Name: _TEACHER'S NOTES_ Period: ____
Over 70% of the Earth’s surface is covered by water. It makes up 50-95% of the weight of living organisms. Although pure water is colorless, odorless, and tasteless, its’ simple chemical structure gives it several properties that make it critical for life.
Chemical Properties:
1. Composition: A water molecule is composed of two atoms of __hydrogen__ “attached” to one atom of __oxygen__. Draw a diagram of a water molecule in the box on the right.
2. Polarity: Due to the chemical nature of an oxygen atom, it holds the “lion’s share” of the electrons bonding it to each hydrogen atom. This gives the oxygen atom a partial __negative__ charge and the hydrogen atoms a partial __positive__ charge. Molecules like this that have an unequal distribution of charges are called __polar__. Add the partial charges to the diagram of water.
3. Adhesion/Cohesion: Since opposite charges attract, water molecules tend to attract each other, making water appear “sticky.” This attraction between a partial negative charge and a partial positive charge is called a __hydrogen__ __bond__. Hydrogen bonds are responsible for the adhesive and cohesive properties of water. __Cohesion__ is the tendency of molecules to “stick” to one another, as with water. __Adhesion__ occurs when molecules “stick” to a different substance, such as the side of a glass.
4. Solubility: Water is called the “universal __solvent__” because it dissolves more substances than any other liquid. It moves valuable chemicals, minerals, and nutrients wherever it goes. Water is the __medium__ within which almost all the chemical reactions of life occur. Many molecules have different solubility, or extent to which they __dissolve__, in liquid water than in air.
Physical Properties:
1. States of Matter: Water is the only natural substance that is found is all three states of matter, _solid_ (ice), _liquid_, and _gas_ (steam or vapor). Water freezes, turns from a liquid to a solid, at _0_˚C (32˚F) and boils, turns from a liquid to a gas, at _100_˚C (212˚F). This boiling point is dependent on atmospheric pressure.
2. Specific Heat: Water has the _second_ highest specific heat capacity of any known substance: it takes much _more_ heat to raise the temperature of a volume of water than the same volume of air. This is the result of the extensive __hydrogen__ bonding between its molecules. This property allows water to regulate Earth's climate by buffering large fluctuations in temperature, especially near the oceans.
3. Density: The maximum density of water occurs at about _4_˚C (39˚F), which is highly unusual because it means that the solid form, ice, is _less_ dense than the liquid form! This special property means the difference between survival and death for many aquatic animals, and affects nutrient availability and biological diversity in temperate lakes.
4. Surface Tension: Another result of water’s high cohesion is a “skin” effect at the interface of water with other substances, called __surface__ __tension__. The attraction between the water molecules creates a strong film, which permits water to hold up heavier and denser substances. Surface tension is responsible for __capillary__ __action__, which allows the movement of water against the force of gravity.
Summary: Water’s __polar__ nature causes many of its special properties!
Demonstrations:
Adhesion: Use a micro-pipettor to apply small amounts of colored water to the inside rim of a Petri dish. The droplets will “stick” to the side by adhesion.
Cohesion: Use a micro-pipette tip to drag one droplet of colored water towards another. When close enough, the droplets will attract each other like magnets and very quickly combine into one larger droplet by cohesion.
Solubility: Dissolve various substances (i.e. food coloring, powdered drink mix) into different flasks of water to demonstrate the range of possible solutes.
States of Matter: Use a hot plate with a beaker of water at boiling temperature to show the transition from liquid to gas.
Density: Use ice cubes floating in a glass to show that they are less dense than the liquid.
Surface Tension: Apply a larger drop of colored water onto the center of a Petri dish. Surface tension holds the drop in a spherical shape, keeping it from spreading out into a thin film.
2 / Minority Science Programs – School of Biological Sciences – University of California, Irvine