1. Unit A: Energy and Matter Exchange in the Biosphere
Chapter 3
Section 3.1
The Hydrological Cycle
(The Water Cycle)

2. Properties of Water All living things need water to survive.
Water is the universal solvent which most reactions take place in.
Understanding the properties of water can help in understanding the water cycle.

3. Water molecules (H20) are made up of 1 oxygen atom and 2 hydrogen atoms.
Water molecules are held together by covalent bonds.
A covalent bond refers to the sharing of electrons between the atoms.
Although they share the electrons, they do not share them evenly.
Oxygen is more electronegative than hydrogen, so therefore the oxygen will pull more strongly on the electrons.

4. Since electrons are negatively charged, and the oxygen molecules are pulling the electrons more strongly toward them, the oxygen atom will have a partially negative charge and the hydrogen atoms will have a partially positive charge.
This type of molecule, with a difference in charge from one end to the other, is referred to as a polar molecule.

5. Water As A Polar Molecule

6. Hydrogen Bonds
When there are many water molecules in close proximity they have a tendency to stick together.
This is because the negative end of one water molecule attracts the positive end of another water molecule.
This attraction forms a weak bond known as a hydrogen bond.

7. Hydrogen Bonds
Negative oxygen end is attracted to the positive hydrogen end on the next molecule = hydrogen bond.

8. Water is Wierd

9. The Hydrological Cycle
The movement of water through the biosphere from the atmosphere to Earth and back is called the hydrological cycle or the water cycle.
Water moves from the Earth to the atmosphere by the following processes:
- Evaporation: water changing from a liquid to water vapour
- Transpiration: the loss of water through plant leaves

10. Transpiration

11. Once water is in the atmosphere it undergoes condensation and forms tiny droplets of liquid water that remain suspended in the atmosphere as clouds.
Condensation is the process of changing from a gas to a liquid.
The clouds form precipitation (rain, snow, sleet, hail) and when the time is right the liquid drops come together and once they are too heavy to remain in the cloud they fall to the Earth.

12. Got Rain?

14. Lets draw it again…

15. Water Beneath the Soil
The fresh water that we use comes from two sources: ground water and surface water.
Precipitation that collects above the ground in lakes, pond, rivers and oceans is called surface water.
Precipitation increases surface runoff from the rivers and lakes to the ocean.

16. The water on the surface filters downwards because of gravity, this downward pull is called percolation.
As water seeps downwards, it carries dissolved organic matter and minerals to the bottom layers of soil, this process is called leaching.
Eventually, water will saturate the lower levels of soil, making the water table.

17. Acid Deposition and The Water Cycle
Our technologies such as factories and cars produce sulphur dioxide and nitric oxide, the most dangerous of the air pollutants.
The sulphur dioxide and nitric oxide enter the atmosphere and combine with water droplets to form acids.
The acids return to the surface of Earth in the form of snow or rain, called “acid rain.”

18. The Effects of Acid
Acid rain can have many effects on our ecosystems, such as:
- killing fish
- killing soil bacteria
- killing aquatic and terrestrial plants
- leaching nutrients from the soil
However, the oxides do not always enter the water cycle.
Depending on weather conditions the particles can remain in the air and settle in their dry state.
This is known as dry acid deposition.
These pollutants then form acids when they combine with surface water.