations (lower atmospheric pressure). For this reason, an egg will take longer to boil at higher altitudes.

The Water Molecule Neutral Charge All Molecules Are Neutral

The Water Molecule Polarity Polarity A water molecule is polar because there is an uneven distribution of electrons between the oxygen and hydrogen atoms. (+) (—)

Hydrogen Bonds Polar water molecules act like magnets and attract each other Polar water molecules act like magnets and attract each other Hydrogen Bonds Hydrogen Bonds The attraction of the Hydrogen end (+) of one molecule for the Oxygen end (-) of another water molecule. The attraction of the Hydrogen end (+) of one molecule for the Oxygen end (-) of another water molecule. They are the strongest bonds that can form between molecules They are the strongest bonds that can form between molecules

Hydrogen Bonds

Cohesion The attraction between molecules of the same substance (e.g. water). Allows some insects and spiders to walk on water.

Adhesion Attraction between molecules of different substances Attraction between molecules of different substances Responsible for Capillary forces in plants Responsible for Capillary forces in plants

High Specific Heat Water resists temperature change, both for heating and cooling. Water can absorb or release large amounts of heat energy with little change in actual temperature.

Homeostasis Steady state Constant physiological condition of cells, organisms global ecosystems a. Makes a good insulator b. Resists temperature change c. Universal solvent d. Coolant e. Ice protects against temperature extremes

Solutions & Suspensions Water is usually part of a mixture. Water is usually part of a mixture. There are two types of mixtures: There are two types of mixtures: Solutions Solutions Suspensions Suspensions

Solution Ionic compounds disperse as ions in water Ionic compounds disperse as ions in water Evenly distributed Evenly distributed SOLUTE SOLUTE Substance that is being dissolved Substance that is being dissolved SOLVENT SOLVENT Substance into which the solute dissolves Substance into which the solute dissolves

Solution

Suspensions Substances that don’t dissolve but separate into tiny pieces. Substances that don’t dissolve but separate into tiny pieces. Water keeps the pieces suspended so they don’t settle out. Water keeps the pieces suspended so they don’t settle out.

Acids, Bases & pH 1 water molecule in 550 million naturally dissociates into a Hydrogen Ion and a Hydroxide Ion 1 water molecule in 550 million naturally dissociates into a Hydrogen Ion and a Hydroxide Ion Hydrogen Ion Hydroxide Ion Acid Base Acid Base H 2 O  H + + OH -

The pH Scale Indicates the concentration of H + ions Ranges from 0 – 14 pH of 7 is neutral pH 0 – 6.99 is acid … H + pH 7.01 – 14 is basic… OH - Each pH unit represents a factor of 10 change in concentration

Acids Strong Acid = pH 1-3 H + ions

Bases Strong Base = pH 11 – 14 OH - ions

Buffers Weak acids or bases that react with strong acids or bases to prevent sharp, sudden changes in pH. Weak acids or bases that react with strong acids or bases to prevent sharp, sudden changes in pH. Weak Acid Weak Base