Properties of Water Water is ESSENTIAL to life

Properties of Water

Water is ESSENTIAL to life. Living organisms are composed mostly of water. Humans – 65% Jellyfish – 90% Tomato – 90% Elephant – 70%

• H atoms are “attached” to one side of the O atom • This results in water molecule having a (+) charge on the side where the H atoms are and a (–) charge on the side where oxygen is. • This uneven distribution of charge is called polarity.

• Opposite electrical charges attract • Water molecules tend to attract each other – Side with H atoms (+) attracts O side (-) of a different water molecule

Oxygen is slightly negative Hydrogen is slightly positive The +/- attraction between the H (+) of one water molecule and the O (-) of the adjacent water molecule.

COHESION • Force that holds molecules of a single substance together is cohesion. • Cohesion is due to hydrogen bonding between water molecules. • Contributes to upward movement of water from roots to leaves in plants.

SURFACE TENSION • Related to cohesion because water molecules are attracted to other water molecules. • Water molecules at the surface are pulled into body of water which causes it to bead up.

• Because of surface tension, water holds its shape and will not spread out • For water to spread out, surface tension must be reduced – Chemicals such as surfactants can reduce the surface tension • Surface tension enables water-striders to run on water without breaking the surface

ADHESION • Attractive force between two particles of different substances – Water molecules and glass molecules • Capillarity is the attraction between molecules that results in rise of liquid when it is in contact with solid

Temperature Moderation • Water can absorb or release large amounts of energy in the form of heat with only a slight change in temperature. – known as Heat Capacity • Another result of the multiple H bonds • A large amount of heat energy is needed for movement of water molecules to increases the temperature

• Hot summer day – water can absorb large amount of energy from sun and can cool the air without large increase in water temp. • At night – gradual cooling water warms the air

High Heat of Vaporization • As liquid evaporates, surface of liquid left behind cools down • Evaporative cooling prevents organisms from overheating – Evaporation of sweat releases body heat and prevents overheating.

Water is Less Dense as a Solid • Which is ice and which is water?

Water is Less Dense as a Solid • Ice is less dense as a solid than as a liquid (ice floats) • Liquid water has hydrogen bonds that are constantly being broken and reformed. • Frozen water forms a crystal-like lattice whereby molecules are set at fixed distances.

Water is Less Dense as a Solid Water Ice

Mixtures • Composed of two or more elements or compounds that are physically mixed together but not chemically combined • Water can make 2 types of mixtures: solutions & suspensions • Suspension: mixtures of water and nondissolved material • Biological example: blood

Solution A mixture of… • Solute – A substance that is put INTO the solvent to dissolve. • Solvent – Substance that does dissolves the solute – Water = Universal Solvent

Acids & Bases • An important aspect of a living system is the degree of acidity or alkalinity • p. H scale ranges from 0 – 14 • Acids – range from 0 – 6 (0 strongest) • Neutral is 7 • Bases - range from 8 – 14 (14 strongest)

Acids, Bases and Buffers • H 2 O H+ + OH _ • Water dissociated into equal amounts of hydrogen ions and hydroxide ions • The p. H of pure water is 7 (neutral) • Acids have more hydrogen ions than hydroxide ions • Bases have more hydroxide ions than hydrogen ions