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Earth’s Water • We live on a planet that is dominated by water. More than 70 % of the Earth's surface is covered with this simple molecule. Scientists estimate that the hydrosphere contains about 1.36 billion cubic kilometers of this substance mostly in the form of a liquid (water) that occupies topographic depressions on the Earth. The second most common form of the water molecule on our planet is ice. If all our planet's ice melted, sea-level would rise by about 70 meters.
Why study water? • There are many organisms in this world that can exist without oxygen, but all living creatures need water in order to survive. Human beings, for example are between sixty-five and seventy percent water. Most of the plants that we use for food are between eighty-five and ninety percent water by weight.
And… • Water is used for life support in a variety of capacities ranging from habitats to reproduction to animal body functions (i.e. digest food, transport wastes, lubricate body joints, regulate temperature, and soften body tissues). It is clear that life as we know it could not exist on Earth without its most abundant and remarkable substance - water.
Water properties • Unique properties based on its structure • Oxygen is more electronegative than hydrogen • Electrons spend more time with oxygen than with hydrogen • This unequal sharing causes the oxygen to be slightly negative and the hydrogens to be slightly positive. • Polar covalent bond • ie unequal sharing
Polarity of Water • Since opposite electrical charges attract, water molecules tend to attract each other, making water kind of "sticky." As the right-side diagram shows, the side with the hydrogen atoms (positive charge) attracts the oxygen side (negative charge) of a different water molecule. • This attraction forms hydrogen bonds.
Why is polarity important? • Polarity causes molecules to interact in unique ways that influence life • Encourages formation of hydrogen bonds
Water is “sticky” • Water is attracted to other water. This is called cohesion. Water can also be attracted to other materials. This is called adhesion.
The walls of alveoli are coated with a thin film of water & this creates a potential problem. Water molecules, including those on the alveolar walls, are more attracted to each other than to air, and this attraction creates a force called surface tension. This surface tension increases as water molecules come closer together, which is what happens when we exhale & our alveoli become smaller (like air leaving a balloon). Potentially, surface tension could cause alveoli to collapse and, in addition, would make it more difficult to 're-expand' the alveoli (when you inhaled). Both of these would represent serious problems: if alveoli collapsed they'd contain no air & no oxygen to diffuse into the blood &, if 're-expansion' was more difficult, inhalation would be very, very difficult if not impossible. Fortunately, our alveoli do not collapse & inhalation is relatively easy because the lungs produce a substance called surfactant that reduces surface tension.
Surfactants Lung cells that produce surfactant
Adhesion & Capillary Action • This phenomenon causes water to stick to the sides of vertical structures despite gravity's downward pull. • Water's high surface tension allows for the formation of water droplets and waves, allows plants to move water (and dissolved nutrients) from their roots to their leaves, and the movement of blood through tiny vessels in the bodies of some animals.
Capillary Action • Surface tension is related to the cohesive properties of water. Capillary action however, is related to the adhesive properties of water. You can see capillary action 'in action' by placing a straw into a glass of water. The water 'climbs' up the straw. What is happening is that the water molecules are attracted to the straw molecules. When one water molecule moves closer to a the straw molecules the other water molecules (which are cohesively attracted to that water molecule) also move up into the straw. Capillary action is limited by gravity and the size of the straw. The thinner the straw or tube the higher up capillary action will pull the water
Water movement in plants • Plants take advantage of capillary action to pull water from the ground into themselves. From the roots water is drawn through the plant by another force, transpiration.
Terminology • Heat= amount of energy associated with the movement of atoms and molecules in matter • Temperature= measures the intensity of heat • Calorie= amount of energy needed to raise 1000g of water 1 degree C
Specific Heat of Water • Water has a high specific heat. Specific heat is the amount of energy required to change the temperature of a substance. Because water has a high specific heat, it can absorb large amounts of heat energy before it begins to get hot. It also means that water releases heat energy slowly when situations cause it to cool. Water's high specific heat allows for the moderation of the Earth's climate and helps organisms regulate their body temperature more effectively.
Water's high specific heat allows for the moderation of the Earth's climate and helps organisms regulate their body temperature more effectively.
An Example… • One way to observe this characteristic is to heat an empty pan over a fire. The pan quickly becomes red hot and may start to bum. Place the same pan over the fire with water in it and the liquid will absorb some of the heat. The pan will again become hot but not as hot as before. The temperature of the water, even if it boils, will rise only a small amount compared to the temperature of the pan.
Simple Experiment • A Pyrex measuring cup was filled with either water, rubbing alcohol, or air, and then heated in a pan of simmering water. • In the case of the air, a plastic covering was fashioned for the measuring cup in order to keep the volume of air in the cup contained. • Initially, the cup and its contents were at room temperature. The temperature of the simmering water in the pan was approximately 200 degrees Fahrenheit. • A thermometer was placed in the measuring cup in order to measure the temperature of the contents of the cup. • Temperature readings were taken every minute for ten minutes. • The cup was then removed from the water and placed in a cold water bath where the temperature of the cup's contents was again measured every minute for ten minutes.
Graphs of the temp. changes • Heating vs. Cooling
This leads to evaporative cooling • A water molecule takes a large amount of energy with it when it evaporates Figure 2.12
Physical properties of water • Density • Density increases as temp decreases • Water max density 4 degrees C • Molecules move apart • Solvent properties • Protects ions from one another • Charged substances dissolve • Universal solvent
as a solid • as a liquid • as a gas • Like no other common substance, water exists in nature in all three physical states: Figure 2.10B
Don’t forget the water cycle! • It is this quality which allows for the water cycle, the process whereby the Earth's water is moved by the energy of the sun and the force of gravity through a cycle of evaporation, condensation, precipitation and accumulation.
Water is less dense as a solid. • Most liquids contract (get smaller) when they get colder. Water is different. Water contracts until it reaches 4°C then it expands until it is solid. Solid water is less dense that liquid water because of this. If water worked like other liquids, then there would be no such thing as an ice berg, the ice in your soft drink would sink to the bottom of the glass, and ponds would freeze from the bottom up (not good if you are a fish)!
Water is less dense as a solid than as a liquid • When water freezes, it forms crystal structure which is spacious –leading to increase in volume • Thus ice floats in water • This insulates the life underneath!
More Characteristics • Pure water has a neutral pH of 7, which is neither acidic nor basic. • Pure water is a very poor conductor of electricity! If that is the case, why do we have to be so careful with our electrical appliances?
Universal Solvent • Water is called the "universal solvent" because it dissolves more substances than any other liquid. This means that wherever water goes, either through the ground or through our bodies, it takes along valuable chemicals, minerals, and nutrients.
