Characteristics of Water Unique Properties of Water • Chemical basis for the unique properties of water. • Life would not exist without these properties. Water • A water molecule (H2O), is made up of three atoms: one oxygen and two hydrogen. • Strong covalent bonds between one hydrogen (H) and two oxygen (O) atoms • Both H atoms on same side of O atom • Dipolar (electrons are not shared equally): there is an uneven distribution of electrons resulting in a molecule with a both a positively and negatively charged region. Hydrogen Bond • Polarity means small negative charge at O end • Small positive charge at H end • Attraction between positive and negative ends of water molecules to each other or other ions • Hydrogen bonds are weaker than covalent bonds but still strong enough to result in • High water surface tension • High solubility of chemical compounds in water • Unusual thermal properties of water • Unusual density of water Adhesion & Cohesion Cohesion Adhesion • Attraction between particles of the same substance • Attraction of water for itself: Due to water’s polarity, water molecules stick to each other forming hydrogen bonds. • Attraction for water to other polar or charged materials • Surface tension: Because of the hydrogen-bonding, water acts as if it were coated with a film. This surface tension causes water to "bead-up" on a hard surface. • Surface tension (a measure of the strength of water’s surface) allows insects to walk on the surface of water. • Due to polarity, water sticks to other surfaces as well (unlike substances, opposites attract). • Capillary action: due to polarity, water is able to move through small spaces, against gravity. Water’s Heat Capacity and Specific Heat • • • • Heat Capacity – amount of heat required to raise the temperature of 1 gram of any substance by 1°C Water has a high heat capacity – can take in or lose much heat without changing temperature: heat energy is first used to break H bonds, leaving less heat energy to increase movement of the molecules which is necessary to increase temperature. This property helps maintain homeostasis (body temperature) and prevents bodies of water form changing drastically. Specific Heat – heat capacity per unit mass water warms slowly and cools slowly High heat of vaporization • Heat of vaporization: The amount of heat required for 1 gram of a substance to be converted from a liquid to a gas. • Hydrogen bonds make it difficult for water molecules to escape the liquid state and are responsible for water's high heat of vaporization. • The hydrogen bonds must be broken before water can evaporate and this requires considerable energy. • Because water has a high heat of vaporization • Helps our bodies and our planet to maintain our temperature within a tolerable range. • When we get hot and sweat, water evaporates from our skin and cools us. Since the evaporation of water requires a considerable amount of energy, it is very effective in cooling us. • Water also helps moderate global climate by absorbing solar radiation and dissipating the heat by evaporation of surface water. Water’s Thermal Properties • Water is solid, liquid, and gas at Earth’s surface. • Water influences Earth’s heat budget. • Density • Cold water is more dense than hot water. • Cold water sinks (H bonds condense), Hot water floats (H bonds expand) • Cause of currents in ocean, different life found at bottom of ocean compared to top, etc. • Energy must be added for molecules to overcome attractions. Water’s Three States of Matter • Unlike most substances, water becomes less dense when it freezes. • H bonds first condense when water is cold but then expands when water freezes leaving air pockets (crystal lattice) between the water molecules. ➔ keeps the molecules further apart than liquid water • Ice floating acts as an insulator to water in a body of water so fish don’t die in the winter. Freezing and Boiling Points • Freezing point = melting point: 0°C (32°F) • Boiling point = condensation point: 100°C (212°F) • Calorie is the amount of heat needed to raise the temperature of 1 gram of water by 1°C. Water Density and Temperature • Density of water increases as temperature decreases.: Thermal contraction • From 4°C to 0°C the density of water decreases as temperature decreases. • Ice is less dense than water. • Changes in molecular packing • Water expands as it freezes. • Increasing pressure or adding dissolved substances decreases the maximum density temperature and reduce the freezing point of water (Most seawater never freezes). Water as Solvent • Water molecules stick to other polar molecules. • Electrostatic attraction produces ionic bond. • Water can dissolve almost anything: Due to polarity, water’s opposite charges attract to the opposite charged ions in salt. • Hydration © 2011 Pearson Education, Inc. pH (potential hydrogen) • Solutions are either an acid (below 7), base (above 7) or neutral (7). • Due to the dissociation of water, H2O can break apart in a solution and become H+ (making a solution acidic) or OH- (making a solution basic) • If there is an equal amount of each ion, the solution is neutral. • Water being neutral prevents tissue from being damaged in living things.
