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.

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