Animals in the water depend in the same way on oxygen as animals do on land and as man does. Where does the oxygen in the water come from?
Gases are soluble in water - also oxygen or carbon dioxide. An important part (1/4 to 1/3) of the CO2 emitted by man is taken up by the oceans.
Therefore we are going to study the solubility of gases in water.
Experimental setup:
Experiment:
1. Fill a beaker with fresh tap water and put a funnel reversly in the water.
2. Adjust a test-tube completely filled with water above the funnel. The opening of the test-tube hat to be dipped into the water.
3. Heat the water, but keep the temperature below the boiling point.
Result:
Air bubbles are formed and ascending in the test-tube if it is heated. The gas must have been dissolved in the water.
Task 1 (Experimental Homework):
Fill tap-water into a clean glass. Allow the glass to stand and observe what's happening. Give an explanation for your observations!
Result:
After a short time gas bubbles can be observed at the wall of the glass. We observe air, previously dissolved in the water, which is set free.
Task 2:
In water investigations the solubility is ususally given in mg/L. The dependence of the solubility on the water temperature becomes obvious, if you plot the values for oxygen from the following table in a diagramm and connect the points afterwards. What do you observe?
| Temperature [°C] | 0 | 5 | 10 | 15 | 20 | 25 | 30 |
| Solubility [mg / L] | 14,2 | 12,4 | 10,9 | 9,8 | 8,8 | 8,1 | 7,5 |
Result:
The solubility of oxygen in water depends on the water temperature. It is obvious that cold water dissolves more gas than warm water.
|
The Christ child - Special task on climate From the introduction of the chapter El Niño: On the west coast of the South American continent, just south of the equator, lies Peru. The cool Humboldt ocean current (also called the Peru current) brings water rich in nutrients to the coast, providing valuable nourishment for the fish. But every year, about Christmas time, a warm current comes and leaves the coastal fishermen with empty nets. Although it leaves the table for Christmas rather bare, the fishermen called this phenomenon "El Niño", meaning Christ Child. Normally on the west coast of South America, where Peru lies, cold water from the deeper layers of the ocean is transported upwards. During El Niño years this ascending water stream is inhibited. The surface water is warmer. Fishes avoid this region and the fishermen make a bad catch. Question: Why do fishes not like the warm surface water? What is missing? |
Task 3:
Think about where to store ideally a bottle with sparkling water.
Answer:
From the results above we conclude, that sparkling water does loose its gas more slowly if it is colder. Therefore a cool place is better for the storage than a warm one.
| Background:
Uptake of carbon dioxide by water Carbon dioxide is a greenhouse gas in the atmosphere. It is the oxide (= product of burning processes) of carbon. Carbon is burnt or oxydized if fossil fuel (charcoal, wood, oil) is burnt, but also if nutrients in our body are burnt. Carbon dioxide is soluble in water. Therefore there is an equilibrium between the CO2 dissolved in the oceans and the amount present in the air. Moreover carbon dioxide is taken up by the vegetation. Equilibrium - what does it mean? If a gentle stream of warm air is blown over the surface of cold water, the air looses heat and the water becomes warmer. If air enriched in CO2 is standing over the oceans, the air will give off CO2 towards the ocean until the flux from the ocean to the air and the flux from the air to the ocean are identical (equilibrium). The following talbe gives estimations for the annual global carbon [billions of tons = Pg = Petagram], which is released as CO2 to the air:
Picture: Partitioning of carbon released as carbon dioxide. Which amount goes where?
|
