Properties of Water Chapter 4, Section 4

Structure of Water: A Polar Molecule  electrons are more likely to be found around the oxygen than the hydrogen because oxygen attracts electrons (greater electronegativity)  the uneven distribution of electrons between the oxygen and hydrogen atoms creates the polarity and gives water its bent shape

Hydrogen Bonds  the positive side of one water molecule is attracted to the negative side of another water molecule  water molecules can form up to four hydrogen bonds with adjacent water molecules

Life-Supporting Properties of Water  Cohesion – attraction between molecules of same substance  Very strong in water due to hydrogen bonding!  Adhesion – attraction between unlike molecules  Because water is a polar molecule, it adheres very well to other polar molecules (See fig 4-13 on p. 82: Capillary action supports trees)

Temperature Moderation Temperature is a measure of the random motion of particles in a substance.  Heating a substance causes its molecules or atoms to move around (vibrate) more quickly  Water heats up slowly because hydrogen bonds keep water molecules from moving around.  Water can absorb a lot of thermal energy before it undergoes a temperature change  As water cools, it gives off heat slowly as hydrogen bonds reform.

Low Density of Ice  Unlike other liquids, water becomes less dense as it freezes.  Hydrogen bonds in ice keep water molecules spaced in a regular pattern

Water Can Dissolve Other Substances  Water dissolves other substances to form solutions  Solutions are uniform mixtures of two or more substances Example: salt (NaCl) dissolved in water Example: salt (NaCl) dissolved in water water is a solvent (forms an aqueous solution)water is a solvent (forms an aqueous solution) dissolved substances (NaCl) are solutesdissolved substances (NaCl) are solutes

 Water can dissolve ionic compounds when positive and negative ions in a solid become attracted to the slightly positive and negative ends of the water molecule  Water can dissolve polar covalent compounds such as sugar because the sugar molecules have polar areas where water molecules can attach and separate the sugar molecules.  Water cannot dissolve non- polar substances such as oils. (see fig 4-16 on p. 84)

Acids, Bases, and pH A small percentage of water molecules break apart into ions in aqueous solutions H 2 O H + + OH - If a solute increases the concentration of H+ ions in the solution, it is an ACID If a solute increases the concentration of H+ ions in the solution, it is an ACID If a solute decreases the concentration of H+ ions in the solution, it is a BASE If a solute decreases the concentration of H+ ions in the solution, it is a BASE The pH Scale describes how acidic or basic a solution is: Vinegar (pH of 3) is 10 times more acidic than tomato juice (pH of 4) Buffers are substance that regulate pH by accepting H+ ions when levels rise or donate H+ ions when levels fall Ex: Human blood contains buffers to prevent blood from getting acidic enough to endanger cell function Ex: Human blood contains buffers to prevent blood from getting acidic enough to endanger cell function