A World of Water
Even though our planet is often referred to as the “blue planet” most of the water on Earth is salt water. We as humans, cannot consume salt water, meaning that we have to rely on the much smaller fresh water supply.
Water and Humans
The goal of this system is to absorb oxygen in exchange for carbon dioxide. Human breath in through inhalation filling their lungs with air containing oxygen. Once oxygen is absorbed the excess air is exhaled or breathed out, which rids our bodies of waste carbon dioxide. You can see the organs involved in the respiratory system in the figure below.
Human beings are constantly gaining and losing water. Every breath you take you lose some water vapour. Your body attempts to keep a consistent 37 degrees celsius, to keep cool you perspire or sweat, of course, you lose water during this process. In order to maintain health water levels we must drink ~2.5L of water each day (about ~10.5 cups).
The Water Cycle
The water cycle uses the states of matter, for example, evaporation is caused from water turning from a liquid to gaseous state, likewise, condensation occurs because of colder altitudes cause water vapour (water gas) to condense into a liquid; sometimes, if it is cold enough, water will even change state from a gas to solid in a process called sublimation.
Water at the surface of the ocean, rivers, and lakes can become water vapor and move into the atmosphere with a little added energy from the Sun through a process called evaporation. Snow and ice can also become water vapor through a process called sublimation. And water vapor gets into the atmosphere from plants by a process called transpiration.
Because air is cooler at higher altitude in the troposphere, water vapor cools as it rises high in the atmosphere and transforms into water droplets by a process called condensation. The water droplets that form make up clouds. Water vapor can also condense into droplets near the ground, forming fog when the ground is cold. If the temperature is cold enough, ice crystals form instead of liquid water droplets.
If the droplets or ice crystals within clouds grow in size, they eventually become too heavy to stay in the air, falling to the ground as rain, snow, and other types of precipitation.
Precipitated water can find its way back to large bodies of water or groundwater by following run-off streams which can lead to ponds, lakes, oceans, and other bodies of water.
As you are most likely aware human beings cannot drink salt water. This can become an issue because salt water is about ~97% of the Earth’s water supply. Instead we must drink fresh water. Fresh water is often found in groundwater, rivers, lakes, ponds, glaciers and ice sheets (see figure below). In fact, most of the world freshwater supply is “trapped” in glaciers in the north and south poles. Of the water that is readily available for human consumption, most of it is found in groundwater. Groundwater is water that can be found underground.
So you may be asking yourself, “Do we have enough water in the world?” The water cycle ensures that the water levels stay relatively the same on Earth, but not all freshwater is distributed evenly around the world (see figure below). Of course, humans consume a lot of water, not only for their daily function, but for other activities like: cooking, eating, washing, farming, manufacturing, and of course many others. Other activities add wastes, referred to as pollutants, into the water. These polluted waters are not long viable sources of freshwater until they have been cleaned.
The information on this page and it associated figures are adapted from the Science Focus 8.Science Focus 8, pp. 364-374