The properties of water
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The properties of water. Life depends on them!. Water is polar. 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.

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The properties of water l.jpg

The properties of water

Life depends on them!



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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.


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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.


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Hydrogen bonds

  • Polar molecules with hydrogen atoms are very strongly attracted to the negative regions of other polar molecules.


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Cohesion

  • Hydrogen bonds forming between water molecules cause them to stick together. This is cohesion.

  • Cohesion creates surface tension.



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Adhesion

  • Water molecules are also attracted to other substances, especially if they carry an electrical charge.


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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://mutuslab.cs.uwindsor.ca/schurko/animations/waterphases/status_water.htm


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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.


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High specific heat

  • This also means that water helps to prevent large rapid temperature changes in the environment.



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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.)


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Sodium chloride dissociates

salt

http://programs.northlandcollege.edu/biology/Biology1111/animations/dissolve.html


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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.


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Water dissolves molecules

  • Water molecules will also surround polar molecules.

  • Even fairly large molecules with charged regions can be surrounded with water and dissolved.



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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.


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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?


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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.


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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.


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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.


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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.


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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?


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Acid rain

  • Increasing acidification of rain has severe environmental consequences.


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Effects of acid rain.

http://www.partnersinair.org/en/images/curr_unit2a_bkgd_figure22.jpg


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