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Water and it’s importance
• Most important compound in living organisms.
• Life processes rely on free movement of molecules and ions.
• This occurs when substances are dissolved in water.
Water Molecule
Oxygen is slightly negative(more electronegative)
Hydrogen is slightly positive(less electronegative)
Neighboringmolecules held by a hydrogen bonds
Polarity of Water• Water is bonded together by polar covalent hydrogen
bonds.• Sharing is not equal; oxygen attracts electrons more
strongly than hydrogen• This gives water an assymetrical distribution of
charge. – Water can dissolve many substances
• Other polar molecules• Other charged molecules (positive and negative)
Water is a versatile solvent
• When water and another solution are mixed, you have an aqueous solution.
• Solute = what is being dissolved• Solvent = what is doing the dissolving• Polarity of water allows a variety of
substances to dissolve easily (ionic compounds and polar molecules)– Substances that repel water include non-polar and
non-ionic substances
Water as a solvent
• Polarity of water attracts other polar molecules – Ex: sugar
• These molecules are considered to be “hydrophilic” or water loving
• Molecules that are non-polar are repelled– Ex: oil
• Non-polar molecules are considered “hydrophobic” or water fearing
Importance to Living Things• The chemistry of living things involves the
study of solutions.• Water is vital to life because the chemical
reactions (metabolism) of living things must take place in aqueous solutions.
• This occurs when substances are dissolved in water.
• Life processes rely on free movement of molecules and ions. (ex: electrical signals in nervous systems)
Examples of aqueous solutions:
• Blood plasma – Liquid portion only– Ions, larger molecules, gases all dissolved in water
• Cytosol– The gelatin-like aqueous solution inside cells.
• Interstitial fluid– Intercellular fluid of multicellular organisms~11 L in average adult human body
• Aquarium water….
Temperature Stabilizer• Cells release heat as a result of metabolism
– Water helps to minimize temperature changes in cells
• Water can absorb great amount of heat before it’s temperature changes considerably– Due to hydrogen bonds– Energy disrupts hydrogen bonds rather than an increase in individual
molecules– Hydrogen bonds are constantly breaking and reforming
• This also allows large bodies of water and large organisms to regulate temperature– If bonds stay broken, water molecules at the surface escape into the
atmosphere• Evaporation• This takes away some energy and cools surface left behind
– This is why sweating cools you down!
High Specific Heat
Substance C (J/g oC)
Air 1.01
Aluminum 0.902
Copper 0.385
Gold 0.129 Iron 0.450
Mercury 0.140
NaCl 0.864 Ice 2.03
Water 4.18
• Water (liquid) has a high specific heat compared to other substances.
• It requires more heat to increase it’s temperature
• 0-100 degrees Celsius range!
Specific Heat Definiton:
• The quantity of heat required to raise the temperature of a substance by one degree Celsius is called the specific heat capacity of the substance.
Phase Changes
• High heat of fusion: Water loses a large amount of heat when it cools.
• High heat of vaporization: Water requires a LARGE amount of heat to change from a liquid to a gas
Water is Cohesive• Cohesion- ability of molecules to stick together; results in
surface tension– Water attracts other water molecules, therefore forming
hydrogen bonds.– Hydrogen bonds are weak bonds – Hydrogen bonds help hold large molecules together-
such as protein.
• Cohesion examples:– Water bugs “walking on water”
Surface Tension• Cohesion allows water to form an interface
with a surface.
• Surface tension is the measure of how difficult it is to break this interface.
• Due to H bonding between water molecules
• Ex: materials are able to rest on water, provided the surface tension is not broken.
Adhesion• Sticking together of one substance to
another.
• Water is very adhesive; sticks to substances like glue.
• Capillary action is adhesion and cohesion in action.– Capillary action is the process of pulling water
up against gravity in plant vessels
Imbibition
• Process of soaking into a hydrophilic substance.
• Water soaking into seeds, paper towels, or sponge.
• Important for plants (seed germination usually requires imbibition)
Freezing and Expansion• Water expands below 4o C.• Molecules are spaced further apart • It is less dense, therefore ice floats in
water.• Ice at surface of lakes and oceans provides a thermal insulator for life below the surface.
