The properties of water
Life depends on them!
Water is polar
Covalent bond
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.
Hydrogen bonds are responsible
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.
Ice and water
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.
Water dissolves molecules
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 of H+
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