Chapter 2-2: Properties of Water
Humans are approximately 60% water by weight and 75% water by volume. As much as 95% of the weight of some plants is due to the water they contain.
Water Formula: H2O: Hydrogen is held to oxygen by covalent bonds.
Water molecules are held to other water molecules by Hydrogen bonds: Weak covalent bonds specific for hydrogen.
Water is a polar molecule. This is an extremely important property of water.
Polarity: A molecule in which the charges (electrons) are unevenly distributed causing the molecule to have poles.
Why is water a polar molecule?
Water is a polar molecule because
• O contains 8 p+ in its nucleus and therefore has a much stronger attraction for e- .
• H contains a single p+ in its nucleus so e- spend less time around H.
This unequal distribution of charge causes the oxygen end to become slightly negatively charged and the hydrogen ends to become slightly positively charged.
How is water unique?
Hydrogen Bonding: water molecules are attracted to each other forming multiple hydrogen bonds between the oxygen of one water molecule and the hydrogen of another water molecule.
Polarity and hydrogen bonding of water causes many unique properties:
1. Cohesion: An attraction between molecules of the same substance causing molecules on the surface of water to be drawn inward.
Ex. Surface tension of insects walking on water and water forming droplets on smooth surfaces.
2. Adhesion: An attraction between molecules of different substances.
Ex. Capillary action of water being drawn up the roots of plants into the stems and leaves and the capillary action of water being drawn up a small glass tube. Hint: Cohesion holds the column of water together but adhesion allows the water to move against gravity.
Ex. Adhesion causes a meniscus in a graduated cylinder.
3. Heat Capacity: the amount of heat energy needed to increase the temperature. Water’s heat capacity is very high.
· Water will boil at 100oC or 212oF.
· The multiple hydrogen bonds need to be broken before water will boil and turn into steam. This requires a lot of heat.
4. Solutions: Water’s polarity gives it the ability to dissolve both ionic compounds and other polar molecules.
· Water dissolves ionic compounds by causing the ions (charged atoms) to break away and water surrounds them.
· This is called dissociation.
Ex. Solute = Salt Solvent = Water
· Water dissolves other polar molecules because “likes dissolve likes”.
· Polar molecules dissolve other polar molecules.
· Polar molecules do NOT dissolve non-polar molecules.
· The slightly negative end of a water molecule is attracted to the slightly positive end of another polar molecule and vice versa.
Ex. Solute = Sugar Solvent = Water
5. Suspensions: Some materials do not dissolve in water when mixed but can separate into small pieces that do not always settle out.
Ex. Blood which contains water and many cells and other molecules and sugar water.