The properties of water Life depends on them!
Water is polar Covalentbond Covalent bond
Polar molecules • Molecules are electrically neutral. • Portions of a molecule can act as though they have an electrical charge if the components have different attractions for electrons.
Polar molecules • In water, the oxygen acts negative and the hydrogens act postive. • In effect, a water molecule has a positive and a negative pole, or end. • As in magnets and ions, opposites attract.
Hydrogen bonds • Polar molecules with hydrogen atoms are very strongly attracted to the negative regions of other polar molecules.
Cohesion • Hydrogen bonds forming between water molecules cause them to stick together. This is cohesion. • Cohesion creates surface tension.
Adhesion • Water molecules are also attracted to other substances, especially if they carry an electrical charge.
High specific heat • The hydrogen bonds between water molecules mean that a great deal of energy must be added or subtracted to cause a state change: solid to liquid or liquid to gas. • http://www.bgfl.org/bgfl/custom/resources_ftp/client_ftp/ks3/science/changing_matter/index.htm
High specific heat • This also means that water helps to prevent large rapid temperature changes in the environment.
Ice is an insulator • Also, ice can act as an insulator since it floats: ice is less dense than water! • Again, hydrogen bonds are responsible.
Solutions • Water is the universal solvent. • This means that it can dissolve many solutes, especially polar molecules. • Water also causes ionic compounds to dissociate (separate into ions.)
Sodium chloride dissociates salt http://programs.northlandcollege.edu/biology/Biology1111/animations/dissolve.html
Dissociation • The “positive” hydrogens in water are attracted to negative chloride ions. • The “negative” oxygen in water is attracted to positive sodium ions. • A shell of water molecules around the ions keeps ionic bonds from reforming.
Water dissolves molecules • Water molecules will also surround polar molecules. • Even fairly large molecules with charged regions can be surrounded with water and dissolved.
Suspensions • Particles that are too large or are hydrophobic nonpolar molecules will not dissolve. • If enough energy is added, the particles may be temporarily surrounded by water molecules, but they eventually separate into a distinct layer.
Liquid mixtures in biology • What are solutions containing dissociated ions good for? • Dissolved substances can be transported easily, and moved across membranes. • What is a biological suspension?
Acids and bases • A small number of water molecules (1 in 550 million or so) will dissociate spontaneously. H20 H+ + OH- • Usually the concentrations ofH+ and OH-are balanced and the solution is neutral.
Acids • If a solution contains an excess of H+ions, the solution is said to be an acid. • The concentration of H+ions is measured by the pH scale. • A higher concentration means a lower pH value.
The pH scale • The pH (power of Hydrogen) scale runs from 0 to 14. • Water, with equal concentrations of H+ and OH- ions, has a pH of 7.
Bases • If there is an excess of OH- ions, the solution is said to be a base, or an alkali. • A basic solution has a very low concentration of H+ ions and a pH value above 7.
The pH scale • http://www.johnkyrk.com/pH.html • What types of materials are bases? • What types are acids? • What is the ideal pH of intravenous solutions?
Acid rain • Increasing acidification of rain has severe environmental consequences.
Effects of acid rain. http://www.partnersinair.org/en/images/curr_unit2a_bkgd_figure22.jpg